Merge #10483: scripted-diff: Use the C++11 keyword nullptr to denote the pointer literal instead of the macro NULL

90d4d89 scripted-diff: Use the C++11 keyword nullptr to denote the pointer literal instead of the macro NULL (practicalswift)

Pull request description:

  Since C++11 the macro `NULL` may be:
  * an integer literal with value zero, or
  * a prvalue of type `std::nullptr_t`

  By using the C++11 keyword `nullptr` we are guaranteed a prvalue of type `std::nullptr_t`.

  For a more thorough discussion, see "A name for the null pointer: nullptr" (Sutter &
  Stroustrup), http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf

  With this patch applied there are no `NULL` macro usages left in the repo:

  ```
  $ git grep NULL -- "*.cpp" "*.h" | egrep -v '(/univalue/|/secp256k1/|/leveldb/|_NULL|NULLDUMMY|torcontrol.*NULL|NULL cert)' | wc -l
  0
  ```

  The road towards `nullptr` (C++11) is split into two PRs:
  * `NULL` → `nullptr` is handled in PR #10483 (scripted, this PR)
  * `0` → `nullptr` is handled in PR #10645 (manual)

Tree-SHA512: 3c395d66f2ad724a8e6fed74b93634de8bfc0c0eafac94e64e5194c939499fefd6e68f047de3083ad0b4eff37df9a8a3a76349aa17d55eabbd8e0412f140a297

104 files changed, 563 insertions, 563 deletions

diff --git a/src/addrman.cpp b/src/addrman.cpp
index 4a408b9beb..a56bb4f9c1 100644
--- a/src/addrman.cpp
+++ b/src/addrman.cpp
@@ -69,13 +69,13 @@ CAddrInfo* CAddrMan::Find(const CNetAddr& addr, int* pnId)
 {
     std::map<CNetAddr, int>::iterator it = mapAddr.find(addr);
     if (it == mapAddr.end())
-        return NULL;
+        return nullptr;
     if (pnId)
         *pnId = (*it).second;
     std::map<int, CAddrInfo>::iterator it2 = mapInfo.find((*it).second);
     if (it2 != mapInfo.end())
         return &(*it2).second;
-    return NULL;
+    return nullptr;
 }
 
 CAddrInfo* CAddrMan::Create(const CAddress& addr, const CNetAddr& addrSource, int* pnId)
diff --git a/src/addrman.h b/src/addrman.h
index 70d907488f..547088aedf 100644
--- a/src/addrman.h
+++ b/src/addrman.h
@@ -220,11 +220,11 @@ protected:
     FastRandomContext insecure_rand;
 
     //! Find an entry.
-    CAddrInfo* Find(const CNetAddr& addr, int *pnId = NULL);
+    CAddrInfo* Find(const CNetAddr& addr, int *pnId = nullptr);
 
     //! find an entry, creating it if necessary.
     //! nTime and nServices of the found node are updated, if necessary.
-    CAddrInfo* Create(const CAddress &addr, const CNetAddr &addrSource, int *pnId = NULL);
+    CAddrInfo* Create(const CAddress &addr, const CNetAddr &addrSource, int *pnId = nullptr);
 
     //! Swap two elements in vRandom.
     void SwapRandom(unsigned int nRandomPos1, unsigned int nRandomPos2);
diff --git a/src/arith_uint256.h b/src/arith_uint256.h
index c7734035df..6223e4afeb 100644
--- a/src/arith_uint256.h
+++ b/src/arith_uint256.h
@@ -283,7 +283,7 @@ public:
      * complexities of the sign bit and using base 256 are probably an
      * implementation accident.
      */
-    arith_uint256& SetCompact(uint32_t nCompact, bool *pfNegative = NULL, bool *pfOverflow = NULL);
+    arith_uint256& SetCompact(uint32_t nCompact, bool *pfNegative = nullptr, bool *pfOverflow = nullptr);
     uint32_t GetCompact(bool fNegative = false) const;
 
     friend uint256 ArithToUint256(const arith_uint256 &);
diff --git a/src/base58.cpp b/src/base58.cpp
index 17022a6bc1..c70e358ebb 100644
--- a/src/base58.cpp
+++ b/src/base58.cpp
@@ -37,7 +37,7 @@ bool DecodeBase58(const char* psz, std::vector<unsigned char>& vch)
     while (*psz && !isspace(*psz)) {
         // Decode base58 character
         const char* ch = strchr(pszBase58, *psz);
-        if (ch == NULL)
+        if (ch == nullptr)
             return false;
         // Apply "b256 = b256 * 58 + ch".
         int carry = ch - pszBase58;
diff --git a/src/base58.h b/src/base58.h
index 4de5cc6ce5..db12208f74 100644
--- a/src/base58.h
+++ b/src/base58.h
@@ -26,7 +26,7 @@
 
 /**
  * Encode a byte sequence as a base58-encoded string.
- * pbegin and pend cannot be NULL, unless both are.
+ * pbegin and pend cannot be nullptr, unless both are.
  */
 std::string EncodeBase58(const unsigned char* pbegin, const unsigned char* pend);
 
@@ -38,7 +38,7 @@ std::string EncodeBase58(const std::vector<unsigned char>& vch);
 /**
  * Decode a base58-encoded string (psz) into a byte vector (vchRet).
  * return true if decoding is successful.
- * psz cannot be NULL.
+ * psz cannot be nullptr.
  */
 bool DecodeBase58(const char* psz, std::vector<unsigned char>& vchRet);
 
diff --git a/src/bench/bench.cpp b/src/bench/bench.cpp
index f424572913..849d924af2 100644
--- a/src/bench/bench.cpp
+++ b/src/bench/bench.cpp
@@ -17,7 +17,7 @@ benchmark::BenchRunner::BenchmarkMap &benchmark::BenchRunner::benchmarks() {
 
 static double gettimedouble(void) {
     struct timeval tv;
-    gettimeofday(&tv, NULL);
+    gettimeofday(&tv, nullptr);
     return tv.tv_usec * 0.000001 + tv.tv_sec;
 }
 
diff --git a/src/bitcoin-cli.cpp b/src/bitcoin-cli.cpp
index c1a7c927e3..0b74f31bfe 100644
--- a/src/bitcoin-cli.cpp
+++ b/src/bitcoin-cli.cpp
@@ -161,8 +161,8 @@ static void http_request_done(struct evhttp_request *req, void *ctx)
 {
     HTTPReply *reply = static_cast<HTTPReply*>(ctx);
 
-    if (req == NULL) {
-        /* If req is NULL, it means an error occurred while connecting: the
+    if (req == nullptr) {
+        /* If req is nullptr, it means an error occurred while connecting: the
          * error code will have been passed to http_error_cb.
          */
         reply->status = 0;
@@ -210,7 +210,7 @@ UniValue CallRPC(const std::string& strMethod, const UniValue& params)
 
     HTTPReply response;
     raii_evhttp_request req = obtain_evhttp_request(http_request_done, (void*)&response);
-    if (req == NULL)
+    if (req == nullptr)
         throw std::runtime_error("create http request failed");
 #if LIBEVENT_VERSION_NUMBER >= 0x02010300
     evhttp_request_set_error_cb(req.get(), http_error_cb);
@@ -370,7 +370,7 @@ int CommandLineRPC(int argc, char *argv[])
         nRet = EXIT_FAILURE;
     }
     catch (...) {
-        PrintExceptionContinue(NULL, "CommandLineRPC()");
+        PrintExceptionContinue(nullptr, "CommandLineRPC()");
         throw;
     }
 
@@ -397,7 +397,7 @@ int main(int argc, char* argv[])
         PrintExceptionContinue(&e, "AppInitRPC()");
         return EXIT_FAILURE;
     } catch (...) {
-        PrintExceptionContinue(NULL, "AppInitRPC()");
+        PrintExceptionContinue(nullptr, "AppInitRPC()");
         return EXIT_FAILURE;
     }
 
@@ -408,7 +408,7 @@ int main(int argc, char* argv[])
     catch (const std::exception& e) {
         PrintExceptionContinue(&e, "CommandLineRPC()");
     } catch (...) {
-        PrintExceptionContinue(NULL, "CommandLineRPC()");
+        PrintExceptionContinue(nullptr, "CommandLineRPC()");
     }
     return ret;
 }
diff --git a/src/bitcoin-tx.cpp b/src/bitcoin-tx.cpp
index 9acb3fd30e..e5ea9f3228 100644
--- a/src/bitcoin-tx.cpp
+++ b/src/bitcoin-tx.cpp
@@ -822,7 +822,7 @@ static int CommandLineRawTx(int argc, char* argv[])
         nRet = EXIT_FAILURE;
     }
     catch (...) {
-        PrintExceptionContinue(NULL, "CommandLineRawTx()");
+        PrintExceptionContinue(nullptr, "CommandLineRawTx()");
         throw;
     }
 
@@ -845,7 +845,7 @@ int main(int argc, char* argv[])
         PrintExceptionContinue(&e, "AppInitRawTx()");
         return EXIT_FAILURE;
     } catch (...) {
-        PrintExceptionContinue(NULL, "AppInitRawTx()");
+        PrintExceptionContinue(nullptr, "AppInitRawTx()");
         return EXIT_FAILURE;
     }
 
@@ -856,7 +856,7 @@ int main(int argc, char* argv[])
     catch (const std::exception& e) {
         PrintExceptionContinue(&e, "CommandLineRawTx()");
     } catch (...) {
-        PrintExceptionContinue(NULL, "CommandLineRawTx()");
+        PrintExceptionContinue(nullptr, "CommandLineRawTx()");
     }
     return ret;
 }
diff --git a/src/bitcoind.cpp b/src/bitcoind.cpp
index ff61b9065b..eaef7a903b 100644
--- a/src/bitcoind.cpp
+++ b/src/bitcoind.cpp
@@ -170,7 +170,7 @@ bool AppInit(int argc, char* argv[])
     catch (const std::exception& e) {
         PrintExceptionContinue(&e, "AppInit()");
     } catch (...) {
-        PrintExceptionContinue(NULL, "AppInit()");
+        PrintExceptionContinue(nullptr, "AppInit()");
     }
 
     if (!fRet)
diff --git a/src/chain.cpp b/src/chain.cpp
index ffd58d471d..47acde882e 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -9,7 +9,7 @@
  * CChain implementation
  */
 void CChain::SetTip(CBlockIndex *pindex) {
-    if (pindex == NULL) {
+    if (pindex == nullptr) {
         vChain.clear();
         return;
     }
@@ -49,8 +49,8 @@ CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
 }
 
 const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
-    if (pindex == NULL) {
-        return NULL;
+    if (pindex == nullptr) {
+        return nullptr;
     }
     if (pindex->nHeight > Height())
         pindex = pindex->GetAncestor(Height());
@@ -63,7 +63,7 @@ CBlockIndex* CChain::FindEarliestAtLeast(int64_t nTime) const
 {
     std::vector<CBlockIndex*>::const_iterator lower = std::lower_bound(vChain.begin(), vChain.end(), nTime,
         [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTimeMax() < time; });
-    return (lower == vChain.end() ? NULL : *lower);
+    return (lower == vChain.end() ? nullptr : *lower);
 }
 
 /** Turn the lowest '1' bit in the binary representation of a number into a '0'. */
@@ -83,14 +83,14 @@ int static inline GetSkipHeight(int height) {
 CBlockIndex* CBlockIndex::GetAncestor(int height)
 {
     if (height > nHeight || height < 0)
-        return NULL;
+        return nullptr;
 
     CBlockIndex* pindexWalk = this;
     int heightWalk = nHeight;
     while (heightWalk > height) {
         int heightSkip = GetSkipHeight(heightWalk);
         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
-        if (pindexWalk->pskip != NULL &&
+        if (pindexWalk->pskip != nullptr &&
             (heightSkip == height ||
              (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
                                        heightSkipPrev >= height)))) {
@@ -150,7 +150,7 @@ int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& fr
 }
 
 /** Find the last common ancestor two blocks have.
- *  Both pa and pb must be non-NULL. */
+ *  Both pa and pb must be non-nullptr. */
 const CBlockIndex* LastCommonAncestor(const CBlockIndex* pa, const CBlockIndex* pb) {
     if (pa->nHeight > pb->nHeight) {
         pa = pa->GetAncestor(pb->nHeight);
diff --git a/src/chain.h b/src/chain.h
index c5304b7d6f..1223539e73 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -221,9 +221,9 @@ public:
 
     void SetNull()
     {
-        phashBlock = NULL;
-        pprev = NULL;
-        pskip = NULL;
+        phashBlock = nullptr;
+        pprev = nullptr;
+        pskip = nullptr;
         nHeight = 0;
         nFile = 0;
         nDataPos = 0;
@@ -437,20 +437,20 @@ private:
     std::vector<CBlockIndex*> vChain;
 
 public:
-    /** Returns the index entry for the genesis block of this chain, or NULL if none. */
+    /** Returns the index entry for the genesis block of this chain, or nullptr if none. */
     CBlockIndex *Genesis() const {
-        return vChain.size() > 0 ? vChain[0] : NULL;
+        return vChain.size() > 0 ? vChain[0] : nullptr;
     }
 
-    /** Returns the index entry for the tip of this chain, or NULL if none. */
+    /** Returns the index entry for the tip of this chain, or nullptr if none. */
     CBlockIndex *Tip() const {
-        return vChain.size() > 0 ? vChain[vChain.size() - 1] : NULL;
+        return vChain.size() > 0 ? vChain[vChain.size() - 1] : nullptr;
     }
 
-    /** Returns the index entry at a particular height in this chain, or NULL if no such height exists. */
+    /** Returns the index entry at a particular height in this chain, or nullptr if no such height exists. */
     CBlockIndex *operator[](int nHeight) const {
         if (nHeight < 0 || nHeight >= (int)vChain.size())
-            return NULL;
+            return nullptr;
         return vChain[nHeight];
     }
 
@@ -465,12 +465,12 @@ public:
         return (*this)[pindex->nHeight] == pindex;
     }
 
-    /** Find the successor of a block in this chain, or NULL if the given index is not found or is the tip. */
+    /** Find the successor of a block in this chain, or nullptr if the given index is not found or is the tip. */
     CBlockIndex *Next(const CBlockIndex *pindex) const {
         if (Contains(pindex))
             return (*this)[pindex->nHeight + 1];
         else
-            return NULL;
+            return nullptr;
     }
 
     /** Return the maximal height in the chain. Is equal to chain.Tip() ? chain.Tip()->nHeight : -1. */
@@ -482,7 +482,7 @@ public:
     void SetTip(CBlockIndex *pindex);
 
     /** Return a CBlockLocator that refers to a block in this chain (by default the tip). */
-    CBlockLocator GetLocator(const CBlockIndex *pindex = NULL) const;
+    CBlockLocator GetLocator(const CBlockIndex *pindex = nullptr) const;
 
     /** Find the last common block between this chain and a block index entry. */
     const CBlockIndex *FindFork(const CBlockIndex *pindex) const;
diff --git a/src/checkpoints.cpp b/src/checkpoints.cpp
index e6b5fb72a7..5afe3e66b6 100644
--- a/src/checkpoints.cpp
+++ b/src/checkpoints.cpp
@@ -26,7 +26,7 @@ namespace Checkpoints {
             if (t != mapBlockIndex.end())
                 return t->second;
         }
-        return NULL;
+        return nullptr;
     }
 
 } // namespace Checkpoints
diff --git a/src/checkqueue.h b/src/checkqueue.h
index 408e278d21..4bc6be45f8 100644
--- a/src/checkqueue.h
+++ b/src/checkqueue.h
@@ -183,15 +183,15 @@ public:
     CCheckQueueControl& operator=(const CCheckQueueControl&) = delete;
     explicit CCheckQueueControl(CCheckQueue<T> * const pqueueIn) : pqueue(pqueueIn), fDone(false)
     {
-        // passed queue is supposed to be unused, or NULL
-        if (pqueue != NULL) {
+        // passed queue is supposed to be unused, or nullptr
+        if (pqueue != nullptr) {
             ENTER_CRITICAL_SECTION(pqueue->ControlMutex);
         }
     }
 
     bool Wait()
     {
-        if (pqueue == NULL)
+        if (pqueue == nullptr)
             return true;
         bool fRet = pqueue->Wait();
         fDone = true;
@@ -200,7 +200,7 @@ public:
 
     void Add(std::vector<T>& vChecks)
     {
-        if (pqueue != NULL)
+        if (pqueue != nullptr)
             pqueue->Add(vChecks);
     }
 
@@ -208,7 +208,7 @@ public:
     {
         if (!fDone)
             Wait();
-        if (pqueue != NULL) {
+        if (pqueue != nullptr) {
             LEAVE_CRITICAL_SECTION(pqueue->ControlMutex);
         }
     }
diff --git a/src/consensus/merkle.cpp b/src/consensus/merkle.cpp
index 1ce5a9d87e..798ce4b5fd 100644
--- a/src/consensus/merkle.cpp
+++ b/src/consensus/merkle.cpp
@@ -132,13 +132,13 @@ static void MerkleComputation(const std::vector<uint256>& leaves, uint256* proot
 
 uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated) {
     uint256 hash;
-    MerkleComputation(leaves, &hash, mutated, -1, NULL);
+    MerkleComputation(leaves, &hash, mutated, -1, nullptr);
     return hash;
 }
 
 std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position) {
     std::vector<uint256> ret;
-    MerkleComputation(leaves, NULL, NULL, position, &ret);
+    MerkleComputation(leaves, nullptr, nullptr, position, &ret);
     return ret;
 }
 
diff --git a/src/consensus/merkle.h b/src/consensus/merkle.h
index 194aea9b75..33764c7460 100644
--- a/src/consensus/merkle.h
+++ b/src/consensus/merkle.h
@@ -12,7 +12,7 @@
 #include "primitives/block.h"
 #include "uint256.h"
 
-uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated = NULL);
+uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated = nullptr);
 std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position);
 uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint256>& branch, uint32_t position);
 
@@ -20,13 +20,13 @@ uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint2
  * Compute the Merkle root of the transactions in a block.
  * *mutated is set to true if a duplicated subtree was found.
  */
-uint256 BlockMerkleRoot(const CBlock& block, bool* mutated = NULL);
+uint256 BlockMerkleRoot(const CBlock& block, bool* mutated = nullptr);
 
 /*
  * Compute the Merkle root of the witness transactions in a block.
  * *mutated is set to true if a duplicated subtree was found.
  */
-uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated = NULL);
+uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated = nullptr);
 
 /*
  * Compute the Merkle branch for the tree of transactions in a block, for a
diff --git a/src/core_write.cpp b/src/core_write.cpp
index 217b491a0d..0eae4703ab 100644
--- a/src/core_write.cpp
+++ b/src/core_write.cpp
@@ -102,7 +102,7 @@ std::string ScriptToAsmStr(const CScript& script, const bool fAttemptSighashDeco
                     // this won't decode correctly formatted public keys in Pubkey or Multisig scripts due to
                     // the restrictions on the pubkey formats (see IsCompressedOrUncompressedPubKey) being incongruous with the
                     // checks in CheckSignatureEncoding.
-                    if (CheckSignatureEncoding(vch, SCRIPT_VERIFY_STRICTENC, NULL)) {
+                    if (CheckSignatureEncoding(vch, SCRIPT_VERIFY_STRICTENC, nullptr)) {
                         const unsigned char chSigHashType = vch.back();
                         if (mapSigHashTypes.count(chSigHashType)) {
                             strSigHashDecode = "[" + mapSigHashTypes.find(chSigHashType)->second + "]";
diff --git a/src/crypto/chacha20.cpp b/src/crypto/chacha20.cpp
index 816ae870e1..4038ae9f86 100644
--- a/src/crypto/chacha20.cpp
+++ b/src/crypto/chacha20.cpp
@@ -75,7 +75,7 @@ void ChaCha20::Output(unsigned char* c, size_t bytes)
 {
     uint32_t x0, x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, x11, x12, x13, x14, x15;
     uint32_t j0, j1, j2, j3, j4, j5, j6, j7, j8, j9, j10, j11, j12, j13, j14, j15;
-    unsigned char *ctarget = NULL;
+    unsigned char *ctarget = nullptr;
     unsigned char tmp[64];
     unsigned int i;
 
diff --git a/src/dbwrapper.cpp b/src/dbwrapper.cpp
index 3626e0177f..27767f90b4 100644
--- a/src/dbwrapper.cpp
+++ b/src/dbwrapper.cpp
@@ -92,7 +92,7 @@ static leveldb::Options GetOptions(size_t nCacheSize)
 
 CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bool fWipe, bool obfuscate)
 {
-    penv = NULL;
+    penv = nullptr;
     readoptions.verify_checksums = true;
     iteroptions.verify_checksums = true;
     iteroptions.fill_cache = false;
@@ -144,15 +144,15 @@ CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bo
 CDBWrapper::~CDBWrapper()
 {
     delete pdb;
-    pdb = NULL;
+    pdb = nullptr;
     delete options.filter_policy;
-    options.filter_policy = NULL;
+    options.filter_policy = nullptr;
     delete options.info_log;
-    options.info_log = NULL;
+    options.info_log = nullptr;
     delete options.block_cache;
-    options.block_cache = NULL;
+    options.block_cache = nullptr;
     delete penv;
-    options.env = NULL;
+    options.env = nullptr;
 }
 
 bool CDBWrapper::WriteBatch(CDBBatch& batch, bool fSync)
diff --git a/src/dbwrapper.h b/src/dbwrapper.h
index d36188652a..7575d207ae 100644
--- a/src/dbwrapper.h
+++ b/src/dbwrapper.h
@@ -177,7 +177,7 @@ class CDBWrapper
 {
     friend const std::vector<unsigned char>& dbwrapper_private::GetObfuscateKey(const CDBWrapper &w);
 private:
-    //! custom environment this database is using (may be NULL in case of default environment)
+    //! custom environment this database is using (may be nullptr in case of default environment)
     leveldb::Env* penv;
 
     //! database options used
diff --git a/src/httpserver.cpp b/src/httpserver.cpp
index ba01255400..5adfca4094 100644
--- a/src/httpserver.cpp
+++ b/src/httpserver.cpp
@@ -292,7 +292,7 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
 static void http_reject_request_cb(struct evhttp_request* req, void*)
 {
     LogPrint(BCLog::HTTP, "Rejecting request while shutting down\n");
-    evhttp_send_error(req, HTTP_SERVUNAVAIL, NULL);
+    evhttp_send_error(req, HTTP_SERVUNAVAIL, nullptr);
 }
 
 /** Event dispatcher thread */
@@ -334,7 +334,7 @@ static bool HTTPBindAddresses(struct evhttp* http)
     // Bind addresses
     for (std::vector<std::pair<std::string, uint16_t> >::iterator i = endpoints.begin(); i != endpoints.end(); ++i) {
         LogPrint(BCLog::HTTP, "Binding RPC on address %s port %i\n", i->first, i->second);
-        evhttp_bound_socket *bind_handle = evhttp_bind_socket_with_handle(http, i->first.empty() ? NULL : i->first.c_str(), i->second);
+        evhttp_bound_socket *bind_handle = evhttp_bind_socket_with_handle(http, i->first.empty() ? nullptr : i->first.c_str(), i->second);
         if (bind_handle) {
             boundSockets.push_back(bind_handle);
         } else {
@@ -404,7 +404,7 @@ bool InitHTTPServer()
     evhttp_set_timeout(http, GetArg("-rpcservertimeout", DEFAULT_HTTP_SERVER_TIMEOUT));
     evhttp_set_max_headers_size(http, MAX_HEADERS_SIZE);
     evhttp_set_max_body_size(http, MAX_SIZE);
-    evhttp_set_gencb(http, http_request_cb, NULL);
+    evhttp_set_gencb(http, http_request_cb, nullptr);
 
     if (!HTTPBindAddresses(http)) {
         LogPrintf("Unable to bind any endpoint for RPC server\n");
@@ -464,7 +464,7 @@ void InterruptHTTPServer()
             evhttp_del_accept_socket(eventHTTP, socket);
         }
         // Reject requests on current connections
-        evhttp_set_gencb(eventHTTP, http_reject_request_cb, NULL);
+        evhttp_set_gencb(eventHTTP, http_reject_request_cb, nullptr);
     }
     if (workQueue)
         workQueue->Interrupt();
@@ -530,7 +530,7 @@ HTTPEvent::~HTTPEvent()
 }
 void HTTPEvent::trigger(struct timeval* tv)
 {
-    if (tv == NULL)
+    if (tv == nullptr)
         event_active(ev, 0, 0); // immediately trigger event in main thread
     else
         evtimer_add(ev, tv); // trigger after timeval passed
@@ -573,7 +573,7 @@ std::string HTTPRequest::ReadBody()
      * abstraction to consume the evbuffer on the fly in the parsing algorithm.
      */
     const char* data = (const char*)evbuffer_pullup(buf, size);
-    if (!data) // returns NULL in case of empty buffer
+    if (!data) // returns nullptr in case of empty buffer
         return "";
     std::string rv(data, size);
     evbuffer_drain(buf, size);
@@ -600,7 +600,7 @@ void HTTPRequest::WriteReply(int nStatus, const std::string& strReply)
     assert(evb);
     evbuffer_add(evb, strReply.data(), strReply.size());
     HTTPEvent* ev = new HTTPEvent(eventBase, true,
-        std::bind(evhttp_send_reply, req, nStatus, (const char*)NULL, (struct evbuffer *)NULL));
+        std::bind(evhttp_send_reply, req, nStatus, (const char*)nullptr, (struct evbuffer *)nullptr));
     ev->trigger(0);
     replySent = true;
     req = 0; // transferred back to main thread
@@ -669,7 +669,7 @@ void UnregisterHTTPHandler(const std::string &prefix, bool exactMatch)
 std::string urlDecode(const std::string &urlEncoded) {
     std::string res;
     if (!urlEncoded.empty()) {
-        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, NULL);
+        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, nullptr);
         if (decoded) {
             res = std::string(decoded);
             free(decoded);
diff --git a/src/init.cpp b/src/init.cpp
index ca62d3e7cc..cddb42c0ee 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -76,7 +76,7 @@ std::unique_ptr<CConnman> g_connman;
 std::unique_ptr<PeerLogicValidation> peerLogic;
 
 #if ENABLE_ZMQ
-static CZMQNotificationInterface* pzmqNotificationInterface = NULL;
+static CZMQNotificationInterface* pzmqNotificationInterface = nullptr;
 #endif
 
 #ifdef WIN32
@@ -153,7 +153,7 @@ public:
     // Writes do not need similar protection, as failure to write is handled by the caller.
 };
 
-static CCoinsViewErrorCatcher *pcoinscatcher = NULL;
+static CCoinsViewErrorCatcher *pcoinscatcher = nullptr;
 static std::unique_ptr<ECCVerifyHandle> globalVerifyHandle;
 
 void Interrupt(boost::thread_group& threadGroup)
@@ -232,17 +232,17 @@ void Shutdown()
 
     {
         LOCK(cs_main);
-        if (pcoinsTip != NULL) {
+        if (pcoinsTip != nullptr) {
             FlushStateToDisk();
         }
         delete pcoinsTip;
-        pcoinsTip = NULL;
+        pcoinsTip = nullptr;
         delete pcoinscatcher;
-        pcoinscatcher = NULL;
+        pcoinscatcher = nullptr;
         delete pcoinsdbview;
-        pcoinsdbview = NULL;
+        pcoinsdbview = nullptr;
         delete pblocktree;
-        pblocktree = NULL;
+        pblocktree = nullptr;
     }
 #ifdef ENABLE_WALLET
     for (CWalletRef pwallet : vpwallets) {
@@ -254,7 +254,7 @@ void Shutdown()
     if (pzmqNotificationInterface) {
         UnregisterValidationInterface(pzmqNotificationInterface);
         delete pzmqNotificationInterface;
-        pzmqNotificationInterface = NULL;
+        pzmqNotificationInterface = nullptr;
     }
 #endif
 
@@ -300,7 +300,7 @@ static void registerSignalHandler(int signal, void(*handler)(int))
     sa.sa_handler = handler;
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = 0;
-    sigaction(signal, &sa, NULL);
+    sigaction(signal, &sa, nullptr);
 }
 #endif
 
@@ -558,7 +558,7 @@ static CConditionVariable condvar_GenesisWait;
 
 static void BlockNotifyGenesisWait(bool, const CBlockIndex *pBlockIndex)
 {
-    if (pBlockIndex != NULL) {
+    if (pBlockIndex != nullptr) {
         {
             boost::unique_lock<boost::mutex> lock_GenesisWait(cs_GenesisWait);
             fHaveGenesis = true;
@@ -843,7 +843,7 @@ bool AppInitBasicSetup()
 #ifdef _MSC_VER
     // Turn off Microsoft heap dump noise
     _CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_FILE);
-    _CrtSetReportFile(_CRT_WARN, CreateFileA("NUL", GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, 0));
+    _CrtSetReportFile(_CRT_WARN, CreateFileA("NUL", GENERIC_WRITE, 0, nullptr, OPEN_EXISTING, 0, 0));
     // Disable confusing "helpful" text message on abort, Ctrl-C
     _set_abort_behavior(0, _WRITE_ABORT_MSG | _CALL_REPORTFAULT);
 #endif
@@ -858,7 +858,7 @@ bool AppInitBasicSetup()
 #endif
     typedef BOOL (WINAPI *PSETPROCDEPPOL)(DWORD);
     PSETPROCDEPPOL setProcDEPPol = (PSETPROCDEPPOL)GetProcAddress(GetModuleHandleA("Kernel32.dll"), "SetProcessDEPPolicy");
-    if (setProcDEPPol != NULL) setProcDEPPol(PROCESS_DEP_ENABLE);
+    if (setProcDEPPol != nullptr) setProcDEPPol(PROCESS_DEP_ENABLE);
 #endif
 
     if (!SetupNetworking())
@@ -1477,7 +1477,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                         strLoadError = _("Error initializing block database");
                         break;
                     }
-                    assert(chainActive.Tip() != NULL);
+                    assert(chainActive.Tip() != nullptr);
                 }
 
                 if (!fReset) {
@@ -1604,7 +1604,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     // Either install a handler to notify us when genesis activates, or set fHaveGenesis directly.
     // No locking, as this happens before any background thread is started.
-    if (chainActive.Tip() == NULL) {
+    if (chainActive.Tip() == nullptr) {
         uiInterface.NotifyBlockTip.connect(BlockNotifyGenesisWait);
     } else {
         fHaveGenesis = true;
diff --git a/src/key.cpp b/src/key.cpp
index 5a991fc1d2..315a3978c8 100644
--- a/src/key.cpp
+++ b/src/key.cpp
@@ -13,7 +13,7 @@
 #include <secp256k1.h>
 #include <secp256k1_recovery.h>
 
-static secp256k1_context* secp256k1_context_sign = NULL;
+static secp256k1_context* secp256k1_context_sign = nullptr;
 
 /** These functions are taken from the libsecp256k1 distribution and are very ugly. */
 static int ec_privkey_import_der(const secp256k1_context* ctx, unsigned char *out32, const unsigned char *privkey, size_t privkeylen) {
@@ -165,7 +165,7 @@ bool CKey::Sign(const uint256 &hash, std::vector<unsigned char>& vchSig, uint32_
     unsigned char extra_entropy[32] = {0};
     WriteLE32(extra_entropy, test_case);
     secp256k1_ecdsa_signature sig;
-    int ret = secp256k1_ecdsa_sign(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, test_case ? extra_entropy : NULL);
+    int ret = secp256k1_ecdsa_sign(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, test_case ? extra_entropy : nullptr);
     assert(ret);
     secp256k1_ecdsa_signature_serialize_der(secp256k1_context_sign, (unsigned char*)vchSig.data(), &nSigLen, &sig);
     vchSig.resize(nSigLen);
@@ -192,7 +192,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
     vchSig.resize(65);
     int rec = -1;
     secp256k1_ecdsa_recoverable_signature sig;
-    int ret = secp256k1_ecdsa_sign_recoverable(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, NULL);
+    int ret = secp256k1_ecdsa_sign_recoverable(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, nullptr);
     assert(ret);
     secp256k1_ecdsa_recoverable_signature_serialize_compact(secp256k1_context_sign, (unsigned char*)&vchSig[1], &rec, &sig);
     assert(ret);
@@ -289,10 +289,10 @@ bool ECC_InitSanityCheck() {
 }
 
 void ECC_Start() {
-    assert(secp256k1_context_sign == NULL);
+    assert(secp256k1_context_sign == nullptr);
 
     secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
-    assert(ctx != NULL);
+    assert(ctx != nullptr);
 
     {
         // Pass in a random blinding seed to the secp256k1 context.
@@ -307,7 +307,7 @@ void ECC_Start() {
 
 void ECC_Stop() {
     secp256k1_context *ctx = secp256k1_context_sign;
-    secp256k1_context_sign = NULL;
+    secp256k1_context_sign = nullptr;
 
     if (ctx) {
         secp256k1_context_destroy(ctx);
diff --git a/src/net.cpp b/src/net.cpp
index ca9a173abe..0ed2196063 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -306,7 +306,7 @@ CNode* CConnman::FindNode(const CNetAddr& ip)
     for (CNode* pnode : vNodes)
         if ((CNetAddr)pnode->addr == ip)
             return (pnode);
-    return NULL;
+    return nullptr;
 }
 
 CNode* CConnman::FindNode(const CSubNet& subNet)
@@ -315,7 +315,7 @@ CNode* CConnman::FindNode(const CSubNet& subNet)
     for (CNode* pnode : vNodes)
     if (subNet.Match((CNetAddr)pnode->addr))
         return (pnode);
-    return NULL;
+    return nullptr;
 }
 
 CNode* CConnman::FindNode(const std::string& addrName)
@@ -326,7 +326,7 @@ CNode* CConnman::FindNode(const std::string& addrName)
             return (pnode);
         }
     }
-    return NULL;
+    return nullptr;
 }
 
 CNode* CConnman::FindNode(const CService& addr)
@@ -335,7 +335,7 @@ CNode* CConnman::FindNode(const CService& addr)
     for (CNode* pnode : vNodes)
         if ((CService)pnode->addr == addr)
             return (pnode);
-    return NULL;
+    return nullptr;
 }
 
 bool CConnman::CheckIncomingNonce(uint64_t nonce)
@@ -366,16 +366,16 @@ static CAddress GetBindAddress(SOCKET sock)
 
 CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCountFailure)
 {
-    if (pszDest == NULL) {
+    if (pszDest == nullptr) {
         if (IsLocal(addrConnect))
-            return NULL;
+            return nullptr;
 
         // Look for an existing connection
         CNode* pnode = FindNode((CService)addrConnect);
         if (pnode)
         {
             LogPrintf("Failed to open new connection, already connected\n");
-            return NULL;
+            return nullptr;
         }
     }
 
@@ -393,7 +393,7 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
         if (!IsSelectableSocket(hSocket)) {
             LogPrintf("Cannot create connection: non-selectable socket created (fd >= FD_SETSIZE ?)\n");
             CloseSocket(hSocket);
-            return NULL;
+            return nullptr;
         }
 
         if (pszDest && addrConnect.IsValid()) {
@@ -408,7 +408,7 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
                 pnode->MaybeSetAddrName(std::string(pszDest));
                 CloseSocket(hSocket);
                 LogPrintf("Failed to open new connection, already connected\n");
-                return NULL;
+                return nullptr;
             }
         }
 
@@ -429,7 +429,7 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
         addrman.Attempt(addrConnect, fCountFailure);
     }
 
-    return NULL;
+    return nullptr;
 }
 
 void CConnman::DumpBanlist()
@@ -966,7 +966,7 @@ bool CConnman::AttemptToEvictConnection()
             NodeEvictionCandidate candidate = {node->GetId(), node->nTimeConnected, node->nMinPingUsecTime,
                                                node->nLastBlockTime, node->nLastTXTime,
                                                (node->nServices & nRelevantServices) == nRelevantServices,
-                                               node->fRelayTxes, node->pfilter != NULL, node->addr, node->nKeyedNetGroup};
+                                               node->fRelayTxes, node->pfilter != nullptr, node->addr, node->nKeyedNetGroup};
             vEvictionCandidates.push_back(candidate);
         }
     }
@@ -1524,7 +1524,7 @@ void ThreadMapPort()
 
 void MapPort(bool fUseUPnP)
 {
-    static boost::thread* upnp_thread = NULL;
+    static boost::thread* upnp_thread = nullptr;
 
     if (fUseUPnP)
     {
@@ -1539,7 +1539,7 @@ void MapPort(bool fUseUPnP)
         upnp_thread->interrupt();
         upnp_thread->join();
         delete upnp_thread;
-        upnp_thread = NULL;
+        upnp_thread = nullptr;
     }
 }
 
@@ -1685,7 +1685,7 @@ void CConnman::ThreadOpenConnections()
             for (const std::string& strAddr : gArgs.GetArgs("-connect"))
             {
                 CAddress addr(CService(), NODE_NONE);
-                OpenNetworkConnection(addr, false, NULL, strAddr.c_str());
+                OpenNetworkConnection(addr, false, nullptr, strAddr.c_str());
                 for (int i = 0; i < 10 && i < nLoop; i++)
                 {
                     if (!interruptNet.sleep_for(std::chrono::milliseconds(500)))
@@ -1841,7 +1841,7 @@ void CConnman::ThreadOpenConnections()
                 LogPrint(BCLog::NET, "Making feeler connection to %s\n", addrConnect.ToString());
             }
 
-            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, NULL, false, fFeeler);
+            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, nullptr, false, fFeeler);
         }
     }
 }
@@ -2153,9 +2153,9 @@ void Discover(boost::thread_group& threadGroup)
     struct ifaddrs* myaddrs;
     if (getifaddrs(&myaddrs) == 0)
     {
-        for (struct ifaddrs* ifa = myaddrs; ifa != NULL; ifa = ifa->ifa_next)
+        for (struct ifaddrs* ifa = myaddrs; ifa != nullptr; ifa = ifa->ifa_next)
         {
-            if (ifa->ifa_addr == NULL) continue;
+            if (ifa->ifa_addr == nullptr) continue;
             if ((ifa->ifa_flags & IFF_UP) == 0) continue;
             if (strcmp(ifa->ifa_name, "lo") == 0) continue;
             if (strcmp(ifa->ifa_name, "lo0") == 0) continue;
@@ -2208,8 +2208,8 @@ CConnman::CConnman(uint64_t nSeed0In, uint64_t nSeed1In) : nSeed0(nSeed0In), nSe
     nLastNodeId = 0;
     nSendBufferMaxSize = 0;
     nReceiveFloodSize = 0;
-    semOutbound = NULL;
-    semAddnode = NULL;
+    semOutbound = nullptr;
+    semAddnode = nullptr;
     flagInterruptMsgProc = false;
 
     Options connOptions;
@@ -2312,11 +2312,11 @@ bool CConnman::Start(CScheduler& scheduler, const Options& connOptions)
 
     fAddressesInitialized = true;
 
-    if (semOutbound == NULL) {
+    if (semOutbound == nullptr) {
         // initialize semaphore
         semOutbound = new CSemaphore(std::min((nMaxOutbound + nMaxFeeler), nMaxConnections));
     }
-    if (semAddnode == NULL) {
+    if (semAddnode == nullptr) {
         // initialize semaphore
         semAddnode = new CSemaphore(nMaxAddnode);
     }
@@ -2434,9 +2434,9 @@ void CConnman::Stop()
     vNodesDisconnected.clear();
     vhListenSocket.clear();
     delete semOutbound;
-    semOutbound = NULL;
+    semOutbound = nullptr;
     delete semAddnode;
-    semAddnode = NULL;
+    semAddnode = nullptr;
 }
 
 void CConnman::DeleteNode(CNode* pnode)
diff --git a/src/net.h b/src/net.h
index 93a76fc093..a32736aa97 100644
--- a/src/net.h
+++ b/src/net.h
@@ -170,7 +170,7 @@ public:
     void Interrupt();
     bool GetNetworkActive() const { return fNetworkActive; };
     void SetNetworkActive(bool active);
-    bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false, bool fFeeler = false, bool fAddnode = false);
+    bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = nullptr, const char *strDest = nullptr, bool fOneShot = false, bool fFeeler = false, bool fAddnode = false);
     bool CheckIncomingNonce(uint64_t nonce);
 
     bool ForNode(NodeId id, std::function<bool(CNode* pnode)> func);
@@ -470,7 +470,7 @@ bool AddLocal(const CNetAddr& addr, int nScore = LOCAL_NONE);
 bool RemoveLocal(const CService& addr);
 bool SeenLocal(const CService& addr);
 bool IsLocal(const CService& addr);
-bool GetLocal(CService &addr, const CNetAddr *paddrPeer = NULL);
+bool GetLocal(CService &addr, const CNetAddr *paddrPeer = nullptr);
 bool IsReachable(enum Network net);
 bool IsReachable(const CNetAddr &addr);
 CAddress GetLocalAddress(const CNetAddr *paddrPeer, ServiceFlags nLocalServices);
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 6aab13fe1c..766618130c 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -202,10 +202,10 @@ struct CNodeState {
         fCurrentlyConnected = false;
         nMisbehavior = 0;
         fShouldBan = false;
-        pindexBestKnownBlock = NULL;
+        pindexBestKnownBlock = nullptr;
         hashLastUnknownBlock.SetNull();
-        pindexLastCommonBlock = NULL;
-        pindexBestHeaderSent = NULL;
+        pindexLastCommonBlock = nullptr;
+        pindexBestHeaderSent = nullptr;
         nUnconnectingHeaders = 0;
         fSyncStarted = false;
         nHeadersSyncTimeout = 0;
@@ -230,7 +230,7 @@ std::map<NodeId, CNodeState> mapNodeState;
 CNodeState *State(NodeId pnode) {
     std::map<NodeId, CNodeState>::iterator it = mapNodeState.find(pnode);
     if (it == mapNodeState.end())
-        return NULL;
+        return nullptr;
     return &it->second;
 }
 
@@ -336,9 +336,9 @@ bool MarkBlockAsReceived(const uint256& hash) {
 // Requires cs_main.
 // returns false, still setting pit, if the block was already in flight from the same peer
 // pit will only be valid as long as the same cs_main lock is being held
-bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const CBlockIndex* pindex = NULL, std::list<QueuedBlock>::iterator** pit = NULL) {
+bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const CBlockIndex* pindex = nullptr, std::list<QueuedBlock>::iterator** pit = nullptr) {
     CNodeState *state = State(nodeid);
-    assert(state != NULL);
+    assert(state != nullptr);
 
     // Short-circuit most stuff in case its from the same node
     std::map<uint256, std::pair<NodeId, std::list<QueuedBlock>::iterator> >::iterator itInFlight = mapBlocksInFlight.find(hash);
@@ -353,14 +353,14 @@ bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const CBlockIndex*
     MarkBlockAsReceived(hash);
 
     std::list<QueuedBlock>::iterator it = state->vBlocksInFlight.insert(state->vBlocksInFlight.end(),
-            {hash, pindex, pindex != NULL, std::unique_ptr<PartiallyDownloadedBlock>(pit ? new PartiallyDownloadedBlock(&mempool) : NULL)});
+            {hash, pindex, pindex != nullptr, std::unique_ptr<PartiallyDownloadedBlock>(pit ? new PartiallyDownloadedBlock(&mempool) : nullptr)});
     state->nBlocksInFlight++;
     state->nBlocksInFlightValidHeaders += it->fValidatedHeaders;
     if (state->nBlocksInFlight == 1) {
         // We're starting a block download (batch) from this peer.
         state->nDownloadingSince = GetTimeMicros();
     }
-    if (state->nBlocksInFlightValidHeaders == 1 && pindex != NULL) {
+    if (state->nBlocksInFlightValidHeaders == 1 && pindex != nullptr) {
         nPeersWithValidatedDownloads++;
     }
     itInFlight = mapBlocksInFlight.insert(std::make_pair(hash, std::make_pair(nodeid, it))).first;
@@ -372,12 +372,12 @@ bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const CBlockIndex*
 /** Check whether the last unknown block a peer advertised is not yet known. */
 void ProcessBlockAvailability(NodeId nodeid) {
     CNodeState *state = State(nodeid);
-    assert(state != NULL);
+    assert(state != nullptr);
 
     if (!state->hashLastUnknownBlock.IsNull()) {
         BlockMap::iterator itOld = mapBlockIndex.find(state->hashLastUnknownBlock);
         if (itOld != mapBlockIndex.end() && itOld->second->nChainWork > 0) {
-            if (state->pindexBestKnownBlock == NULL || itOld->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
+            if (state->pindexBestKnownBlock == nullptr || itOld->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
                 state->pindexBestKnownBlock = itOld->second;
             state->hashLastUnknownBlock.SetNull();
         }
@@ -387,14 +387,14 @@ void ProcessBlockAvailability(NodeId nodeid) {
 /** Update tracking information about which blocks a peer is assumed to have. */
 void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
     CNodeState *state = State(nodeid);
-    assert(state != NULL);
+    assert(state != nullptr);
 
     ProcessBlockAvailability(nodeid);
 
     BlockMap::iterator it = mapBlockIndex.find(hash);
     if (it != mapBlockIndex.end() && it->second->nChainWork > 0) {
         // An actually better block was announced.
-        if (state->pindexBestKnownBlock == NULL || it->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
+        if (state->pindexBestKnownBlock == nullptr || it->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
             state->pindexBestKnownBlock = it->second;
     } else {
         // An unknown block was announced; just assume that the latest one is the best one.
@@ -461,17 +461,17 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<con
 
     vBlocks.reserve(vBlocks.size() + count);
     CNodeState *state = State(nodeid);
-    assert(state != NULL);
+    assert(state != nullptr);
 
     // Make sure pindexBestKnownBlock is up to date, we'll need it.
     ProcessBlockAvailability(nodeid);
 
-    if (state->pindexBestKnownBlock == NULL || state->pindexBestKnownBlock->nChainWork < chainActive.Tip()->nChainWork || state->pindexBestKnownBlock->nChainWork < UintToArith256(consensusParams.nMinimumChainWork)) {
+    if (state->pindexBestKnownBlock == nullptr || state->pindexBestKnownBlock->nChainWork < chainActive.Tip()->nChainWork || state->pindexBestKnownBlock->nChainWork < UintToArith256(consensusParams.nMinimumChainWork)) {
         // This peer has nothing interesting.
         return;
     }
 
-    if (state->pindexLastCommonBlock == NULL) {
+    if (state->pindexLastCommonBlock == nullptr) {
         // Bootstrap quickly by guessing a parent of our best tip is the forking point.
         // Guessing wrong in either direction is not a problem.
         state->pindexLastCommonBlock = chainActive[std::min(state->pindexBestKnownBlock->nHeight, chainActive.Height())];
@@ -546,7 +546,7 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<con
 bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
     LOCK(cs_main);
     CNodeState *state = State(nodeid);
-    if (state == NULL)
+    if (state == nullptr)
         return false;
     stats.nMisbehavior = state->nMisbehavior;
     stats.nSyncHeight = state->pindexBestKnownBlock ? state->pindexBestKnownBlock->nHeight : -1;
@@ -700,7 +700,7 @@ void Misbehaving(NodeId pnode, int howmuch)
         return;
 
     CNodeState *state = State(pnode);
-    if (state == NULL)
+    if (state == nullptr)
         return;
 
     state->nMisbehavior += howmuch;
@@ -1006,7 +1006,7 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
                         // To prevent fingerprinting attacks, only send blocks outside of the active
                         // chain if they are valid, and no more than a month older (both in time, and in
                         // best equivalent proof of work) than the best header chain we know about.
-                        send = mi->second->IsValid(BLOCK_VALID_SCRIPTS) && (pindexBestHeader != NULL) &&
+                        send = mi->second->IsValid(BLOCK_VALID_SCRIPTS) && (pindexBestHeader != nullptr) &&
                             (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() < nOneMonth) &&
                             (GetBlockProofEquivalentTime(*pindexBestHeader, *mi->second, *pindexBestHeader, consensusParams) < nOneMonth);
                         if (!send) {
@@ -1017,7 +1017,7 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
                 // disconnect node in case we have reached the outbound limit for serving historical blocks
                 // never disconnect whitelisted nodes
                 static const int nOneWeek = 7 * 24 * 60 * 60; // assume > 1 week = historical
-                if (send && connman.OutboundTargetReached(true) && ( ((pindexBestHeader != NULL) && (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() > nOneWeek)) || inv.type == MSG_FILTERED_BLOCK) && !pfrom->fWhitelisted)
+                if (send && connman.OutboundTargetReached(true) && ( ((pindexBestHeader != nullptr) && (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() > nOneWeek)) || inv.type == MSG_FILTERED_BLOCK) && !pfrom->fWhitelisted)
                 {
                     LogPrint(BCLog::NET, "historical block serving limit reached, disconnect peer=%d\n", pfrom->GetId());
 
@@ -1734,7 +1734,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         }
 
         CNodeState *nodestate = State(pfrom->GetId());
-        const CBlockIndex* pindex = NULL;
+        const CBlockIndex* pindex = nullptr;
         if (locator.IsNull())
         {
             // If locator is null, return the hashStop block
@@ -1761,7 +1761,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             if (--nLimit <= 0 || pindex->GetBlockHash() == hashStop)
                 break;
         }
-        // pindex can be NULL either if we sent chainActive.Tip() OR
+        // pindex can be nullptr either if we sent chainActive.Tip() OR
         // if our peer has chainActive.Tip() (and thus we are sending an empty
         // headers message). In both cases it's safe to update
         // pindexBestHeaderSent to be our tip.
@@ -1980,7 +1980,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         }
         }
 
-        const CBlockIndex *pindex = NULL;
+        const CBlockIndex *pindex = nullptr;
         CValidationState state;
         if (!ProcessNewBlockHeaders({cmpctblock.header}, state, chainparams, &pindex)) {
             int nDoS;
@@ -2052,7 +2052,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         if (pindex->nHeight <= chainActive.Height() + 2) {
             if ((!fAlreadyInFlight && nodestate->nBlocksInFlight < MAX_BLOCKS_IN_TRANSIT_PER_PEER) ||
                  (fAlreadyInFlight && blockInFlightIt->second.first == pfrom->GetId())) {
-                std::list<QueuedBlock>::iterator* queuedBlockIt = NULL;
+                std::list<QueuedBlock>::iterator* queuedBlockIt = nullptr;
                 if (!MarkBlockAsInFlight(pfrom->GetId(), pindex->GetBlockHash(), pindex, &queuedBlockIt)) {
                     if (!(*queuedBlockIt)->partialBlock)
                         (*queuedBlockIt)->partialBlock.reset(new PartiallyDownloadedBlock(&mempool));
@@ -2257,7 +2257,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             return true;
         }
 
-        const CBlockIndex *pindexLast = NULL;
+        const CBlockIndex *pindexLast = nullptr;
         {
         LOCK(cs_main);
         CNodeState *nodestate = State(pfrom->GetId());
@@ -2766,7 +2766,7 @@ bool ProcessMessages(CNode* pfrom, CConnman& connman, const std::atomic<bool>& i
     catch (const std::exception& e) {
         PrintExceptionContinue(&e, "ProcessMessages()");
     } catch (...) {
-        PrintExceptionContinue(NULL, "ProcessMessages()");
+        PrintExceptionContinue(nullptr, "ProcessMessages()");
     }
 
     if (!fRet) {
@@ -2881,7 +2881,7 @@ bool SendMessages(CNode* pto, CConnman& connman, const std::atomic<bool>& interr
         }
 
         // Start block sync
-        if (pindexBestHeader == NULL)
+        if (pindexBestHeader == nullptr)
             pindexBestHeader = chainActive.Tip();
         bool fFetch = state.fPreferredDownload || (nPreferredDownload == 0 && !pto->fClient && !pto->fOneShot); // Download if this is a nice peer, or we have no nice peers and this one might do.
         if (!state.fSyncStarted && !pto->fClient && !fImporting && !fReindex) {
@@ -2929,7 +2929,7 @@ bool SendMessages(CNode* pto, CConnman& connman, const std::atomic<bool>& interr
             bool fRevertToInv = ((!state.fPreferHeaders &&
                                  (!state.fPreferHeaderAndIDs || pto->vBlockHashesToAnnounce.size() > 1)) ||
                                 pto->vBlockHashesToAnnounce.size() > MAX_BLOCKS_TO_ANNOUNCE);
-            const CBlockIndex *pBestIndex = NULL; // last header queued for delivery
+            const CBlockIndex *pBestIndex = nullptr; // last header queued for delivery
             ProcessBlockAvailability(pto->GetId()); // ensure pindexBestKnownBlock is up-to-date
 
             if (!fRevertToInv) {
@@ -2946,7 +2946,7 @@ bool SendMessages(CNode* pto, CConnman& connman, const std::atomic<bool>& interr
                         fRevertToInv = true;
                         break;
                     }
-                    if (pBestIndex != NULL && pindex->pprev != pBestIndex) {
+                    if (pBestIndex != nullptr && pindex->pprev != pBestIndex) {
                         // This means that the list of blocks to announce don't
                         // connect to each other.
                         // This shouldn't really be possible to hit during
@@ -2967,7 +2967,7 @@ bool SendMessages(CNode* pto, CConnman& connman, const std::atomic<bool>& interr
                         vHeaders.push_back(pindex->GetBlockHeader());
                     } else if (PeerHasHeader(&state, pindex)) {
                         continue; // keep looking for the first new block
-                    } else if (pindex->pprev == NULL || PeerHasHeader(&state, pindex->pprev)) {
+                    } else if (pindex->pprev == nullptr || PeerHasHeader(&state, pindex->pprev)) {
                         // Peer doesn't have this header but they do have the prior one.
                         // Start sending headers.
                         fFoundStartingHeader = true;
diff --git a/src/netaddress.cpp b/src/netaddress.cpp
index f31b2fc49b..b8a261c921 100644
--- a/src/netaddress.cpp
+++ b/src/netaddress.cpp
@@ -271,7 +271,7 @@ std::string CNetAddr::ToStringIP() const
     socklen_t socklen = sizeof(sockaddr);
     if (serv.GetSockAddr((struct sockaddr*)&sockaddr, &socklen)) {
         char name[1025] = "";
-        if (!getnameinfo((const struct sockaddr*)&sockaddr, socklen, name, sizeof(name), NULL, 0, NI_NUMERICHOST))
+        if (!getnameinfo((const struct sockaddr*)&sockaddr, socklen, name, sizeof(name), nullptr, 0, NI_NUMERICHOST))
             return std::string(name);
     }
     if (IsIPv4())
@@ -407,7 +407,7 @@ static const int NET_UNKNOWN = NET_MAX + 0;
 static const int NET_TEREDO  = NET_MAX + 1;
 int static GetExtNetwork(const CNetAddr *addr)
 {
-    if (addr == NULL)
+    if (addr == nullptr)
         return NET_UNKNOWN;
     if (addr->IsRFC4380())
         return NET_TEREDO;
diff --git a/src/netaddress.h b/src/netaddress.h
index 61afe0f1fe..57ce897db5 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -80,7 +80,7 @@ class CNetAddr
         uint64_t GetHash() const;
         bool GetInAddr(struct in_addr* pipv4Addr) const;
         std::vector<unsigned char> GetGroup() const;
-        int GetReachabilityFrom(const CNetAddr *paddrPartner = NULL) const;
+        int GetReachabilityFrom(const CNetAddr *paddrPartner = nullptr) const;
 
         CNetAddr(const struct in6_addr& pipv6Addr, const uint32_t scope = 0);
         bool GetIn6Addr(struct in6_addr* pipv6Addr) const;
diff --git a/src/netbase.cpp b/src/netbase.cpp
index 8952468ecd..05f9f6961c 100644
--- a/src/netbase.cpp
+++ b/src/netbase.cpp
@@ -81,13 +81,13 @@ bool static LookupIntern(const char *pszName, std::vector<CNetAddr>& vIP, unsign
 #else
     aiHint.ai_flags = fAllowLookup ? AI_ADDRCONFIG : AI_NUMERICHOST;
 #endif
-    struct addrinfo *aiRes = NULL;
-    int nErr = getaddrinfo(pszName, NULL, &aiHint, &aiRes);
+    struct addrinfo *aiRes = nullptr;
+    int nErr = getaddrinfo(pszName, nullptr, &aiHint, &aiRes);
     if (nErr)
         return false;
 
     struct addrinfo *aiTrav = aiRes;
-    while (aiTrav != NULL && (nMaxSolutions == 0 || vIP.size() < nMaxSolutions))
+    while (aiTrav != nullptr && (nMaxSolutions == 0 || vIP.size() < nMaxSolutions))
     {
         CNetAddr resolved;
         if (aiTrav->ai_family == AF_INET)
@@ -227,7 +227,7 @@ static IntrRecvError InterruptibleRecv(char* data, size_t len, int timeout, cons
                 fd_set fdset;
                 FD_ZERO(&fdset);
                 FD_SET(hSocket, &fdset);
-                int nRet = select(hSocket + 1, &fdset, NULL, NULL, &tval);
+                int nRet = select(hSocket + 1, &fdset, nullptr, nullptr, &tval);
                 if (nRet == SOCKET_ERROR) {
                     return IntrRecvError::NetworkError;
                 }
@@ -439,7 +439,7 @@ bool static ConnectSocketDirectly(const CService &addrConnect, SOCKET& hSocketRe
             fd_set fdset;
             FD_ZERO(&fdset);
             FD_SET(hSocket, &fdset);
-            int nRet = select(hSocket + 1, NULL, &fdset, NULL, &timeout);
+            int nRet = select(hSocket + 1, nullptr, &fdset, nullptr, &timeout);
             if (nRet == 0)
             {
                 LogPrint(BCLog::NET, "connection to %s timeout\n", addrConnect.ToString());
@@ -642,8 +642,8 @@ std::string NetworkErrorString(int err)
     char buf[256];
     buf[0] = 0;
     if(FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_MAX_WIDTH_MASK,
-            NULL, err, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
-            buf, sizeof(buf), NULL))
+            nullptr, err, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+            buf, sizeof(buf), nullptr))
     {
         return strprintf("%s (%d)", buf, err);
     }
diff --git a/src/pow.cpp b/src/pow.cpp
index e06d9662e6..7d87c6bbb7 100644
--- a/src/pow.cpp
+++ b/src/pow.cpp
@@ -12,7 +12,7 @@
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
 {
-    assert(pindexLast != NULL);
+    assert(pindexLast != nullptr);
     unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();
 
     // Only change once per difficulty adjustment interval
diff --git a/src/prevector.h b/src/prevector.h
index 46640d6fff..f7bde8911a 100644
--- a/src/prevector.h
+++ b/src/prevector.h
@@ -448,7 +448,7 @@ public:
         }
         if (!is_direct()) {
             free(_union.indirect);
-            _union.indirect = NULL;
+            _union.indirect = nullptr;
         }
     }
 
diff --git a/src/pubkey.cpp b/src/pubkey.cpp
index 91af4e56f2..2da7be783f 100644
--- a/src/pubkey.cpp
+++ b/src/pubkey.cpp
@@ -10,7 +10,7 @@
 namespace
 {
 /* Global secp256k1_context object used for verification. */
-secp256k1_context* secp256k1_context_verify = NULL;
+secp256k1_context* secp256k1_context_verify = nullptr;
 } // namespace
 
 /** This function is taken from the libsecp256k1 distribution and implements
@@ -274,7 +274,7 @@ bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
     if (!ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig, vchSig.data(), vchSig.size())) {
         return false;
     }
-    return (!secp256k1_ecdsa_signature_normalize(secp256k1_context_verify, NULL, &sig));
+    return (!secp256k1_ecdsa_signature_normalize(secp256k1_context_verify, nullptr, &sig));
 }
 
 /* static */ int ECCVerifyHandle::refcount = 0;
@@ -282,9 +282,9 @@ bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
 ECCVerifyHandle::ECCVerifyHandle()
 {
     if (refcount == 0) {
-        assert(secp256k1_context_verify == NULL);
+        assert(secp256k1_context_verify == nullptr);
         secp256k1_context_verify = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY);
-        assert(secp256k1_context_verify != NULL);
+        assert(secp256k1_context_verify != nullptr);
     }
     refcount++;
 }
@@ -293,8 +293,8 @@ ECCVerifyHandle::~ECCVerifyHandle()
 {
     refcount--;
     if (refcount == 0) {
-        assert(secp256k1_context_verify != NULL);
+        assert(secp256k1_context_verify != nullptr);
         secp256k1_context_destroy(secp256k1_context_verify);
-        secp256k1_context_verify = NULL;
+        secp256k1_context_verify = nullptr;
     }
 }
diff --git a/src/qt/addressbookpage.cpp b/src/qt/addressbookpage.cpp
index cebac46b95..f295bd4689 100644
--- a/src/qt/addressbookpage.cpp
+++ b/src/qt/addressbookpage.cpp
@@ -273,7 +273,7 @@ void AddressBookPage::on_exportButton_clicked()
     // CSV is currently the only supported format
     QString filename = GUIUtil::getSaveFileName(this,
         tr("Export Address List"), QString(),
-        tr("Comma separated file (*.csv)"), NULL);
+        tr("Comma separated file (*.csv)"), nullptr);
 
     if (filename.isNull())
         return;
diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
index 4a4116c670..b6cfdf8887 100644
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -305,7 +305,7 @@ void BitcoinCore::initialize()
     } catch (const std::exception& e) {
         handleRunawayException(&e);
     } catch (...) {
-        handleRunawayException(NULL);
+        handleRunawayException(nullptr);
     }
 }
 
@@ -322,7 +322,7 @@ void BitcoinCore::shutdown()
     } catch (const std::exception& e) {
         handleRunawayException(&e);
     } catch (...) {
-        handleRunawayException(NULL);
+        handleRunawayException(nullptr);
     }
 }
 
@@ -383,7 +383,7 @@ void BitcoinApplication::createPaymentServer()
 
 void BitcoinApplication::createOptionsModel(bool resetSettings)
 {
-    optionsModel = new OptionsModel(NULL, resetSettings);
+    optionsModel = new OptionsModel(nullptr, resetSettings);
 }
 
 void BitcoinApplication::createWindow(const NetworkStyle *networkStyle)
@@ -608,7 +608,7 @@ int main(int argc, char *argv[])
     // but before showing splash screen.
     if (IsArgSet("-?") || IsArgSet("-h") || IsArgSet("-help") || IsArgSet("-version"))
     {
-        HelpMessageDialog help(NULL, IsArgSet("-version"));
+        HelpMessageDialog help(nullptr, IsArgSet("-version"));
         help.showOrPrint();
         return EXIT_SUCCESS;
     }
@@ -723,7 +723,7 @@ int main(int argc, char *argv[])
         PrintExceptionContinue(&e, "Runaway exception");
         app.handleRunawayException(QString::fromStdString(GetWarnings("gui")));
     } catch (...) {
-        PrintExceptionContinue(NULL, "Runaway exception");
+        PrintExceptionContinue(nullptr, "Runaway exception");
         app.handleRunawayException(QString::fromStdString(GetWarnings("gui")));
     }
     return rv;
diff --git a/src/qt/bitcoingui.cpp b/src/qt/bitcoingui.cpp
index 429c18cba8..e3970298e6 100644
--- a/src/qt/bitcoingui.cpp
+++ b/src/qt/bitcoingui.cpp
@@ -479,7 +479,7 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel)
         connect(_clientModel, SIGNAL(networkActiveChanged(bool)), this, SLOT(setNetworkActive(bool)));
 
         modalOverlay->setKnownBestHeight(_clientModel->getHeaderTipHeight(), QDateTime::fromTime_t(_clientModel->getHeaderTipTime()));
-        setNumBlocks(_clientModel->getNumBlocks(), _clientModel->getLastBlockDate(), _clientModel->getVerificationProgress(NULL), false);
+        setNumBlocks(_clientModel->getNumBlocks(), _clientModel->getLastBlockDate(), _clientModel->getVerificationProgress(nullptr), false);
         connect(_clientModel, SIGNAL(numBlocksChanged(int,QDateTime,double,bool)), this, SLOT(setNumBlocks(int,QDateTime,double,bool)));
 
         // Receive and report messages from client model
@@ -922,7 +922,7 @@ void BitcoinGUI::message(const QString &title, const QString &message, unsigned
         showNormalIfMinimized();
         QMessageBox mBox((QMessageBox::Icon)nMBoxIcon, strTitle, message, buttons, this);
         int r = mBox.exec();
-        if (ret != NULL)
+        if (ret != nullptr)
             *ret = r == QMessageBox::Ok;
     }
     else
diff --git a/src/qt/bitcoingui.h b/src/qt/bitcoingui.h
index 8731caafc7..aa45ea1f0a 100644
--- a/src/qt/bitcoingui.h
+++ b/src/qt/bitcoingui.h
@@ -168,7 +168,7 @@ public Q_SLOTS:
                             @see CClientUIInterface::MessageBoxFlags
        @param[in] ret       pointer to a bool that will be modified to whether Ok was clicked (modal only)
     */
-    void message(const QString &title, const QString &message, unsigned int style, bool *ret = NULL);
+    void message(const QString &title, const QString &message, unsigned int style, bool *ret = nullptr);
 
 #ifdef ENABLE_WALLET
     /** Set the encryption status as shown in the UI.
diff --git a/src/qt/guiutil.cpp b/src/qt/guiutil.cpp
index bffa81137b..28ffda9981 100644
--- a/src/qt/guiutil.cpp
+++ b/src/qt/guiutil.cpp
@@ -634,11 +634,11 @@ bool SetStartOnSystemStartup(bool fAutoStart)
 
     if (fAutoStart)
     {
-        CoInitialize(NULL);
+        CoInitialize(nullptr);
 
         // Get a pointer to the IShellLink interface.
-        IShellLink* psl = NULL;
-        HRESULT hres = CoCreateInstance(CLSID_ShellLink, NULL,
+        IShellLink* psl = nullptr;
+        HRESULT hres = CoCreateInstance(CLSID_ShellLink, nullptr,
             CLSCTX_INPROC_SERVER, IID_IShellLink,
             reinterpret_cast<void**>(&psl));
 
@@ -646,7 +646,7 @@ bool SetStartOnSystemStartup(bool fAutoStart)
         {
             // Get the current executable path
             TCHAR pszExePath[MAX_PATH];
-            GetModuleFileName(NULL, pszExePath, sizeof(pszExePath));
+            GetModuleFileName(nullptr, pszExePath, sizeof(pszExePath));
 
             // Start client minimized
             QString strArgs = "-min";
@@ -674,7 +674,7 @@ bool SetStartOnSystemStartup(bool fAutoStart)
 
             // Query IShellLink for the IPersistFile interface for
             // saving the shortcut in persistent storage.
-            IPersistFile* ppf = NULL;
+            IPersistFile* ppf = nullptr;
             hres = psl->QueryInterface(IID_IPersistFile, reinterpret_cast<void**>(&ppf));
             if (SUCCEEDED(hres))
             {
@@ -781,21 +781,21 @@ LSSharedFileListItemRef findStartupItemInList(LSSharedFileListRef list, CFURLRef
 LSSharedFileListItemRef findStartupItemInList(LSSharedFileListRef list, CFURLRef findUrl)
 {
     // loop through the list of startup items and try to find the bitcoin app
-    CFArrayRef listSnapshot = LSSharedFileListCopySnapshot(list, NULL);
+    CFArrayRef listSnapshot = LSSharedFileListCopySnapshot(list, nullptr);
     for(int i = 0; i < CFArrayGetCount(listSnapshot); i++) {
         LSSharedFileListItemRef item = (LSSharedFileListItemRef)CFArrayGetValueAtIndex(listSnapshot, i);
         UInt32 resolutionFlags = kLSSharedFileListNoUserInteraction | kLSSharedFileListDoNotMountVolumes;
-        CFURLRef currentItemURL = NULL;
+        CFURLRef currentItemURL = nullptr;
 
 #if defined(MAC_OS_X_VERSION_MAX_ALLOWED) && MAC_OS_X_VERSION_MAX_ALLOWED >= 10100
     if(&LSSharedFileListItemCopyResolvedURL)
-        currentItemURL = LSSharedFileListItemCopyResolvedURL(item, resolutionFlags, NULL);
+        currentItemURL = LSSharedFileListItemCopyResolvedURL(item, resolutionFlags, nullptr);
 #if defined(MAC_OS_X_VERSION_MIN_REQUIRED) && MAC_OS_X_VERSION_MIN_REQUIRED < 10100
     else
-        LSSharedFileListItemResolve(item, resolutionFlags, &currentItemURL, NULL);
+        LSSharedFileListItemResolve(item, resolutionFlags, &currentItemURL, nullptr);
 #endif
 #else
-    LSSharedFileListItemResolve(item, resolutionFlags, &currentItemURL, NULL);
+    LSSharedFileListItemResolve(item, resolutionFlags, &currentItemURL, nullptr);
 #endif
 
         if(currentItemURL && CFEqual(currentItemURL, findUrl)) {
@@ -807,13 +807,13 @@ LSSharedFileListItemRef findStartupItemInList(LSSharedFileListRef list, CFURLRef
             CFRelease(currentItemURL);
         }
     }
-    return NULL;
+    return nullptr;
 }
 
 bool GetStartOnSystemStartup()
 {
     CFURLRef bitcoinAppUrl = CFBundleCopyBundleURL(CFBundleGetMainBundle());
-    LSSharedFileListRef loginItems = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
+    LSSharedFileListRef loginItems = LSSharedFileListCreate(nullptr, kLSSharedFileListSessionLoginItems, nullptr);
     LSSharedFileListItemRef foundItem = findStartupItemInList(loginItems, bitcoinAppUrl);
     return !!foundItem; // return boolified object
 }
@@ -821,12 +821,12 @@ bool GetStartOnSystemStartup()
 bool SetStartOnSystemStartup(bool fAutoStart)
 {
     CFURLRef bitcoinAppUrl = CFBundleCopyBundleURL(CFBundleGetMainBundle());
-    LSSharedFileListRef loginItems = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
+    LSSharedFileListRef loginItems = LSSharedFileListCreate(nullptr, kLSSharedFileListSessionLoginItems, nullptr);
     LSSharedFileListItemRef foundItem = findStartupItemInList(loginItems, bitcoinAppUrl);
 
     if(fAutoStart && !foundItem) {
         // add bitcoin app to startup item list
-        LSSharedFileListInsertItemURL(loginItems, kLSSharedFileListItemBeforeFirst, NULL, NULL, bitcoinAppUrl, NULL, NULL);
+        LSSharedFileListInsertItemURL(loginItems, kLSSharedFileListItemBeforeFirst, nullptr, nullptr, bitcoinAppUrl, nullptr, nullptr);
     }
     else if(!fAutoStart && foundItem) {
         // remove item
diff --git a/src/qt/openuridialog.cpp b/src/qt/openuridialog.cpp
index 5a66161346..3ee656d470 100644
--- a/src/qt/openuridialog.cpp
+++ b/src/qt/openuridialog.cpp
@@ -44,7 +44,7 @@ void OpenURIDialog::accept()
 
 void OpenURIDialog::on_selectFileButton_clicked()
 {
-    QString filename = GUIUtil::getOpenFileName(this, tr("Select payment request file to open"), "", "", NULL);
+    QString filename = GUIUtil::getOpenFileName(this, tr("Select payment request file to open"), "", "", nullptr);
     if(filename.isEmpty())
         return;
     QUrl fileUri = QUrl::fromLocalFile(filename);
diff --git a/src/qt/paymentrequestplus.cpp b/src/qt/paymentrequestplus.cpp
index 01ec416613..897762b1a7 100644
--- a/src/qt/paymentrequestplus.cpp
+++ b/src/qt/paymentrequestplus.cpp
@@ -66,7 +66,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
 
     // One day we'll support more PKI types, but just
     // x509 for now:
-    const EVP_MD* digestAlgorithm = NULL;
+    const EVP_MD* digestAlgorithm = nullptr;
     if (paymentRequest.pki_type() == "x509+sha256") {
         digestAlgorithm = EVP_sha256();
     }
@@ -104,7 +104,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
         }
 #endif
         const unsigned char *data = (const unsigned char *)certChain.certificate(i).data();
-        X509 *cert = d2i_X509(NULL, &data, certChain.certificate(i).size());
+        X509 *cert = d2i_X509(nullptr, &data, certChain.certificate(i).size());
         if (cert)
             certs.push_back(cert);
     }
@@ -129,7 +129,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
         return false;
     }
 
-    char *website = NULL;
+    char *website = nullptr;
     bool fResult = true;
     try
     {
@@ -169,7 +169,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
 #endif
         EVP_PKEY *pubkey = X509_get_pubkey(signing_cert);
         EVP_MD_CTX_init(ctx);
-        if (!EVP_VerifyInit_ex(ctx, digestAlgorithm, NULL) ||
+        if (!EVP_VerifyInit_ex(ctx, digestAlgorithm, nullptr) ||
             !EVP_VerifyUpdate(ctx, data_to_verify.data(), data_to_verify.size()) ||
             !EVP_VerifyFinal(ctx, (const unsigned char*)paymentRequest.signature().data(), (unsigned int)paymentRequest.signature().size(), pubkey)) {
             throw SSLVerifyError("Bad signature, invalid payment request.");
@@ -179,7 +179,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
 #endif
 
         // OpenSSL API for getting human printable strings from certs is baroque.
-        int textlen = X509_NAME_get_text_by_NID(certname, NID_commonName, NULL, 0);
+        int textlen = X509_NAME_get_text_by_NID(certname, NID_commonName, nullptr, 0);
         website = new char[textlen + 1];
         if (X509_NAME_get_text_by_NID(certname, NID_commonName, website, textlen + 1) == textlen && textlen > 0) {
             merchant = website;
diff --git a/src/qt/paymentserver.cpp b/src/qt/paymentserver.cpp
index 132ee32748..c5e2b78c54 100644
--- a/src/qt/paymentserver.cpp
+++ b/src/qt/paymentserver.cpp
@@ -274,7 +274,7 @@ bool PaymentServer::ipcSendCommandLine()
         if (!socket->waitForConnected(BITCOIN_IPC_CONNECT_TIMEOUT))
         {
             delete socket;
-            socket = NULL;
+            socket = nullptr;
             return false;
         }
 
@@ -290,7 +290,7 @@ bool PaymentServer::ipcSendCommandLine()
         socket->disconnectFromServer();
 
         delete socket;
-        socket = NULL;
+        socket = nullptr;
         fResult = true;
     }
 
@@ -364,7 +364,7 @@ void PaymentServer::initNetManager()
 {
     if (!optionsModel)
         return;
-    if (netManager != NULL)
+    if (netManager != nullptr)
         delete netManager;
 
     // netManager is used to fetch paymentrequests given in bitcoin: URIs
diff --git a/src/qt/paymentserver.h b/src/qt/paymentserver.h
index 7c6d4507fe..2fe6e3e7b1 100644
--- a/src/qt/paymentserver.h
+++ b/src/qt/paymentserver.h
@@ -75,12 +75,12 @@ public:
     PaymentServer(QObject* parent, bool startLocalServer = true);
     ~PaymentServer();
 
-    // Load root certificate authorities. Pass NULL (default)
+    // Load root certificate authorities. Pass nullptr (default)
     // to read from the file specified in the -rootcertificates setting,
     // or, if that's not set, to use the system default root certificates.
     // If you pass in a store, you should not X509_STORE_free it: it will be
     // freed either at exit or when another set of CAs are loaded.
-    static void LoadRootCAs(X509_STORE* store = NULL);
+    static void LoadRootCAs(X509_STORE* store = nullptr);
 
     // Return certificate store
     static X509_STORE* getCertStore();
diff --git a/src/qt/receiverequestdialog.cpp b/src/qt/receiverequestdialog.cpp
index 3752fa4b66..4aa6375d8a 100644
--- a/src/qt/receiverequestdialog.cpp
+++ b/src/qt/receiverequestdialog.cpp
@@ -68,7 +68,7 @@ void QRImageWidget::saveImage()
 {
     if(!pixmap())
         return;
-    QString fn = GUIUtil::getSaveFileName(this, tr("Save QR Code"), QString(), tr("PNG Image (*.png)"), NULL);
+    QString fn = GUIUtil::getSaveFileName(this, tr("Save QR Code"), QString(), tr("PNG Image (*.png)"), nullptr);
     if (!fn.isEmpty())
     {
         exportImage().save(fn);
diff --git a/src/qt/recentrequeststablemodel.cpp b/src/qt/recentrequeststablemodel.cpp
index 4e88c8802c..1c4f7aca86 100644
--- a/src/qt/recentrequeststablemodel.cpp
+++ b/src/qt/recentrequeststablemodel.cpp
@@ -123,7 +123,7 @@ void RecentRequestsTableModel::updateAmountColumnTitle()
 /** Gets title for amount column including current display unit if optionsModel reference available. */
 QString RecentRequestsTableModel::getAmountTitle()
 {
-    return (this->walletModel->getOptionsModel() != NULL) ? tr("Requested") + " ("+BitcoinUnits::name(this->walletModel->getOptionsModel()->getDisplayUnit()) + ")" : "";
+    return (this->walletModel->getOptionsModel() != nullptr) ? tr("Requested") + " ("+BitcoinUnits::name(this->walletModel->getOptionsModel()->getDisplayUnit()) + ")" : "";
 }
 
 QModelIndex RecentRequestsTableModel::index(int row, int column, const QModelIndex &parent) const
diff --git a/src/qt/rpcconsole.cpp b/src/qt/rpcconsole.cpp
index 232068bf45..3590a98efa 100644
--- a/src/qt/rpcconsole.cpp
+++ b/src/qt/rpcconsole.cpp
@@ -60,7 +60,7 @@ const struct {
     {"cmd-reply", ":/icons/tx_output"},
     {"cmd-error", ":/icons/tx_output"},
     {"misc", ":/icons/tx_inout"},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };
 
 namespace {
@@ -532,7 +532,7 @@ void RPCConsole::setClientModel(ClientModel *model)
         setNumConnections(model->getNumConnections());
         connect(model, SIGNAL(numConnectionsChanged(int)), this, SLOT(setNumConnections(int)));
 
-        setNumBlocks(model->getNumBlocks(), model->getLastBlockDate(), model->getVerificationProgress(NULL), false);
+        setNumBlocks(model->getNumBlocks(), model->getLastBlockDate(), model->getVerificationProgress(nullptr), false);
         connect(model, SIGNAL(numBlocksChanged(int,QDateTime,double,bool)), this, SLOT(setNumBlocks(int,QDateTime,double,bool)));
 
         updateNetworkState();
@@ -982,7 +982,7 @@ void RPCConsole::peerLayoutChanged()
     if (!clientModel || !clientModel->getPeerTableModel())
         return;
 
-    const CNodeCombinedStats *stats = NULL;
+    const CNodeCombinedStats *stats = nullptr;
     bool fUnselect = false;
     bool fReselect = false;
 
diff --git a/src/qt/rpcconsole.h b/src/qt/rpcconsole.h
index ec531c99c8..da06818f87 100644
--- a/src/qt/rpcconsole.h
+++ b/src/qt/rpcconsole.h
@@ -36,8 +36,8 @@ public:
     explicit RPCConsole(const PlatformStyle *platformStyle, QWidget *parent);
     ~RPCConsole();
 
-    static bool RPCParseCommandLine(std::string &strResult, const std::string &strCommand, bool fExecute, std::string * const pstrFilteredOut = NULL);
-    static bool RPCExecuteCommandLine(std::string &strResult, const std::string &strCommand, std::string * const pstrFilteredOut = NULL) {
+    static bool RPCParseCommandLine(std::string &strResult, const std::string &strCommand, bool fExecute, std::string * const pstrFilteredOut = nullptr);
+    static bool RPCExecuteCommandLine(std::string &strResult, const std::string &strCommand, std::string * const pstrFilteredOut = nullptr) {
         return RPCParseCommandLine(strResult, strCommand, true, pstrFilteredOut);
     }
 
diff --git a/src/qt/test/paymentservertests.cpp b/src/qt/test/paymentservertests.cpp
index b9a8ad6e28..d9fb869821 100644
--- a/src/qt/test/paymentservertests.cpp
+++ b/src/qt/test/paymentservertests.cpp
@@ -25,7 +25,7 @@ X509 *parse_b64der_cert(const char* cert_data)
     std::vector<unsigned char> data = DecodeBase64(cert_data);
     assert(data.size() > 0);
     const unsigned char* dptr = &data[0];
-    X509 *cert = d2i_X509(NULL, &dptr, data.size());
+    X509 *cert = d2i_X509(nullptr, &dptr, data.size());
     assert(cert);
     return cert;
 }
@@ -66,7 +66,7 @@ void PaymentServerTests::paymentServerTests()
 {
     SelectParams(CBaseChainParams::MAIN);
     OptionsModel optionsModel;
-    PaymentServer* server = new PaymentServer(NULL, false);
+    PaymentServer* server = new PaymentServer(nullptr, false);
     X509_STORE* caStore = X509_STORE_new();
     X509_STORE_add_cert(caStore, parse_b64der_cert(caCert1_BASE64));
     PaymentServer::LoadRootCAs(caStore);
diff --git a/src/qt/transactionrecord.cpp b/src/qt/transactionrecord.cpp
index 03fd734e92..2ece0d0f28 100644
--- a/src/qt/transactionrecord.cpp
+++ b/src/qt/transactionrecord.cpp
@@ -167,7 +167,7 @@ void TransactionRecord::updateStatus(const CWalletTx &wtx)
     // Determine transaction status
 
     // Find the block the tx is in
-    CBlockIndex* pindex = NULL;
+    CBlockIndex* pindex = nullptr;
     BlockMap::iterator mi = mapBlockIndex.find(wtx.hashBlock);
     if (mi != mapBlockIndex.end())
         pindex = (*mi).second;
diff --git a/src/qt/transactionview.cpp b/src/qt/transactionview.cpp
index 43d6e8826b..53c38da9db 100644
--- a/src/qt/transactionview.cpp
+++ b/src/qt/transactionview.cpp
@@ -343,7 +343,7 @@ void TransactionView::exportClicked()
     // CSV is currently the only supported format
     QString filename = GUIUtil::getSaveFileName(this,
         tr("Export Transaction History"), QString(),
-        tr("Comma separated file (*.csv)"), NULL);
+        tr("Comma separated file (*.csv)"), nullptr);
 
     if (filename.isNull())
         return;
diff --git a/src/qt/walletmodel.h b/src/qt/walletmodel.h
index 5258dc6699..6be36a57e2 100644
--- a/src/qt/walletmodel.h
+++ b/src/qt/walletmodel.h
@@ -129,7 +129,7 @@ public:
     TransactionTableModel *getTransactionTableModel();
     RecentRequestsTableModel *getRecentRequestsTableModel();
 
-    CAmount getBalance(const CCoinControl *coinControl = NULL) const;
+    CAmount getBalance(const CCoinControl *coinControl = nullptr) const;
     CAmount getUnconfirmedBalance() const;
     CAmount getImmatureBalance() const;
     bool haveWatchOnly() const;
diff --git a/src/qt/walletview.cpp b/src/qt/walletview.cpp
index 4a18c0bd4d..971f5e0e1a 100644
--- a/src/qt/walletview.cpp
+++ b/src/qt/walletview.cpp
@@ -246,7 +246,7 @@ void WalletView::backupWallet()
 {
     QString filename = GUIUtil::getSaveFileName(this,
         tr("Backup Wallet"), QString(),
-        tr("Wallet Data (*.dat)"), NULL);
+        tr("Wallet Data (*.dat)"), nullptr);
 
     if (filename.isEmpty())
         return;
diff --git a/src/qt/winshutdownmonitor.cpp b/src/qt/winshutdownmonitor.cpp
index d6f40c38b8..d78d9a2358 100644
--- a/src/qt/winshutdownmonitor.cpp
+++ b/src/qt/winshutdownmonitor.cpp
@@ -57,7 +57,7 @@ void WinShutdownMonitor::registerShutdownBlockReason(const QString& strReason, c
 {
     typedef BOOL (WINAPI *PSHUTDOWNBRCREATE)(HWND, LPCWSTR);
     PSHUTDOWNBRCREATE shutdownBRCreate = (PSHUTDOWNBRCREATE)GetProcAddress(GetModuleHandleA("User32.dll"), "ShutdownBlockReasonCreate");
-    if (shutdownBRCreate == NULL) {
+    if (shutdownBRCreate == nullptr) {
         qWarning() << "registerShutdownBlockReason: GetProcAddress for ShutdownBlockReasonCreate failed";
         return;
     }
diff --git a/src/random.cpp b/src/random.cpp
index e07ef44471..b004bfa91e 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -155,7 +155,7 @@ static void RandAddSeedPerfmon()
     const size_t nMaxSize = 10000000; // Bail out at more than 10MB of performance data
     while (true) {
         nSize = vData.size();
-        ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", NULL, NULL, vData.data(), &nSize);
+        ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", nullptr, nullptr, vData.data(), &nSize);
         if (ret != ERROR_MORE_DATA || vData.size() >= nMaxSize)
             break;
         vData.resize(std::max((vData.size() * 3) / 2, nMaxSize)); // Grow size of buffer exponentially
@@ -203,7 +203,7 @@ void GetOSRand(unsigned char *ent32)
 {
 #if defined(WIN32)
     HCRYPTPROV hProvider;
-    int ret = CryptAcquireContextW(&hProvider, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
+    int ret = CryptAcquireContextW(&hProvider, nullptr, nullptr, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
     if (!ret) {
         RandFailure();
     }
@@ -257,7 +257,7 @@ void GetOSRand(unsigned char *ent32)
     int have = 0;
     do {
         size_t len = NUM_OS_RANDOM_BYTES - have;
-        if (sysctl(name, ARRAYLEN(name), ent32 + have, &len, NULL, 0) != 0) {
+        if (sysctl(name, ARRAYLEN(name), ent32 + have, &len, nullptr, 0) != 0) {
             RandFailure();
         }
         have += len;
diff --git a/src/rest.cpp b/src/rest.cpp
index 33e3fb4529..6a4b005f90 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -134,7 +134,7 @@ static bool rest_headers(HTTPRequest* req,
     if (path.size() != 2)
         return RESTERR(req, HTTP_BAD_REQUEST, "No header count specified. Use /rest/headers/<count>/<hash>.<ext>.");
 
-    long count = strtol(path[0].c_str(), NULL, 10);
+    long count = strtol(path[0].c_str(), nullptr, 10);
     if (count < 1 || count > 2000)
         return RESTERR(req, HTTP_BAD_REQUEST, "Header count out of range: " + path[0]);
 
@@ -148,8 +148,8 @@ static bool rest_headers(HTTPRequest* req,
     {
         LOCK(cs_main);
         BlockMap::const_iterator it = mapBlockIndex.find(hash);
-        const CBlockIndex *pindex = (it != mapBlockIndex.end()) ? it->second : NULL;
-        while (pindex != NULL && chainActive.Contains(pindex)) {
+        const CBlockIndex *pindex = (it != mapBlockIndex.end()) ? it->second : nullptr;
+        while (pindex != nullptr && chainActive.Contains(pindex)) {
             headers.push_back(pindex);
             if (headers.size() == (unsigned long)count)
                 break;
@@ -209,7 +209,7 @@ static bool rest_block(HTTPRequest* req,
         return RESTERR(req, HTTP_BAD_REQUEST, "Invalid hash: " + hashStr);
 
     CBlock block;
-    CBlockIndex* pblockindex = NULL;
+    CBlockIndex* pblockindex = nullptr;
     {
         LOCK(cs_main);
         if (mapBlockIndex.count(hash) == 0)
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 607262f770..701608e132 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -48,9 +48,9 @@ extern void TxToJSON(const CTransaction& tx, const uint256 hashBlock, UniValue&
 
 double GetDifficulty(const CBlockIndex* blockindex)
 {
-    if (blockindex == NULL)
+    if (blockindex == nullptr)
     {
-        if (chainActive.Tip() == NULL)
+        if (chainActive.Tip() == nullptr)
             return 1.0;
         else
             blockindex = chainActive.Tip();
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index 7d292a4635..f498b5c8ea 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -52,7 +52,7 @@ UniValue GetNetworkHashPS(int lookup, int height) {
     if (height >= 0 && height < chainActive.Height())
         pb = chainActive[height];
 
-    if (pb == NULL || !pb->nHeight)
+    if (pb == nullptr || !pb->nHeight)
         return 0;
 
     // If lookup is -1, then use blocks since last difficulty change.
@@ -139,7 +139,7 @@ UniValue generateBlocks(std::shared_ptr<CReserveScript> coinbaseScript, int nGen
             continue;
         }
         std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(*pblock);
-        if (!ProcessNewBlock(Params(), shared_pblock, true, NULL))
+        if (!ProcessNewBlock(Params(), shared_pblock, true, nullptr))
             throw JSONRPCError(RPC_INTERNAL_ERROR, "ProcessNewBlock, block not accepted");
         ++nHeight;
         blockHashes.push_back(pblock->GetHash().GetHex());
@@ -754,7 +754,7 @@ UniValue submitblock(const JSONRPCRequest& request)
 
     submitblock_StateCatcher sc(block.GetHash());
     RegisterValidationInterface(&sc);
-    bool fAccepted = ProcessNewBlock(Params(), blockptr, true, NULL);
+    bool fAccepted = ProcessNewBlock(Params(), blockptr, true, nullptr);
     UnregisterValidationInterface(&sc);
     if (fBlockPresent) {
         if (fAccepted && !sc.found) {
diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
index cd93919eb5..efff4a99ae 100644
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -78,7 +78,7 @@ UniValue getinfo(const JSONRPCRequest& request)
 #ifdef ENABLE_WALLET
     CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
 
-    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : NULL);
+    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : nullptr);
 #else
     LOCK(cs_main);
 #endif
@@ -202,7 +202,7 @@ UniValue validateaddress(const JSONRPCRequest& request)
 #ifdef ENABLE_WALLET
     CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
 
-    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : NULL);
+    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : nullptr);
 #else
     LOCK(cs_main);
 #endif
@@ -322,7 +322,7 @@ UniValue createmultisig(const JSONRPCRequest& request)
 #ifdef ENABLE_WALLET
     CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
 #else
-    CWallet * const pwallet = NULL;
+    CWallet * const pwallet = nullptr;
 #endif
 
     if (request.fHelp || request.params.size() < 2 || request.params.size() > 2)
diff --git a/src/rpc/net.cpp b/src/rpc/net.cpp
index 6271e0cc83..e463a4eda4 100644
--- a/src/rpc/net.cpp
+++ b/src/rpc/net.cpp
@@ -217,7 +217,7 @@ UniValue addnode(const JSONRPCRequest& request)
     if (strCommand == "onetry")
     {
         CAddress addr;
-        g_connman->OpenNetworkConnection(addr, false, NULL, strNode.c_str());
+        g_connman->OpenNetworkConnection(addr, false, nullptr, strNode.c_str());
         return NullUniValue;
     }
 
diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
index b967f2dbf8..8100511854 100644
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -208,7 +208,7 @@ UniValue gettxoutproof(const JSONRPCRequest& request)
 
     LOCK(cs_main);
 
-    CBlockIndex* pblockindex = NULL;
+    CBlockIndex* pblockindex = nullptr;
 
     uint256 hashBlock;
     if (!request.params[1].isNull())
@@ -228,7 +228,7 @@ UniValue gettxoutproof(const JSONRPCRequest& request)
         }
     }
 
-    if (pblockindex == NULL)
+    if (pblockindex == nullptr)
     {
         CTransactionRef tx;
         if (!GetTransaction(oneTxid, tx, Params().GetConsensus(), hashBlock, false) || hashBlock.IsNull())
@@ -707,7 +707,7 @@ UniValue signrawtransaction(const JSONRPCRequest& request)
         );
 
 #ifdef ENABLE_WALLET
-    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : NULL);
+    LOCK2(cs_main, pwallet ? &pwallet->cs_wallet : nullptr);
 #else
     LOCK(cs_main);
 #endif
@@ -937,7 +937,7 @@ UniValue sendrawtransaction(const JSONRPCRequest& request)
         CValidationState state;
         bool fMissingInputs;
         bool fLimitFree = true;
-        if (!AcceptToMemoryPool(mempool, state, std::move(tx), fLimitFree, &fMissingInputs, NULL, false, nMaxRawTxFee)) {
+        if (!AcceptToMemoryPool(mempool, state, std::move(tx), fLimitFree, &fMissingInputs, nullptr, false, nMaxRawTxFee)) {
             if (state.IsInvalid()) {
                 throw JSONRPCError(RPC_TRANSACTION_REJECTED, strprintf("%i: %s", state.GetRejectCode(), state.GetRejectReason()));
             } else {
diff --git a/src/rpc/server.cpp b/src/rpc/server.cpp
index 58640d69d6..1799c8e98e 100644
--- a/src/rpc/server.cpp
+++ b/src/rpc/server.cpp
@@ -30,7 +30,7 @@ static bool fRPCInWarmup = true;
 static std::string rpcWarmupStatus("RPC server started");
 static CCriticalSection cs_rpcWarmup;
 /* Timer-creating functions */
-static RPCTimerInterface* timerInterface = NULL;
+static RPCTimerInterface* timerInterface = nullptr;
 /* Map of name to timer. */
 static std::map<std::string, std::unique_ptr<RPCTimerBase> > deadlineTimers;
 
@@ -291,7 +291,7 @@ const CRPCCommand *CRPCTable::operator[](const std::string &name) const
 {
     std::map<std::string, const CRPCCommand*>::const_iterator it = mapCommands.find(name);
     if (it == mapCommands.end())
-        return NULL;
+        return nullptr;
     return (*it).second;
 }
 
@@ -537,7 +537,7 @@ void RPCSetTimerInterface(RPCTimerInterface *iface)
 void RPCUnsetTimerInterface(RPCTimerInterface *iface)
 {
     if (timerInterface == iface)
-        timerInterface = NULL;
+        timerInterface = nullptr;
 }
 
 void RPCRunLater(const std::string& name, std::function<void(void)> func, int64_t nSeconds)
diff --git a/src/script/bitcoinconsensus.cpp b/src/script/bitcoinconsensus.cpp
index 4b71a42cdf..03128917fd 100644
--- a/src/script/bitcoinconsensus.cpp
+++ b/src/script/bitcoinconsensus.cpp
@@ -28,10 +28,10 @@ public:
         if (nSize > m_remaining)
             throw std::ios_base::failure(std::string(__func__) + ": end of data");
 
-        if (pch == NULL)
+        if (pch == nullptr)
             throw std::ios_base::failure(std::string(__func__) + ": bad destination buffer");
 
-        if (m_data == NULL)
+        if (m_data == nullptr)
             throw std::ios_base::failure(std::string(__func__) + ": bad source buffer");
 
         memcpy(pch, m_data, nSize);
@@ -95,7 +95,7 @@ static int verify_script(const unsigned char *scriptPubKey, unsigned int scriptP
         set_error(err, bitcoinconsensus_ERR_OK);
 
         PrecomputedTransactionData txdata(tx);
-        return VerifyScript(tx.vin[nIn].scriptSig, CScript(scriptPubKey, scriptPubKey + scriptPubKeyLen), &tx.vin[nIn].scriptWitness, flags, TransactionSignatureChecker(&tx, nIn, amount, txdata), NULL);
+        return VerifyScript(tx.vin[nIn].scriptSig, CScript(scriptPubKey, scriptPubKey + scriptPubKeyLen), &tx.vin[nIn].scriptWitness, flags, TransactionSignatureChecker(&tx, nIn, amount, txdata), nullptr);
     } catch (const std::exception&) {
         return set_error(err, bitcoinconsensus_ERR_TX_DESERIALIZE); // Error deserializing
     }
diff --git a/src/script/bitcoinconsensus.h b/src/script/bitcoinconsensus.h
index 1bef4fe9e9..33bf80e5a7 100644
--- a/src/script/bitcoinconsensus.h
+++ b/src/script/bitcoinconsensus.h
@@ -63,7 +63,7 @@ enum
 /// Returns 1 if the input nIn of the serialized transaction pointed to by
 /// txTo correctly spends the scriptPubKey pointed to by scriptPubKey under
 /// the additional constraints specified by flags.
-/// If not NULL, err will contain an error/success code for the operation
+/// If not nullptr, err will contain an error/success code for the operation
 EXPORT_SYMBOL int bitcoinconsensus_verify_script(const unsigned char *scriptPubKey, unsigned int scriptPubKeyLen,
                                                  const unsigned char *txTo        , unsigned int txToLen,
                                                  unsigned int nIn, unsigned int flags, bitcoinconsensus_error* err);
diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 8a121774a0..f9716dfc66 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -1407,7 +1407,7 @@ static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion,
 bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
     static const CScriptWitness emptyWitness;
-    if (witness == NULL) {
+    if (witness == nullptr) {
         witness = &emptyWitness;
     }
     bool hadWitness = false;
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index ab1dc4e681..437826b5dd 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -123,7 +123,7 @@ enum SigVersion
     SIGVERSION_WITNESS_V0 = 1,
 };
 
-uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType, const CAmount& amount, SigVersion sigversion, const PrecomputedTransactionData* cache = NULL);
+uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType, const CAmount& amount, SigVersion sigversion, const PrecomputedTransactionData* cache = nullptr);
 
 class BaseSignatureChecker
 {
@@ -158,7 +158,7 @@ protected:
     virtual bool VerifySignature(const std::vector<unsigned char>& vchSig, const CPubKey& vchPubKey, const uint256& sighash) const;
 
 public:
-    TransactionSignatureChecker(const CTransaction* txToIn, unsigned int nInIn, const CAmount& amountIn) : txTo(txToIn), nIn(nInIn), amount(amountIn), txdata(NULL) {}
+    TransactionSignatureChecker(const CTransaction* txToIn, unsigned int nInIn, const CAmount& amountIn) : txTo(txToIn), nIn(nInIn), amount(amountIn), txdata(nullptr) {}
     TransactionSignatureChecker(const CTransaction* txToIn, unsigned int nInIn, const CAmount& amountIn, const PrecomputedTransactionData& txdataIn) : txTo(txToIn), nIn(nInIn), amount(amountIn), txdata(&txdataIn) {}
     bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override;
     bool CheckLockTime(const CScriptNum& nLockTime) const override;
@@ -174,8 +174,8 @@ public:
     MutableTransactionSignatureChecker(const CMutableTransaction* txToIn, unsigned int nInIn, const CAmount& amountIn) : TransactionSignatureChecker(&txTo, nInIn, amountIn), txTo(*txToIn) {}
 };
 
-bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* error = NULL);
-bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror = NULL);
+bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* error = nullptr);
+bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror = nullptr);
 
 size_t CountWitnessSigOps(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags);
 
diff --git a/src/script/script.h b/src/script/script.h
index d16bfd0e00..711ffa97f8 100644
--- a/src/script/script.h
+++ b/src/script/script.h
@@ -498,7 +498,7 @@ public:
     bool GetOp(iterator& pc, opcodetype& opcodeRet)
     {
          const_iterator pc2 = pc;
-         bool fRet = GetOp2(pc2, opcodeRet, NULL);
+         bool fRet = GetOp2(pc2, opcodeRet, nullptr);
          pc = begin() + (pc2 - begin());
          return fRet;
     }
@@ -510,7 +510,7 @@ public:
 
     bool GetOp(const_iterator& pc, opcodetype& opcodeRet) const
     {
-        return GetOp2(pc, opcodeRet, NULL);
+        return GetOp2(pc, opcodeRet, nullptr);
     }
 
     bool GetOp2(const_iterator& pc, opcodetype& opcodeRet, std::vector<unsigned char>* pvchRet) const
diff --git a/src/script/standard.cpp b/src/script/standard.cpp
index 760a5305e5..9570b8c8d9 100644
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -31,7 +31,7 @@ const char* GetTxnOutputType(txnouttype t)
     case TX_WITNESS_V0_KEYHASH: return "witness_v0_keyhash";
     case TX_WITNESS_V0_SCRIPTHASH: return "witness_v0_scripthash";
     }
-    return NULL;
+    return nullptr;
 }
 
 /**
diff --git a/src/streams.h b/src/streams.h
index 245fb9cd8f..a9668b68bc 100644
--- a/src/streams.h
+++ b/src/streams.h
@@ -479,7 +479,7 @@ public:
     {
         if (file) {
             ::fclose(file);
-            file = NULL;
+            file = nullptr;
         }
     }
 
@@ -487,7 +487,7 @@ public:
      * @note This will invalidate the CAutoFile object, and makes it the responsibility of the caller
      * of this function to clean up the returned FILE*.
      */
-    FILE* release()             { FILE* ret = file; file = NULL; return ret; }
+    FILE* release()             { FILE* ret = file; file = nullptr; return ret; }
 
     /** Get wrapped FILE* without transfer of ownership.
      * @note Ownership of the FILE* will remain with this class. Use this only if the scope of the
@@ -495,9 +495,9 @@ public:
      */
     FILE* Get() const           { return file; }
 
-    /** Return true if the wrapped FILE* is NULL, false otherwise.
+    /** Return true if the wrapped FILE* is nullptr, false otherwise.
      */
-    bool IsNull() const         { return (file == NULL); }
+    bool IsNull() const         { return (file == nullptr); }
 
     //
     // Stream subset
@@ -508,7 +508,7 @@ public:
     void read(char* pch, size_t nSize)
     {
         if (!file)
-            throw std::ios_base::failure("CAutoFile::read: file handle is NULL");
+            throw std::ios_base::failure("CAutoFile::read: file handle is nullptr");
         if (fread(pch, 1, nSize, file) != nSize)
             throw std::ios_base::failure(feof(file) ? "CAutoFile::read: end of file" : "CAutoFile::read: fread failed");
     }
@@ -516,7 +516,7 @@ public:
     void ignore(size_t nSize)
     {
         if (!file)
-            throw std::ios_base::failure("CAutoFile::ignore: file handle is NULL");
+            throw std::ios_base::failure("CAutoFile::ignore: file handle is nullptr");
         unsigned char data[4096];
         while (nSize > 0) {
             size_t nNow = std::min<size_t>(nSize, sizeof(data));
@@ -529,7 +529,7 @@ public:
     void write(const char* pch, size_t nSize)
     {
         if (!file)
-            throw std::ios_base::failure("CAutoFile::write: file handle is NULL");
+            throw std::ios_base::failure("CAutoFile::write: file handle is nullptr");
         if (fwrite(pch, 1, nSize, file) != nSize)
             throw std::ios_base::failure("CAutoFile::write: write failed");
     }
@@ -539,7 +539,7 @@ public:
     {
         // Serialize to this stream
         if (!file)
-            throw std::ios_base::failure("CAutoFile::operator<<: file handle is NULL");
+            throw std::ios_base::failure("CAutoFile::operator<<: file handle is nullptr");
         ::Serialize(*this, obj);
         return (*this);
     }
@@ -549,7 +549,7 @@ public:
     {
         // Unserialize from this stream
         if (!file)
-            throw std::ios_base::failure("CAutoFile::operator>>: file handle is NULL");
+            throw std::ios_base::failure("CAutoFile::operator>>: file handle is nullptr");
         ::Unserialize(*this, obj);
         return (*this);
     }
@@ -616,7 +616,7 @@ public:
     {
         if (src) {
             ::fclose(src);
-            src = NULL;
+            src = nullptr;
         }
     }
 
diff --git a/src/support/allocators/secure.h b/src/support/allocators/secure.h
index 9daba86ef3..f20f424941 100644
--- a/src/support/allocators/secure.h
+++ b/src/support/allocators/secure.h
@@ -45,7 +45,7 @@ struct secure_allocator : public std::allocator<T> {
 
     void deallocate(T* p, std::size_t n)
     {
-        if (p != NULL) {
+        if (p != nullptr) {
             memory_cleanse(p, sizeof(T) * n);
         }
         LockedPoolManager::Instance().free(p);
diff --git a/src/support/allocators/zeroafterfree.h b/src/support/allocators/zeroafterfree.h
index 28a940ad1b..581d5d6318 100644
--- a/src/support/allocators/zeroafterfree.h
+++ b/src/support/allocators/zeroafterfree.h
@@ -36,7 +36,7 @@ struct zero_after_free_allocator : public std::allocator<T> {
 
     void deallocate(T* p, std::size_t n)
     {
-        if (p != NULL)
+        if (p != nullptr)
             memory_cleanse(p, sizeof(T) * n);
         std::allocator<T>::deallocate(p, n);
     }
diff --git a/src/support/events.h b/src/support/events.h
index 90690876ee..cc6d29aecd 100644
--- a/src/support/events.h
+++ b/src/support/events.h
@@ -47,7 +47,7 @@ inline raii_evhttp_request obtain_evhttp_request(void(*cb)(struct evhttp_request
 }
 
 inline raii_evhttp_connection obtain_evhttp_connection_base(struct event_base* base, std::string host, uint16_t port) {
-    auto result = raii_evhttp_connection(evhttp_connection_base_new(base, NULL, host.c_str(), port));
+    auto result = raii_evhttp_connection(evhttp_connection_base_new(base, nullptr, host.c_str(), port));
     if (!result.get())
         throw std::runtime_error("create connection failed");
     return result;
diff --git a/src/support/lockedpool.cpp b/src/support/lockedpool.cpp
index 2df6b84a59..2ead72185f 100644
--- a/src/support/lockedpool.cpp
+++ b/src/support/lockedpool.cpp
@@ -28,7 +28,7 @@
 
 #include <algorithm>
 
-LockedPoolManager* LockedPoolManager::_instance = NULL;
+LockedPoolManager* LockedPoolManager::_instance = nullptr;
 std::once_flag LockedPoolManager::init_flag;
 
 /*******************************************************************************/
@@ -87,7 +87,7 @@ template <class Iterator, class Pair> bool extend(Iterator it, const Pair& other
 
 void Arena::free(void *ptr)
 {
-    // Freeing the NULL pointer is OK.
+    // Freeing the nullptr pointer is OK.
     if (ptr == nullptr) {
         return;
     }
diff --git a/src/sync.cpp b/src/sync.cpp
index c359e8220b..b82f3770e4 100644
--- a/src/sync.cpp
+++ b/src/sync.cpp
@@ -100,7 +100,7 @@ static void potential_deadlock_detected(const std::pair<void*, void*>& mismatch,
 
 static void push_lock(void* c, const CLockLocation& locklocation, bool fTry)
 {
-    if (lockstack.get() == NULL)
+    if (lockstack.get() == nullptr)
         lockstack.reset(new LockStack);
 
     boost::unique_lock<boost::mutex> lock(lockdata.dd_mutex);
diff --git a/src/sync.h b/src/sync.h
index 20974f5fbc..4921aedf39 100644
--- a/src/sync.h
+++ b/src/sync.h
@@ -265,7 +265,7 @@ public:
         fHaveGrant = false;
     }
 
-    CSemaphoreGrant() : sem(NULL), fHaveGrant(false) {}
+    CSemaphoreGrant() : sem(nullptr), fHaveGrant(false) {}
 
     CSemaphoreGrant(CSemaphore& sema, bool fTry = false) : sem(&sema), fHaveGrant(false)
     {
diff --git a/src/test/addrman_tests.cpp b/src/test/addrman_tests.cpp
index c53744b495..2ad22d34ad 100644
--- a/src/test/addrman_tests.cpp
+++ b/src/test/addrman_tests.cpp
@@ -38,12 +38,12 @@ public:
         return (unsigned int)(state % nMax);
     }
 
-    CAddrInfo* Find(const CNetAddr& addr, int* pnId = NULL)
+    CAddrInfo* Find(const CNetAddr& addr, int* pnId = nullptr)
     {
         return CAddrMan::Find(addr, pnId);
     }
 
-    CAddrInfo* Create(const CAddress& addr, const CNetAddr& addrSource, int* pnId = NULL)
+    CAddrInfo* Create(const CAddress& addr, const CNetAddr& addrSource, int* pnId = nullptr)
     {
         return CAddrMan::Create(addr, addrSource, pnId);
     }
@@ -340,7 +340,7 @@ BOOST_AUTO_TEST_CASE(addrman_delete)
     addrman.Delete(nId);
     BOOST_CHECK_EQUAL(addrman.size(), 0);
     CAddrInfo* info2 = addrman.Find(addr1);
-    BOOST_CHECK(info2 == NULL);
+    BOOST_CHECK(info2 == nullptr);
 }
 
 BOOST_AUTO_TEST_CASE(addrman_getaddr)
diff --git a/src/test/miner_tests.cpp b/src/test/miner_tests.cpp
index eaff2ac70d..9fa9a8509c 100644
--- a/src/test/miner_tests.cpp
+++ b/src/test/miner_tests.cpp
@@ -236,7 +236,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
         pblock->hashMerkleRoot = BlockMerkleRoot(*pblock);
         pblock->nNonce = blockinfo[i].nonce;
         std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(*pblock);
-        BOOST_CHECK(ProcessNewBlock(chainparams, shared_pblock, true, NULL));
+        BOOST_CHECK(ProcessNewBlock(chainparams, shared_pblock, true, nullptr));
         pblock->hashPrevBlock = pblock->GetHash();
     }
 
diff --git a/src/test/multisig_tests.cpp b/src/test/multisig_tests.cpp
index c686f679c2..5e89ef60d2 100644
--- a/src/test/multisig_tests.cpp
+++ b/src/test/multisig_tests.cpp
@@ -79,20 +79,20 @@ BOOST_AUTO_TEST_CASE(multisig_verify)
     keys.assign(1,key[0]);
     keys.push_back(key[1]);
     s = sign_multisig(a_and_b, keys, txTo[0], 0);
-    BOOST_CHECK(VerifyScript(s, a_and_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err));
+    BOOST_CHECK(VerifyScript(s, a_and_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
 
     for (int i = 0; i < 4; i++)
     {
         keys.assign(1,key[i]);
         s = sign_multisig(a_and_b, keys, txTo[0], 0);
-        BOOST_CHECK_MESSAGE(!VerifyScript(s, a_and_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err), strprintf("a&b 1: %d", i));
+        BOOST_CHECK_MESSAGE(!VerifyScript(s, a_and_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err), strprintf("a&b 1: %d", i));
         BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_INVALID_STACK_OPERATION, ScriptErrorString(err));
 
         keys.assign(1,key[1]);
         keys.push_back(key[i]);
         s = sign_multisig(a_and_b, keys, txTo[0], 0);
-        BOOST_CHECK_MESSAGE(!VerifyScript(s, a_and_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err), strprintf("a&b 2: %d", i));
+        BOOST_CHECK_MESSAGE(!VerifyScript(s, a_and_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[0], 0, amount), &err), strprintf("a&b 2: %d", i));
         BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
     }
 
@@ -103,18 +103,18 @@ BOOST_AUTO_TEST_CASE(multisig_verify)
         s = sign_multisig(a_or_b, keys, txTo[1], 0);
         if (i == 0 || i == 1)
         {
-            BOOST_CHECK_MESSAGE(VerifyScript(s, a_or_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err), strprintf("a|b: %d", i));
+            BOOST_CHECK_MESSAGE(VerifyScript(s, a_or_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err), strprintf("a|b: %d", i));
             BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
         }
         else
         {
-            BOOST_CHECK_MESSAGE(!VerifyScript(s, a_or_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err), strprintf("a|b: %d", i));
+            BOOST_CHECK_MESSAGE(!VerifyScript(s, a_or_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err), strprintf("a|b: %d", i));
             BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
         }
     }
     s.clear();
     s << OP_0 << OP_1;
-    BOOST_CHECK(!VerifyScript(s, a_or_b, NULL, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err));
+    BOOST_CHECK(!VerifyScript(s, a_or_b, nullptr, flags, MutableTransactionSignatureChecker(&txTo[1], 0, amount), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_SIG_DER, ScriptErrorString(err));
 
 
@@ -126,12 +126,12 @@ BOOST_AUTO_TEST_CASE(multisig_verify)
             s = sign_multisig(escrow, keys, txTo[2], 0);
             if (i < j && i < 3 && j < 3)
             {
-                BOOST_CHECK_MESSAGE(VerifyScript(s, escrow, NULL, flags, MutableTransactionSignatureChecker(&txTo[2], 0, amount), &err), strprintf("escrow 1: %d %d", i, j));
+                BOOST_CHECK_MESSAGE(VerifyScript(s, escrow, nullptr, flags, MutableTransactionSignatureChecker(&txTo[2], 0, amount), &err), strprintf("escrow 1: %d %d", i, j));
                 BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
             }
             else
             {
-                BOOST_CHECK_MESSAGE(!VerifyScript(s, escrow, NULL, flags, MutableTransactionSignatureChecker(&txTo[2], 0, amount), &err), strprintf("escrow 2: %d %d", i, j));
+                BOOST_CHECK_MESSAGE(!VerifyScript(s, escrow, nullptr, flags, MutableTransactionSignatureChecker(&txTo[2], 0, amount), &err), strprintf("escrow 2: %d %d", i, j));
                 BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
             }
         }
diff --git a/src/test/pow_tests.cpp b/src/test/pow_tests.cpp
index b9fabd02e4..b13f2625aa 100644
--- a/src/test/pow_tests.cpp
+++ b/src/test/pow_tests.cpp
@@ -66,7 +66,7 @@ BOOST_AUTO_TEST_CASE(GetBlockProofEquivalentTime_test)
     const auto chainParams = CreateChainParams(CBaseChainParams::MAIN);
     std::vector<CBlockIndex> blocks(10000);
     for (int i = 0; i < 10000; i++) {
-        blocks[i].pprev = i ? &blocks[i - 1] : NULL;
+        blocks[i].pprev = i ? &blocks[i - 1] : nullptr;
         blocks[i].nHeight = i;
         blocks[i].nTime = 1269211443 + i * chainParams->GetConsensus().nPowTargetSpacing;
         blocks[i].nBits = 0x207fffff; /* target 0x7fffff000... */
diff --git a/src/test/raii_event_tests.cpp b/src/test/raii_event_tests.cpp
index 0f40874f55..0d541ec7d4 100644
--- a/src/test/raii_event_tests.cpp
+++ b/src/test/raii_event_tests.cpp
@@ -42,7 +42,7 @@ BOOST_AUTO_TEST_CASE(raii_event_creation)
 {
     event_set_mem_functions(tag_malloc, realloc, tag_free);
     
-    void* base_ptr = NULL;
+    void* base_ptr = nullptr;
     {
         auto base = obtain_event_base();
         base_ptr = (void*)base.get();
@@ -50,10 +50,10 @@ BOOST_AUTO_TEST_CASE(raii_event_creation)
     }
     BOOST_CHECK(tags[base_ptr] == 0);
     
-    void* event_ptr = NULL;
+    void* event_ptr = nullptr;
     {
         auto base = obtain_event_base();
-        auto event = obtain_event(base.get(), -1, 0, NULL, NULL);
+        auto event = obtain_event(base.get(), -1, 0, nullptr, nullptr);
 
         base_ptr = (void*)base.get();
         event_ptr = (void*)event.get();
@@ -71,11 +71,11 @@ BOOST_AUTO_TEST_CASE(raii_event_order)
 {
     event_set_mem_functions(tag_malloc, realloc, tag_free);
     
-    void* base_ptr = NULL;
-    void* event_ptr = NULL;
+    void* base_ptr = nullptr;
+    void* event_ptr = nullptr;
     {
         auto base = obtain_event_base();
-        auto event = obtain_event(base.get(), -1, 0, NULL, NULL);
+        auto event = obtain_event(base.get(), -1, 0, nullptr, nullptr);
 
         base_ptr = (void*)base.get();
         event_ptr = (void*)event.get();
diff --git a/src/test/script_P2SH_tests.cpp b/src/test/script_P2SH_tests.cpp
index 0789b2e80c..efd0f77d9f 100644
--- a/src/test/script_P2SH_tests.cpp
+++ b/src/test/script_P2SH_tests.cpp
@@ -42,7 +42,7 @@ Verify(const CScript& scriptSig, const CScript& scriptPubKey, bool fStrict, Scri
     txTo.vin[0].scriptSig = scriptSig;
     txTo.vout[0].nValue = 1;
 
-    return VerifyScript(scriptSig, scriptPubKey, NULL, fStrict ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE, MutableTransactionSignatureChecker(&txTo, 0, txFrom.vout[0].nValue), &err);
+    return VerifyScript(scriptSig, scriptPubKey, nullptr, fStrict ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE, MutableTransactionSignatureChecker(&txTo, 0, txFrom.vout[0].nValue), &err);
 }
 
 
diff --git a/src/test/script_tests.cpp b/src/test/script_tests.cpp
index a18471588a..06b8274f2d 100644
--- a/src/test/script_tests.cpp
+++ b/src/test/script_tests.cpp
@@ -172,10 +172,10 @@ void DoTest(const CScript& scriptPubKey, const CScript& scriptSig, const CScript
     int libconsensus_flags = flags & bitcoinconsensus_SCRIPT_FLAGS_VERIFY_ALL;
     if (libconsensus_flags == flags) {
         if (flags & bitcoinconsensus_SCRIPT_FLAGS_VERIFY_WITNESS) {
-            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script_with_amount(scriptPubKey.data(), scriptPubKey.size(), txCredit.vout[0].nValue, (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, NULL) == expect, message);
+            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script_with_amount(scriptPubKey.data(), scriptPubKey.size(), txCredit.vout[0].nValue, (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, nullptr) == expect, message);
         } else {
-            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script_with_amount(scriptPubKey.data(), scriptPubKey.size(), 0, (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, NULL) == expect, message);
-            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script(scriptPubKey.data(), scriptPubKey.size(), (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, NULL) == expect,message);
+            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script_with_amount(scriptPubKey.data(), scriptPubKey.size(), 0, (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, nullptr) == expect, message);
+            BOOST_CHECK_MESSAGE(bitcoinconsensus_verify_script(scriptPubKey.data(), scriptPubKey.size(), (const unsigned char*)&stream[0], stream.size(), 0, libconsensus_flags, nullptr) == expect,message);
         }
     }
 #endif
@@ -1064,18 +1064,18 @@ BOOST_AUTO_TEST_CASE(script_CHECKMULTISIG12)
     CMutableTransaction txTo12 = BuildSpendingTransaction(CScript(), CScriptWitness(), txFrom12);
 
     CScript goodsig1 = sign_multisig(scriptPubKey12, key1, txTo12);
-    BOOST_CHECK(VerifyScript(goodsig1, scriptPubKey12, NULL, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
+    BOOST_CHECK(VerifyScript(goodsig1, scriptPubKey12, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
     txTo12.vout[0].nValue = 2;
-    BOOST_CHECK(!VerifyScript(goodsig1, scriptPubKey12, NULL, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(goodsig1, scriptPubKey12, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     CScript goodsig2 = sign_multisig(scriptPubKey12, key2, txTo12);
-    BOOST_CHECK(VerifyScript(goodsig2, scriptPubKey12, NULL, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
+    BOOST_CHECK(VerifyScript(goodsig2, scriptPubKey12, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
 
     CScript badsig1 = sign_multisig(scriptPubKey12, key3, txTo12);
-    BOOST_CHECK(!VerifyScript(badsig1, scriptPubKey12, NULL, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig1, scriptPubKey12, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo12, 0, txFrom12.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 }
 
@@ -1097,54 +1097,54 @@ BOOST_AUTO_TEST_CASE(script_CHECKMULTISIG23)
     std::vector<CKey> keys;
     keys.push_back(key1); keys.push_back(key2);
     CScript goodsig1 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(VerifyScript(goodsig1, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(VerifyScript(goodsig1, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key1); keys.push_back(key3);
     CScript goodsig2 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(VerifyScript(goodsig2, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(VerifyScript(goodsig2, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key2); keys.push_back(key3);
     CScript goodsig3 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(VerifyScript(goodsig3, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(VerifyScript(goodsig3, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key2); keys.push_back(key2); // Can't re-use sig
     CScript badsig1 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig1, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig1, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key2); keys.push_back(key1); // sigs must be in correct order
     CScript badsig2 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig2, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig2, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key3); keys.push_back(key2); // sigs must be in correct order
     CScript badsig3 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig3, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig3, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key4); keys.push_back(key2); // sigs must match pubkeys
     CScript badsig4 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig4, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig4, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     keys.clear();
     keys.push_back(key1); keys.push_back(key4); // sigs must match pubkeys
     CScript badsig5 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig5, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig5, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EVAL_FALSE, ScriptErrorString(err));
 
     keys.clear(); // Must have signatures
     CScript badsig6 = sign_multisig(scriptPubKey23, keys, txTo23);
-    BOOST_CHECK(!VerifyScript(badsig6, scriptPubKey23, NULL, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
+    BOOST_CHECK(!VerifyScript(badsig6, scriptPubKey23, nullptr, gFlags, MutableTransactionSignatureChecker(&txTo23, 0, txFrom23.vout[0].nValue), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_INVALID_STACK_OPERATION, ScriptErrorString(err));
 }
 
@@ -1265,7 +1265,7 @@ BOOST_AUTO_TEST_CASE(script_standard_push)
         CScript script;
         script << i;
         BOOST_CHECK_MESSAGE(script.IsPushOnly(), "Number " << i << " is not pure push.");
-        BOOST_CHECK_MESSAGE(VerifyScript(script, CScript() << OP_1, NULL, SCRIPT_VERIFY_MINIMALDATA, BaseSignatureChecker(), &err), "Number " << i << " push is not minimal data.");
+        BOOST_CHECK_MESSAGE(VerifyScript(script, CScript() << OP_1, nullptr, SCRIPT_VERIFY_MINIMALDATA, BaseSignatureChecker(), &err), "Number " << i << " push is not minimal data.");
         BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
     }
 
@@ -1274,7 +1274,7 @@ BOOST_AUTO_TEST_CASE(script_standard_push)
         CScript script;
         script << data;
         BOOST_CHECK_MESSAGE(script.IsPushOnly(), "Length " << i << " is not pure push.");
-        BOOST_CHECK_MESSAGE(VerifyScript(script, CScript() << OP_1, NULL, SCRIPT_VERIFY_MINIMALDATA, BaseSignatureChecker(), &err), "Length " << i << " push is not minimal data.");
+        BOOST_CHECK_MESSAGE(VerifyScript(script, CScript() << OP_1, nullptr, SCRIPT_VERIFY_MINIMALDATA, BaseSignatureChecker(), &err), "Length " << i << " push is not minimal data.");
         BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_OK, ScriptErrorString(err));
     }
 }
diff --git a/src/test/skiplist_tests.cpp b/src/test/skiplist_tests.cpp
index 77c321cdf6..e3654e67ad 100644
--- a/src/test/skiplist_tests.cpp
+++ b/src/test/skiplist_tests.cpp
@@ -20,7 +20,7 @@ BOOST_AUTO_TEST_CASE(skiplist_test)
 
     for (int i=0; i<SKIPLIST_LENGTH; i++) {
         vIndex[i].nHeight = i;
-        vIndex[i].pprev = (i == 0) ? NULL : &vIndex[i - 1];
+        vIndex[i].pprev = (i == 0) ? nullptr : &vIndex[i - 1];
         vIndex[i].BuildSkip();
     }
 
@@ -29,7 +29,7 @@ BOOST_AUTO_TEST_CASE(skiplist_test)
             BOOST_CHECK(vIndex[i].pskip == &vIndex[vIndex[i].pskip->nHeight]);
             BOOST_CHECK(vIndex[i].pskip->nHeight < i);
         } else {
-            BOOST_CHECK(vIndex[i].pskip == NULL);
+            BOOST_CHECK(vIndex[i].pskip == nullptr);
         }
     }
 
@@ -51,11 +51,11 @@ BOOST_AUTO_TEST_CASE(getlocator_test)
     for (unsigned int i=0; i<vBlocksMain.size(); i++) {
         vHashMain[i] = ArithToUint256(i); // Set the hash equal to the height, so we can quickly check the distances.
         vBlocksMain[i].nHeight = i;
-        vBlocksMain[i].pprev = i ? &vBlocksMain[i - 1] : NULL;
+        vBlocksMain[i].pprev = i ? &vBlocksMain[i - 1] : nullptr;
         vBlocksMain[i].phashBlock = &vHashMain[i];
         vBlocksMain[i].BuildSkip();
         BOOST_CHECK_EQUAL((int)UintToArith256(vBlocksMain[i].GetBlockHash()).GetLow64(), vBlocksMain[i].nHeight);
-        BOOST_CHECK(vBlocksMain[i].pprev == NULL || vBlocksMain[i].nHeight == vBlocksMain[i].pprev->nHeight + 1);
+        BOOST_CHECK(vBlocksMain[i].pprev == nullptr || vBlocksMain[i].nHeight == vBlocksMain[i].pprev->nHeight + 1);
     }
 
     // Build a branch that splits off at block 49999, 50000 blocks long.
@@ -68,7 +68,7 @@ BOOST_AUTO_TEST_CASE(getlocator_test)
         vBlocksSide[i].phashBlock = &vHashSide[i];
         vBlocksSide[i].BuildSkip();
         BOOST_CHECK_EQUAL((int)UintToArith256(vBlocksSide[i].GetBlockHash()).GetLow64(), vBlocksSide[i].nHeight);
-        BOOST_CHECK(vBlocksSide[i].pprev == NULL || vBlocksSide[i].nHeight == vBlocksSide[i].pprev->nHeight + 1);
+        BOOST_CHECK(vBlocksSide[i].pprev == nullptr || vBlocksSide[i].nHeight == vBlocksSide[i].pprev->nHeight + 1);
     }
 
     // Build a CChain for the main branch.
@@ -106,7 +106,7 @@ BOOST_AUTO_TEST_CASE(findearliestatleast_test)
     for (unsigned int i=0; i<vBlocksMain.size(); i++) {
         vHashMain[i] = ArithToUint256(i); // Set the hash equal to the height
         vBlocksMain[i].nHeight = i;
-        vBlocksMain[i].pprev = i ? &vBlocksMain[i - 1] : NULL;
+        vBlocksMain[i].pprev = i ? &vBlocksMain[i - 1] : nullptr;
         vBlocksMain[i].phashBlock = &vHashMain[i];
         vBlocksMain[i].BuildSkip();
         if (i < 10) {
@@ -138,7 +138,7 @@ BOOST_AUTO_TEST_CASE(findearliestatleast_test)
         int64_t test_time = vBlocksMain[r].nTime;
         CBlockIndex *ret = chain.FindEarliestAtLeast(test_time);
         BOOST_CHECK(ret->nTimeMax >= test_time);
-        BOOST_CHECK((ret->pprev==NULL) || ret->pprev->nTimeMax < test_time);
+        BOOST_CHECK((ret->pprev==nullptr) || ret->pprev->nTimeMax < test_time);
         BOOST_CHECK(vBlocksMain[r].GetAncestor(ret->nHeight) == ret);
     }
 }
diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
index e2e0a9668f..72d7f1b19f 100644
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -140,7 +140,7 @@ TestChain100Setup::CreateAndProcessBlock(const std::vector<CMutableTransaction>&
     while (!CheckProofOfWork(block.GetHash(), block.nBits, chainparams.GetConsensus())) ++block.nNonce;
 
     std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(block);
-    ProcessNewBlock(chainparams, shared_pblock, true, NULL);
+    ProcessNewBlock(chainparams, shared_pblock, true, nullptr);
 
     CBlock result = block;
     return result;
diff --git a/src/test/txvalidationcache_tests.cpp b/src/test/txvalidationcache_tests.cpp
index f609cb1af4..2d25cb96c8 100644
--- a/src/test/txvalidationcache_tests.cpp
+++ b/src/test/txvalidationcache_tests.cpp
@@ -29,7 +29,7 @@ ToMemPool(CMutableTransaction& tx)
     LOCK(cs_main);
 
     CValidationState state;
-    return AcceptToMemoryPool(mempool, state, MakeTransactionRef(tx), false, NULL, NULL, true, 0);
+    return AcceptToMemoryPool(mempool, state, MakeTransactionRef(tx), false, nullptr, nullptr, true, 0);
 }
 
 BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, TestChain100Setup)
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 5e9dfb730b..5679086969 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -328,7 +328,7 @@ BOOST_AUTO_TEST_CASE(test_ParseInt32)
 {
     int32_t n;
     // Valid values
-    BOOST_CHECK(ParseInt32("1234", NULL));
+    BOOST_CHECK(ParseInt32("1234", nullptr));
     BOOST_CHECK(ParseInt32("0", &n) && n == 0);
     BOOST_CHECK(ParseInt32("1234", &n) && n == 1234);
     BOOST_CHECK(ParseInt32("01234", &n) && n == 1234); // no octal
@@ -347,17 +347,17 @@ BOOST_AUTO_TEST_CASE(test_ParseInt32)
     std::string teststr(test_bytes, sizeof(test_bytes));
     BOOST_CHECK(!ParseInt32(teststr, &n)); // no embedded NULs
     // Overflow and underflow
-    BOOST_CHECK(!ParseInt32("-2147483649", NULL));
-    BOOST_CHECK(!ParseInt32("2147483648", NULL));
-    BOOST_CHECK(!ParseInt32("-32482348723847471234", NULL));
-    BOOST_CHECK(!ParseInt32("32482348723847471234", NULL));
+    BOOST_CHECK(!ParseInt32("-2147483649", nullptr));
+    BOOST_CHECK(!ParseInt32("2147483648", nullptr));
+    BOOST_CHECK(!ParseInt32("-32482348723847471234", nullptr));
+    BOOST_CHECK(!ParseInt32("32482348723847471234", nullptr));
 }
 
 BOOST_AUTO_TEST_CASE(test_ParseInt64)
 {
     int64_t n;
     // Valid values
-    BOOST_CHECK(ParseInt64("1234", NULL));
+    BOOST_CHECK(ParseInt64("1234", nullptr));
     BOOST_CHECK(ParseInt64("0", &n) && n == 0LL);
     BOOST_CHECK(ParseInt64("1234", &n) && n == 1234LL);
     BOOST_CHECK(ParseInt64("01234", &n) && n == 1234LL); // no octal
@@ -377,17 +377,17 @@ BOOST_AUTO_TEST_CASE(test_ParseInt64)
     std::string teststr(test_bytes, sizeof(test_bytes));
     BOOST_CHECK(!ParseInt64(teststr, &n)); // no embedded NULs
     // Overflow and underflow
-    BOOST_CHECK(!ParseInt64("-9223372036854775809", NULL));
-    BOOST_CHECK(!ParseInt64("9223372036854775808", NULL));
-    BOOST_CHECK(!ParseInt64("-32482348723847471234", NULL));
-    BOOST_CHECK(!ParseInt64("32482348723847471234", NULL));
+    BOOST_CHECK(!ParseInt64("-9223372036854775809", nullptr));
+    BOOST_CHECK(!ParseInt64("9223372036854775808", nullptr));
+    BOOST_CHECK(!ParseInt64("-32482348723847471234", nullptr));
+    BOOST_CHECK(!ParseInt64("32482348723847471234", nullptr));
 }
 
 BOOST_AUTO_TEST_CASE(test_ParseUInt32)
 {
     uint32_t n;
     // Valid values
-    BOOST_CHECK(ParseUInt32("1234", NULL));
+    BOOST_CHECK(ParseUInt32("1234", nullptr));
     BOOST_CHECK(ParseUInt32("0", &n) && n == 0);
     BOOST_CHECK(ParseUInt32("1234", &n) && n == 1234);
     BOOST_CHECK(ParseUInt32("01234", &n) && n == 1234); // no octal
@@ -410,15 +410,15 @@ BOOST_AUTO_TEST_CASE(test_ParseUInt32)
     BOOST_CHECK(!ParseUInt32("-2147483648", &n));
     BOOST_CHECK(!ParseUInt32("4294967296", &n));
     BOOST_CHECK(!ParseUInt32("-1234", &n));
-    BOOST_CHECK(!ParseUInt32("-32482348723847471234", NULL));
-    BOOST_CHECK(!ParseUInt32("32482348723847471234", NULL));
+    BOOST_CHECK(!ParseUInt32("-32482348723847471234", nullptr));
+    BOOST_CHECK(!ParseUInt32("32482348723847471234", nullptr));
 }
 
 BOOST_AUTO_TEST_CASE(test_ParseUInt64)
 {
     uint64_t n;
     // Valid values
-    BOOST_CHECK(ParseUInt64("1234", NULL));
+    BOOST_CHECK(ParseUInt64("1234", nullptr));
     BOOST_CHECK(ParseUInt64("0", &n) && n == 0LL);
     BOOST_CHECK(ParseUInt64("1234", &n) && n == 1234LL);
     BOOST_CHECK(ParseUInt64("01234", &n) && n == 1234LL); // no octal
@@ -438,9 +438,9 @@ BOOST_AUTO_TEST_CASE(test_ParseUInt64)
     std::string teststr(test_bytes, sizeof(test_bytes));
     BOOST_CHECK(!ParseUInt64(teststr, &n)); // no embedded NULs
     // Overflow and underflow
-    BOOST_CHECK(!ParseUInt64("-9223372036854775809", NULL));
-    BOOST_CHECK(!ParseUInt64("18446744073709551616", NULL));
-    BOOST_CHECK(!ParseUInt64("-32482348723847471234", NULL));
+    BOOST_CHECK(!ParseUInt64("-9223372036854775809", nullptr));
+    BOOST_CHECK(!ParseUInt64("18446744073709551616", nullptr));
+    BOOST_CHECK(!ParseUInt64("-32482348723847471234", nullptr));
     BOOST_CHECK(!ParseUInt64("-2147483648", &n));
     BOOST_CHECK(!ParseUInt64("-9223372036854775808", &n));
     BOOST_CHECK(!ParseUInt64("-1234", &n));
@@ -450,7 +450,7 @@ BOOST_AUTO_TEST_CASE(test_ParseDouble)
 {
     double n;
     // Valid values
-    BOOST_CHECK(ParseDouble("1234", NULL));
+    BOOST_CHECK(ParseDouble("1234", nullptr));
     BOOST_CHECK(ParseDouble("0", &n) && n == 0.0);
     BOOST_CHECK(ParseDouble("1234", &n) && n == 1234.0);
     BOOST_CHECK(ParseDouble("01234", &n) && n == 1234.0); // no octal
@@ -470,8 +470,8 @@ BOOST_AUTO_TEST_CASE(test_ParseDouble)
     std::string teststr(test_bytes, sizeof(test_bytes));
     BOOST_CHECK(!ParseDouble(teststr, &n)); // no embedded NULs
     // Overflow and underflow
-    BOOST_CHECK(!ParseDouble("-1e10000", NULL));
-    BOOST_CHECK(!ParseDouble("1e10000", NULL));
+    BOOST_CHECK(!ParseDouble("-1e10000", nullptr));
+    BOOST_CHECK(!ParseDouble("1e10000", nullptr));
 }
 
 BOOST_AUTO_TEST_CASE(test_FormatParagraph)
diff --git a/src/test/versionbits_tests.cpp b/src/test/versionbits_tests.cpp
index 722f6ae059..f433aad889 100644
--- a/src/test/versionbits_tests.cpp
+++ b/src/test/versionbits_tests.cpp
@@ -69,7 +69,7 @@ public:
         while (vpblock.size() < height) {
             CBlockIndex* pindex = new CBlockIndex();
             pindex->nHeight = vpblock.size();
-            pindex->pprev = vpblock.size() > 0 ? vpblock.back() : NULL;
+            pindex->pprev = vpblock.size() > 0 ? vpblock.back() : nullptr;
             pindex->nTime = nTime;
             pindex->nVersion = nVersion;
             pindex->BuildSkip();
@@ -81,7 +81,7 @@ public:
     VersionBitsTester& TestStateSinceHeight(int height) {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateSinceHeightFor(vpblock.empty() ? NULL : vpblock.back()) == height, strprintf("Test %i for StateSinceHeight", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateSinceHeightFor(vpblock.empty() ? nullptr : vpblock.back()) == height, strprintf("Test %i for StateSinceHeight", num));
             }
         }
         num++;
@@ -91,7 +91,7 @@ public:
     VersionBitsTester& TestDefined() {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? NULL : vpblock.back()) == THRESHOLD_DEFINED, strprintf("Test %i for DEFINED", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? nullptr : vpblock.back()) == THRESHOLD_DEFINED, strprintf("Test %i for DEFINED", num));
             }
         }
         num++;
@@ -101,7 +101,7 @@ public:
     VersionBitsTester& TestStarted() {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? NULL : vpblock.back()) == THRESHOLD_STARTED, strprintf("Test %i for STARTED", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? nullptr : vpblock.back()) == THRESHOLD_STARTED, strprintf("Test %i for STARTED", num));
             }
         }
         num++;
@@ -111,7 +111,7 @@ public:
     VersionBitsTester& TestLockedIn() {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? NULL : vpblock.back()) == THRESHOLD_LOCKED_IN, strprintf("Test %i for LOCKED_IN", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? nullptr : vpblock.back()) == THRESHOLD_LOCKED_IN, strprintf("Test %i for LOCKED_IN", num));
             }
         }
         num++;
@@ -121,7 +121,7 @@ public:
     VersionBitsTester& TestActive() {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? NULL : vpblock.back()) == THRESHOLD_ACTIVE, strprintf("Test %i for ACTIVE", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? nullptr : vpblock.back()) == THRESHOLD_ACTIVE, strprintf("Test %i for ACTIVE", num));
             }
         }
         num++;
@@ -131,14 +131,14 @@ public:
     VersionBitsTester& TestFailed() {
         for (int i = 0; i < CHECKERS; i++) {
             if (InsecureRandBits(i) == 0) {
-                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? NULL : vpblock.back()) == THRESHOLD_FAILED, strprintf("Test %i for FAILED", num));
+                BOOST_CHECK_MESSAGE(checker[i].GetStateFor(vpblock.empty() ? nullptr : vpblock.back()) == THRESHOLD_FAILED, strprintf("Test %i for FAILED", num));
             }
         }
         num++;
         return *this;
     }
 
-    CBlockIndex * Tip() { return vpblock.size() ? vpblock.back() : NULL; }
+    CBlockIndex * Tip() { return vpblock.size() ? vpblock.back() : nullptr; }
 };
 
 BOOST_FIXTURE_TEST_SUITE(versionbits_tests, TestingSetup)
@@ -255,7 +255,7 @@ BOOST_AUTO_TEST_CASE(versionbits_computeblockversion)
 
     // Before MedianTimePast of the chain has crossed nStartTime, the bit
     // should not be set.
-    CBlockIndex *lastBlock = NULL;
+    CBlockIndex *lastBlock = nullptr;
     lastBlock = firstChain.Mine(2016, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
     BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
 
diff --git a/src/torcontrol.cpp b/src/torcontrol.cpp
index ac13f73e70..57c22537b2 100644
--- a/src/torcontrol.cpp
+++ b/src/torcontrol.cpp
@@ -138,8 +138,8 @@ void TorControlConnection::readcb(struct bufferevent *bev, void *ctx)
     size_t n_read_out = 0;
     char *line;
     assert(input);
-    //  If there is not a whole line to read, evbuffer_readln returns NULL
-    while((line = evbuffer_readln(input, &n_read_out, EVBUFFER_EOL_CRLF)) != NULL)
+    //  If there is not a whole line to read, evbuffer_readln returns nullptr
+    while((line = evbuffer_readln(input, &n_read_out, EVBUFFER_EOL_CRLF)) != nullptr)
     {
         std::string s(line, n_read_out);
         free(line);
@@ -210,7 +210,7 @@ bool TorControlConnection::Connect(const std::string &target, const ConnectionCB
     b_conn = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
     if (!b_conn)
         return false;
-    bufferevent_setcb(b_conn, TorControlConnection::readcb, NULL, TorControlConnection::eventcb, this);
+    bufferevent_setcb(b_conn, TorControlConnection::readcb, nullptr, TorControlConnection::eventcb, this);
     bufferevent_enable(b_conn, EV_READ|EV_WRITE);
     this->connected = _connected;
     this->disconnected = _disconnected;
@@ -333,7 +333,7 @@ static std::map<std::string,std::string> ParseTorReplyMapping(const std::string
                         if (j == 3 && value[i] > '3') {
                             j--;
                         }
-                        escaped_value.push_back(strtol(value.substr(i, j).c_str(), NULL, 8));
+                        escaped_value.push_back(strtol(value.substr(i, j).c_str(), nullptr, 8));
                         // Account for automatic incrementing at loop end
                         i += j - 1;
                     } else {
@@ -367,7 +367,7 @@ static std::map<std::string,std::string> ParseTorReplyMapping(const std::string
 static std::pair<bool,std::string> ReadBinaryFile(const fs::path &filename, size_t maxsize=std::numeric_limits<size_t>::max())
 {
     FILE *f = fsbridge::fopen(filename, "rb");
-    if (f == NULL)
+    if (f == nullptr)
         return std::make_pair(false,"");
     std::string retval;
     char buffer[128];
@@ -393,7 +393,7 @@ static std::pair<bool,std::string> ReadBinaryFile(const fs::path &filename, size
 static bool WriteBinaryFile(const fs::path &filename, const std::string &data)
 {
     FILE *f = fsbridge::fopen(filename, "wb");
-    if (f == NULL)
+    if (f == nullptr)
         return false;
     if (fwrite(data.data(), 1, data.size(), f) != data.size()) {
         fclose(f);
diff --git a/src/txmempool.h b/src/txmempool.h
index d272114a7c..6723ea8e6c 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -45,7 +45,7 @@ struct LockPoints
     // values are still valid even after a reorg.
     CBlockIndex* maxInputBlock;
 
-    LockPoints() : height(0), time(0), maxInputBlock(NULL) { }
+    LockPoints() : height(0), time(0), maxInputBlock(nullptr) { }
 };
 
 class CTxMemPool;
@@ -592,7 +592,7 @@ public:
       *  pvNoSpendsRemaining, if set, will be populated with the list of outpoints
       *  which are not in mempool which no longer have any spends in this mempool.
       */
-    void TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpendsRemaining=NULL);
+    void TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpendsRemaining=nullptr);
 
     /** Expire all transaction (and their dependencies) in the mempool older than time. Return the number of removed transactions. */
     int Expire(int64_t time);
diff --git a/src/util.cpp b/src/util.cpp
index b76c173f90..fc75b7ef85 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -144,7 +144,7 @@ public:
         // Securely erase the memory used by the PRNG
         RAND_cleanup();
         // Shutdown OpenSSL library multithreading support
-        CRYPTO_set_locking_callback(NULL);
+        CRYPTO_set_locking_callback(nullptr);
         // Clear the set of locks now to maintain symmetry with the constructor.
         ppmutexOpenSSL.reset();
     }
@@ -173,8 +173,8 @@ static boost::once_flag debugPrintInitFlag = BOOST_ONCE_INIT;
  * the OS/libc. When the shutdown sequence is fully audited and
  * tested, explicit destruction of these objects can be implemented.
  */
-static FILE* fileout = NULL;
-static boost::mutex* mutexDebugLog = NULL;
+static FILE* fileout = nullptr;
+static boost::mutex* mutexDebugLog = nullptr;
 static std::list<std::string>* vMsgsBeforeOpenLog;
 
 static int FileWriteStr(const std::string &str, FILE *fp)
@@ -184,7 +184,7 @@ static int FileWriteStr(const std::string &str, FILE *fp)
 
 static void DebugPrintInit()
 {
-    assert(mutexDebugLog == NULL);
+    assert(mutexDebugLog == nullptr);
     mutexDebugLog = new boost::mutex();
     vMsgsBeforeOpenLog = new std::list<std::string>;
 }
@@ -194,12 +194,12 @@ void OpenDebugLog()
     boost::call_once(&DebugPrintInit, debugPrintInitFlag);
     boost::mutex::scoped_lock scoped_lock(*mutexDebugLog);
 
-    assert(fileout == NULL);
+    assert(fileout == nullptr);
     assert(vMsgsBeforeOpenLog);
     fs::path pathDebug = GetDataDir() / "debug.log";
     fileout = fsbridge::fopen(pathDebug, "a");
     if (fileout) {
-        setbuf(fileout, NULL); // unbuffered
+        setbuf(fileout, nullptr); // unbuffered
         // dump buffered messages from before we opened the log
         while (!vMsgsBeforeOpenLog->empty()) {
             FileWriteStr(vMsgsBeforeOpenLog->front(), fileout);
@@ -208,7 +208,7 @@ void OpenDebugLog()
     }
 
     delete vMsgsBeforeOpenLog;
-    vMsgsBeforeOpenLog = NULL;
+    vMsgsBeforeOpenLog = nullptr;
 }
 
 struct CLogCategoryDesc
@@ -344,7 +344,7 @@ int LogPrintStr(const std::string &str)
         boost::mutex::scoped_lock scoped_lock(*mutexDebugLog);
 
         // buffer if we haven't opened the log yet
-        if (fileout == NULL) {
+        if (fileout == nullptr) {
             assert(vMsgsBeforeOpenLog);
             ret = strTimestamped.length();
             vMsgsBeforeOpenLog->push_back(strTimestamped);
@@ -355,8 +355,8 @@ int LogPrintStr(const std::string &str)
             if (fReopenDebugLog) {
                 fReopenDebugLog = false;
                 fs::path pathDebug = GetDataDir() / "debug.log";
-                if (fsbridge::freopen(pathDebug,"a",fileout) != NULL)
-                    setbuf(fileout, NULL); // unbuffered
+                if (fsbridge::freopen(pathDebug,"a",fileout) != nullptr)
+                    setbuf(fileout, nullptr); // unbuffered
             }
 
             ret = FileWriteStr(strTimestamped, fileout);
@@ -503,7 +503,7 @@ static std::string FormatException(const std::exception* pex, const char* pszThr
 {
 #ifdef WIN32
     char pszModule[MAX_PATH] = "";
-    GetModuleFileNameA(NULL, pszModule, sizeof(pszModule));
+    GetModuleFileNameA(nullptr, pszModule, sizeof(pszModule));
 #else
     const char* pszModule = "bitcoin";
 #endif
@@ -534,7 +534,7 @@ fs::path GetDefaultDataDir()
 #else
     fs::path pathRet;
     char* pszHome = getenv("HOME");
-    if (pszHome == NULL || strlen(pszHome) == 0)
+    if (pszHome == nullptr || strlen(pszHome) == 0)
         pathRet = fs::path("/");
     else
         pathRet = fs::path(pszHome);
@@ -791,7 +791,7 @@ void ShrinkDebugFile()
             fclose(file);
         }
     }
-    else if (file != NULL)
+    else if (file != nullptr)
         fclose(file);
 }
 
@@ -800,7 +800,7 @@ fs::path GetSpecialFolderPath(int nFolder, bool fCreate)
 {
     char pszPath[MAX_PATH] = "";
 
-    if(SHGetSpecialFolderPathA(NULL, pszPath, nFolder, fCreate))
+    if(SHGetSpecialFolderPathA(nullptr, pszPath, nFolder, fCreate))
     {
         return fs::path(pszPath);
     }
diff --git a/src/util.h b/src/util.h
index e1bdfb1988..b01606cb8f 100644
--- a/src/util.h
+++ b/src/util.h
@@ -358,7 +358,7 @@ template <typename Callable> void TraceThread(const char* name,  Callable func)
         throw;
     }
     catch (...) {
-        PrintExceptionContinue(NULL, name);
+        PrintExceptionContinue(nullptr, name);
         throw;
     }
 }
diff --git a/src/utilstrencodings.cpp b/src/utilstrencodings.cpp
index 9ee14070a2..fd233f6757 100644
--- a/src/utilstrencodings.cpp
+++ b/src/utilstrencodings.cpp
@@ -452,7 +452,7 @@ bool ParseInt32(const std::string& str, int32_t *out)
 {
     if (!ParsePrechecks(str))
         return false;
-    char *endp = NULL;
+    char *endp = nullptr;
     errno = 0; // strtol will not set errno if valid
     long int n = strtol(str.c_str(), &endp, 10);
     if(out) *out = (int32_t)n;
@@ -468,7 +468,7 @@ bool ParseInt64(const std::string& str, int64_t *out)
 {
     if (!ParsePrechecks(str))
         return false;
-    char *endp = NULL;
+    char *endp = nullptr;
     errno = 0; // strtoll will not set errno if valid
     long long int n = strtoll(str.c_str(), &endp, 10);
     if(out) *out = (int64_t)n;
@@ -485,7 +485,7 @@ bool ParseUInt32(const std::string& str, uint32_t *out)
         return false;
     if (str.size() >= 1 && str[0] == '-') // Reject negative values, unfortunately strtoul accepts these by default if they fit in the range
         return false;
-    char *endp = NULL;
+    char *endp = nullptr;
     errno = 0; // strtoul will not set errno if valid
     unsigned long int n = strtoul(str.c_str(), &endp, 10);
     if(out) *out = (uint32_t)n;
@@ -502,7 +502,7 @@ bool ParseUInt64(const std::string& str, uint64_t *out)
         return false;
     if (str.size() >= 1 && str[0] == '-') // Reject negative values, unfortunately strtoull accepts these by default if they fit in the range
         return false;
-    char *endp = NULL;
+    char *endp = nullptr;
     errno = 0; // strtoull will not set errno if valid
     unsigned long long int n = strtoull(str.c_str(), &endp, 10);
     if(out) *out = (uint64_t)n;
@@ -583,7 +583,7 @@ int64_t atoi64(const char* psz)
 #ifdef _MSC_VER
     return _atoi64(psz);
 #else
-    return strtoll(psz, NULL, 10);
+    return strtoll(psz, nullptr, 10);
 #endif
 }
 
@@ -592,7 +592,7 @@ int64_t atoi64(const std::string& str)
 #ifdef _MSC_VER
     return _atoi64(str.c_str());
 #else
-    return strtoll(str.c_str(), NULL, 10);
+    return strtoll(str.c_str(), nullptr, 10);
 #endif
 }
 
diff --git a/src/utilstrencodings.h b/src/utilstrencodings.h
index 707fdaad16..53da60e8f1 100644
--- a/src/utilstrencodings.h
+++ b/src/utilstrencodings.h
@@ -39,11 +39,11 @@ std::vector<unsigned char> ParseHex(const char* psz);
 std::vector<unsigned char> ParseHex(const std::string& str);
 signed char HexDigit(char c);
 bool IsHex(const std::string& str);
-std::vector<unsigned char> DecodeBase64(const char* p, bool* pfInvalid = NULL);
+std::vector<unsigned char> DecodeBase64(const char* p, bool* pfInvalid = nullptr);
 std::string DecodeBase64(const std::string& str);
 std::string EncodeBase64(const unsigned char* pch, size_t len);
 std::string EncodeBase64(const std::string& str);
-std::vector<unsigned char> DecodeBase32(const char* p, bool* pfInvalid = NULL);
+std::vector<unsigned char> DecodeBase32(const char* p, bool* pfInvalid = nullptr);
 std::string DecodeBase32(const std::string& str);
 std::string EncodeBase32(const unsigned char* pch, size_t len);
 std::string EncodeBase32(const std::string& str);
diff --git a/src/utiltime.cpp b/src/utiltime.cpp
index e07069125d..4cc77dbfeb 100644
--- a/src/utiltime.cpp
+++ b/src/utiltime.cpp
@@ -21,7 +21,7 @@ int64_t GetTime()
     int64_t mocktime = nMockTime.load(std::memory_order_relaxed);
     if (mocktime) return mocktime;
 
-    time_t now = time(NULL);
+    time_t now = time(nullptr);
     assert(now > 0);
     return now;
 }
diff --git a/src/validation.cpp b/src/validation.cpp
index 0f9f6436d8..f21700c673 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -61,7 +61,7 @@ CCriticalSection cs_main;
 
 BlockMap mapBlockIndex;
 CChain chainActive;
-CBlockIndex *pindexBestHeader = NULL;
+CBlockIndex *pindexBestHeader = nullptr;
 CWaitableCriticalSection csBestBlock;
 CConditionVariable cvBlockChange;
 int nScriptCheckThreads = 0;
@@ -177,9 +177,9 @@ CBlockIndex* FindForkInGlobalIndex(const CChain& chain, const CBlockLocator& loc
     return chain.Genesis();
 }
 
-CCoinsViewDB *pcoinsdbview = NULL;
-CCoinsViewCache *pcoinsTip = NULL;
-CBlockTreeDB *pblocktree = NULL;
+CCoinsViewDB *pcoinsdbview = nullptr;
+CCoinsViewCache *pcoinsTip = nullptr;
+CBlockTreeDB *pblocktree = nullptr;
 
 enum FlushStateMode {
     FLUSH_STATE_NONE,
@@ -378,7 +378,7 @@ void UpdateMempoolForReorg(DisconnectedBlockTransactions &disconnectpool, bool f
     while (it != disconnectpool.queuedTx.get<insertion_order>().rend()) {
         // ignore validation errors in resurrected transactions
         CValidationState stateDummy;
-        if (!fAddToMempool || (*it)->IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, *it, false, NULL, NULL, true)) {
+        if (!fAddToMempool || (*it)->IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, *it, false, nullptr, nullptr, true)) {
             // If the transaction doesn't make it in to the mempool, remove any
             // transactions that depend on it (which would now be orphans).
             mempool.removeRecursive(**it, MemPoolRemovalReason::REORG);
@@ -899,7 +899,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 /** Return transaction in txOut, and if it was found inside a block, its hash is placed in hashBlock */
 bool GetTransaction(const uint256 &hash, CTransactionRef &txOut, const Consensus::Params& consensusParams, uint256 &hashBlock, bool fAllowSlow)
 {
-    CBlockIndex *pindexSlow = NULL;
+    CBlockIndex *pindexSlow = nullptr;
 
     LOCK(cs_main);
 
@@ -1045,7 +1045,7 @@ bool IsInitialBlockDownload()
         return false;
     if (fImporting || fReindex)
         return true;
-    if (chainActive.Tip() == NULL)
+    if (chainActive.Tip() == nullptr)
         return true;
     if (chainActive.Tip()->nChainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
         return true;
@@ -1056,7 +1056,7 @@ bool IsInitialBlockDownload()
     return false;
 }
 
-CBlockIndex *pindexBestForkTip = NULL, *pindexBestForkBase = NULL;
+CBlockIndex *pindexBestForkTip = nullptr, *pindexBestForkBase = nullptr;
 
 static void AlertNotify(const std::string& strMessage)
 {
@@ -1086,7 +1086,7 @@ static void CheckForkWarningConditions()
     // If our best fork is no longer within 72 blocks (+/- 12 hours if no one mines it)
     // of our head, drop it
     if (pindexBestForkTip && chainActive.Height() - pindexBestForkTip->nHeight >= 72)
-        pindexBestForkTip = NULL;
+        pindexBestForkTip = nullptr;
 
     if (pindexBestForkTip || (pindexBestInvalid && pindexBestInvalid->nChainWork > chainActive.Tip()->nChainWork + (GetBlockProof(*chainActive.Tip()) * 6)))
     {
@@ -1226,7 +1226,7 @@ void InitScriptExecutionCache() {
  * Check whether all inputs of this transaction are valid (no double spends, scripts & sigs, amounts)
  * This does not modify the UTXO set.
  *
- * If pvChecks is not NULL, script checks are pushed onto it instead of being performed inline. Any
+ * If pvChecks is not nullptr, script checks are pushed onto it instead of being performed inline. Any
  * script checks which are not necessary (eg due to script execution cache hits) are, obviously,
  * not pushed onto pvChecks/run.
  *
@@ -1633,7 +1633,7 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
     AssertLockHeld(cs_main);
     assert(pindex);
     // pindex->phashBlock can be null if called by CreateNewBlock/TestBlockValidity
-    assert((pindex->phashBlock == NULL) ||
+    assert((pindex->phashBlock == nullptr) ||
            (*pindex->phashBlock == block.GetHash()));
     int64_t nTimeStart = GetTimeMicros();
 
@@ -1642,7 +1642,7 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
         return error("%s: Consensus::CheckBlock: %s", __func__, FormatStateMessage(state));
 
     // verify that the view's current state corresponds to the previous block
-    uint256 hashPrevBlock = pindex->pprev == NULL ? uint256() : pindex->pprev->GetBlockHash();
+    uint256 hashPrevBlock = pindex->pprev == nullptr ? uint256() : pindex->pprev->GetBlockHash();
     assert(hashPrevBlock == view.GetBestBlock());
 
     // Special case for the genesis block, skipping connection of its transactions
@@ -1733,7 +1733,7 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
 
     CBlockUndo blockundo;
 
-    CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
+    CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : nullptr);
 
     std::vector<int> prevheights;
     CAmount nFees = 0;
@@ -1787,7 +1787,7 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
 
             std::vector<CScriptCheck> vChecks;
             bool fCacheResults = fJustCheck; /* Don't cache results if we're actually connecting blocks (still consult the cache, though) */
-            if (!CheckInputs(tx, state, view, fScriptChecks, flags, fCacheResults, fCacheResults, txdata[i], nScriptCheckThreads ? &vChecks : NULL))
+            if (!CheckInputs(tx, state, view, fScriptChecks, flags, fCacheResults, fCacheResults, txdata[i], nScriptCheckThreads ? &vChecks : nullptr))
                 return error("ConnectBlock(): CheckInputs on %s failed with %s",
                     tx.GetHash().ToString(), FormatStateMessage(state));
             control.Add(vChecks);
@@ -2019,7 +2019,7 @@ void static UpdateTip(CBlockIndex *pindexNew, const CChainParams& chainParams) {
             }
         }
         // Check the version of the last 100 blocks to see if we need to upgrade:
-        for (int i = 0; i < 100 && pindex != NULL; i++)
+        for (int i = 0; i < 100 && pindex != nullptr; i++)
         {
             int32_t nExpectedVersion = ComputeBlockVersion(pindex->pprev, chainParams.GetConsensus());
             if (pindex->nVersion > VERSIONBITS_LAST_OLD_BLOCK_VERSION && (pindex->nVersion & ~nExpectedVersion) != 0)
@@ -2052,7 +2052,7 @@ void static UpdateTip(CBlockIndex *pindexNew, const CChainParams& chainParams) {
   * should make the mempool consistent again by calling UpdateMempoolForReorg.
   * with cs_main held.
   *
-  * If disconnectpool is NULL, then no disconnected transactions are added to
+  * If disconnectpool is nullptr, then no disconnected transactions are added to
   * disconnectpool (note that the caller is responsible for mempool consistency
   * in any case).
   */
@@ -2108,7 +2108,7 @@ static int64_t nTimeChainState = 0;
 static int64_t nTimePostConnect = 0;
 
 struct PerBlockConnectTrace {
-    CBlockIndex* pindex = NULL;
+    CBlockIndex* pindex = nullptr;
     std::shared_ptr<const CBlock> pblock;
     std::shared_ptr<std::vector<CTransactionRef>> conflictedTxs;
     PerBlockConnectTrace() : conflictedTxs(std::make_shared<std::vector<CTransactionRef>>()) {}
@@ -2173,7 +2173,7 @@ public:
 };
 
 /**
- * Connect a new block to chainActive. pblock is either NULL or a pointer to a CBlock
+ * Connect a new block to chainActive. pblock is either nullptr or a pointer to a CBlock
  * corresponding to pindexNew, to bypass loading it again from disk.
  *
  * The block is added to connectTrace if connection succeeds.
@@ -2238,13 +2238,13 @@ bool static ConnectTip(CValidationState& state, const CChainParams& chainparams,
  */
 static CBlockIndex* FindMostWorkChain() {
     do {
-        CBlockIndex *pindexNew = NULL;
+        CBlockIndex *pindexNew = nullptr;
 
         // Find the best candidate header.
         {
             std::set<CBlockIndex*, CBlockIndexWorkComparator>::reverse_iterator it = setBlockIndexCandidates.rbegin();
             if (it == setBlockIndexCandidates.rend())
-                return NULL;
+                return nullptr;
             pindexNew = *it;
         }
 
@@ -2263,7 +2263,7 @@ static CBlockIndex* FindMostWorkChain() {
             bool fMissingData = !(pindexTest->nStatus & BLOCK_HAVE_DATA);
             if (fFailedChain || fMissingData) {
                 // Candidate chain is not usable (either invalid or missing data)
-                if (fFailedChain && (pindexBestInvalid == NULL || pindexNew->nChainWork > pindexBestInvalid->nChainWork))
+                if (fFailedChain && (pindexBestInvalid == nullptr || pindexNew->nChainWork > pindexBestInvalid->nChainWork))
                     pindexBestInvalid = pindexNew;
                 CBlockIndex *pindexFailed = pindexNew;
                 // Remove the entire chain from the set.
@@ -2304,7 +2304,7 @@ static void PruneBlockIndexCandidates() {
 
 /**
  * Try to make some progress towards making pindexMostWork the active block.
- * pblock is either NULL or a pointer to a CBlock corresponding to pindexMostWork.
+ * pblock is either nullptr or a pointer to a CBlock corresponding to pindexMostWork.
  */
 static bool ActivateBestChainStep(CValidationState& state, const CChainParams& chainparams, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace)
 {
@@ -2390,8 +2390,8 @@ static bool ActivateBestChainStep(CValidationState& state, const CChainParams& c
 static void NotifyHeaderTip() {
     bool fNotify = false;
     bool fInitialBlockDownload = false;
-    static CBlockIndex* pindexHeaderOld = NULL;
-    CBlockIndex* pindexHeader = NULL;
+    static CBlockIndex* pindexHeaderOld = nullptr;
+    CBlockIndex* pindexHeader = nullptr;
     {
         LOCK(cs_main);
         pindexHeader = pindexBestHeader;
@@ -2410,7 +2410,7 @@ static void NotifyHeaderTip() {
 
 /**
  * Make the best chain active, in multiple steps. The result is either failure
- * or an activated best chain. pblock is either NULL or a pointer to a block
+ * or an activated best chain. pblock is either nullptr or a pointer to a block
  * that is already loaded (to avoid loading it again from disk).
  */
 bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock) {
@@ -2419,8 +2419,8 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
     // us in the middle of ProcessNewBlock - do not assume pblock is set
     // sanely for performance or correctness!
 
-    CBlockIndex *pindexMostWork = NULL;
-    CBlockIndex *pindexNewTip = NULL;
+    CBlockIndex *pindexMostWork = nullptr;
+    CBlockIndex *pindexNewTip = nullptr;
     int nStopAtHeight = GetArg("-stopatheight", DEFAULT_STOPATHEIGHT);
     do {
         boost::this_thread::interruption_point();
@@ -2434,12 +2434,12 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
             ConnectTrace connectTrace(mempool); // Destructed before cs_main is unlocked
 
             CBlockIndex *pindexOldTip = chainActive.Tip();
-            if (pindexMostWork == NULL) {
+            if (pindexMostWork == nullptr) {
                 pindexMostWork = FindMostWorkChain();
             }
 
             // Whether we have anything to do at all.
-            if (pindexMostWork == NULL || pindexMostWork == chainActive.Tip())
+            if (pindexMostWork == nullptr || pindexMostWork == chainActive.Tip())
                 return true;
 
             bool fInvalidFound = false;
@@ -2449,7 +2449,7 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
 
             if (fInvalidFound) {
                 // Wipe cache, we may need another branch now.
-                pindexMostWork = NULL;
+                pindexMostWork = nullptr;
             }
             pindexNewTip = chainActive.Tip();
             pindexFork = chainActive.FindFork(pindexOldTip);
@@ -2574,14 +2574,14 @@ bool ResetBlockFailureFlags(CBlockIndex *pindex) {
             }
             if (it->second == pindexBestInvalid) {
                 // Reset invalid block marker if it was pointing to one of those.
-                pindexBestInvalid = NULL;
+                pindexBestInvalid = nullptr;
             }
         }
         it++;
     }
 
     // Remove the invalidity flag from all ancestors too.
-    while (pindex != NULL) {
+    while (pindex != nullptr) {
         if (pindex->nStatus & BLOCK_FAILED_MASK) {
             pindex->nStatus &= ~BLOCK_FAILED_MASK;
             setDirtyBlockIndex.insert(pindex);
@@ -2618,7 +2618,7 @@ static CBlockIndex* AddToBlockIndex(const CBlockHeader& block)
     pindexNew->nTimeMax = (pindexNew->pprev ? std::max(pindexNew->pprev->nTimeMax, pindexNew->nTime) : pindexNew->nTime);
     pindexNew->nChainWork = (pindexNew->pprev ? pindexNew->pprev->nChainWork : 0) + GetBlockProof(*pindexNew);
     pindexNew->RaiseValidity(BLOCK_VALID_TREE);
-    if (pindexBestHeader == NULL || pindexBestHeader->nChainWork < pindexNew->nChainWork)
+    if (pindexBestHeader == nullptr || pindexBestHeader->nChainWork < pindexNew->nChainWork)
         pindexBestHeader = pindexNew;
 
     setDirtyBlockIndex.insert(pindexNew);
@@ -2641,7 +2641,7 @@ static bool ReceivedBlockTransactions(const CBlock &block, CValidationState& sta
     pindexNew->RaiseValidity(BLOCK_VALID_TRANSACTIONS);
     setDirtyBlockIndex.insert(pindexNew);
 
-    if (pindexNew->pprev == NULL || pindexNew->pprev->nChainTx) {
+    if (pindexNew->pprev == nullptr || pindexNew->pprev->nChainTx) {
         // If pindexNew is the genesis block or all parents are BLOCK_VALID_TRANSACTIONS.
         std::deque<CBlockIndex*> queue;
         queue.push_back(pindexNew);
@@ -2655,7 +2655,7 @@ static bool ReceivedBlockTransactions(const CBlock &block, CValidationState& sta
                 LOCK(cs_nBlockSequenceId);
                 pindex->nSequenceId = nBlockSequenceId++;
             }
-            if (chainActive.Tip() == NULL || !setBlockIndexCandidates.value_comp()(pindex, chainActive.Tip())) {
+            if (chainActive.Tip() == nullptr || !setBlockIndexCandidates.value_comp()(pindex, chainActive.Tip())) {
                 setBlockIndexCandidates.insert(pindex);
             }
             std::pair<std::multimap<CBlockIndex*, CBlockIndex*>::iterator, std::multimap<CBlockIndex*, CBlockIndex*>::iterator> range = mapBlocksUnlinked.equal_range(pindex);
@@ -2875,7 +2875,7 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
     std::vector<unsigned char> ret(32, 0x00);
     if (consensusParams.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout != 0) {
         if (commitpos == -1) {
-            uint256 witnessroot = BlockWitnessMerkleRoot(block, NULL);
+            uint256 witnessroot = BlockWitnessMerkleRoot(block, nullptr);
             CHash256().Write(witnessroot.begin(), 32).Write(ret.data(), 32).Finalize(witnessroot.begin());
             CTxOut out;
             out.nValue = 0;
@@ -2902,7 +2902,7 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
  *  set; UTXO-related validity checks are done in ConnectBlock(). */
 static bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, const CChainParams& params, const CBlockIndex* pindexPrev, int64_t nAdjustedTime)
 {
-    assert(pindexPrev != NULL);
+    assert(pindexPrev != nullptr);
     const int nHeight = pindexPrev->nHeight + 1;
 
     // Check proof of work
@@ -2941,7 +2941,7 @@ static bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationSta
 
 static bool ContextualCheckBlock(const CBlock& block, CValidationState& state, const Consensus::Params& consensusParams, const CBlockIndex* pindexPrev)
 {
-    const int nHeight = pindexPrev == NULL ? 0 : pindexPrev->nHeight + 1;
+    const int nHeight = pindexPrev == nullptr ? 0 : pindexPrev->nHeight + 1;
 
     // Start enforcing BIP113 (Median Time Past) using versionbits logic.
     int nLockTimeFlags = 0;
@@ -3026,7 +3026,7 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
     // Check for duplicate
     uint256 hash = block.GetHash();
     BlockMap::iterator miSelf = mapBlockIndex.find(hash);
-    CBlockIndex *pindex = NULL;
+    CBlockIndex *pindex = nullptr;
     if (hash != chainparams.GetConsensus().hashGenesisBlock) {
 
         if (miSelf != mapBlockIndex.end()) {
@@ -3043,7 +3043,7 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
             return error("%s: Consensus::CheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state));
 
         // Get prev block index
-        CBlockIndex* pindexPrev = NULL;
+        CBlockIndex* pindexPrev = nullptr;
         BlockMap::iterator mi = mapBlockIndex.find(block.hashPrevBlock);
         if (mi == mapBlockIndex.end())
             return state.DoS(10, error("%s: prev block not found", __func__), 0, "prev-blk-not-found");
@@ -3053,7 +3053,7 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
         if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime()))
             return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state));
     }
-    if (pindex == NULL)
+    if (pindex == nullptr)
         pindex = AddToBlockIndex(block);
 
     if (ppindex)
@@ -3070,7 +3070,7 @@ bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidatio
     {
         LOCK(cs_main);
         for (const CBlockHeader& header : headers) {
-            CBlockIndex *pindex = NULL; // Use a temp pindex instead of ppindex to avoid a const_cast
+            CBlockIndex *pindex = nullptr; // Use a temp pindex instead of ppindex to avoid a const_cast
             if (!AcceptBlockHeader(header, state, chainparams, &pindex)) {
                 return false;
             }
@@ -3083,7 +3083,7 @@ bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidatio
     return true;
 }
 
-/** Store block on disk. If dbp is non-NULL, the file is known to already reside on disk */
+/** Store block on disk. If dbp is non-nullptr, the file is known to already reside on disk */
 static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
 {
     const CBlock& block = *pblock;
@@ -3091,7 +3091,7 @@ static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidation
     if (fNewBlock) *fNewBlock = false;
     AssertLockHeld(cs_main);
 
-    CBlockIndex *pindexDummy = NULL;
+    CBlockIndex *pindexDummy = nullptr;
     CBlockIndex *&pindex = ppindex ? *ppindex : pindexDummy;
 
     if (!AcceptBlockHeader(block, state, chainparams, &pindex))
@@ -3144,11 +3144,11 @@ static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidation
     try {
         unsigned int nBlockSize = ::GetSerializeSize(block, SER_DISK, CLIENT_VERSION);
         CDiskBlockPos blockPos;
-        if (dbp != NULL)
+        if (dbp != nullptr)
             blockPos = *dbp;
-        if (!FindBlockPos(state, blockPos, nBlockSize+8, nHeight, block.GetBlockTime(), dbp != NULL))
+        if (!FindBlockPos(state, blockPos, nBlockSize+8, nHeight, block.GetBlockTime(), dbp != nullptr))
             return error("AcceptBlock(): FindBlockPos failed");
-        if (dbp == NULL)
+        if (dbp == nullptr)
             if (!WriteBlockToDisk(block, blockPos, chainparams.MessageStart()))
                 AbortNode(state, "Failed to write block");
         if (!ReceivedBlockTransactions(block, state, pindex, blockPos, chainparams.GetConsensus()))
@@ -3166,7 +3166,7 @@ static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidation
 bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<const CBlock> pblock, bool fForceProcessing, bool *fNewBlock)
 {
     {
-        CBlockIndex *pindex = NULL;
+        CBlockIndex *pindex = nullptr;
         if (fNewBlock) *fNewBlock = false;
         CValidationState state;
         // Ensure that CheckBlock() passes before calling AcceptBlock, as
@@ -3177,7 +3177,7 @@ bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<cons
 
         if (ret) {
             // Store to disk
-            ret = AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, NULL, fNewBlock);
+            ret = AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, nullptr, fNewBlock);
         }
         CheckBlockIndex(chainparams.GetConsensus());
         if (!ret) {
@@ -3281,7 +3281,7 @@ static void FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPr
     assert(fPruneMode && nManualPruneHeight > 0);
 
     LOCK2(cs_main, cs_LastBlockFile);
-    if (chainActive.Tip() == NULL)
+    if (chainActive.Tip() == nullptr)
         return;
 
     // last block to prune is the lesser of (user-specified height, MIN_BLOCKS_TO_KEEP from the tip)
@@ -3323,7 +3323,7 @@ void PruneBlockFilesManual(int nManualPruneHeight)
 static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight)
 {
     LOCK2(cs_main, cs_LastBlockFile);
-    if (chainActive.Tip() == NULL || nPruneTarget == 0) {
+    if (chainActive.Tip() == nullptr || nPruneTarget == 0) {
         return;
     }
     if ((uint64_t)chainActive.Tip()->nHeight <= nPruneAfterHeight) {
@@ -3381,7 +3381,7 @@ bool CheckDiskSpace(uint64_t nAdditionalBytes)
 static FILE* OpenDiskFile(const CDiskBlockPos &pos, const char *prefix, bool fReadOnly)
 {
     if (pos.IsNull())
-        return NULL;
+        return nullptr;
     fs::path path = GetBlockPosFilename(pos, prefix);
     fs::create_directories(path.parent_path());
     FILE* file = fsbridge::fopen(path, "rb+");
@@ -3389,13 +3389,13 @@ static FILE* OpenDiskFile(const CDiskBlockPos &pos, const char *prefix, bool fRe
         file = fsbridge::fopen(path, "wb+");
     if (!file) {
         LogPrintf("Unable to open file %s\n", path.string());
-        return NULL;
+        return nullptr;
     }
     if (pos.nPos) {
         if (fseek(file, pos.nPos, SEEK_SET)) {
             LogPrintf("Unable to seek to position %u of %s\n", pos.nPos, path.string());
             fclose(file);
-            return NULL;
+            return nullptr;
         }
     }
     return file;
@@ -3418,7 +3418,7 @@ fs::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix)
 CBlockIndex * InsertBlockIndex(uint256 hash)
 {
     if (hash.IsNull())
-        return NULL;
+        return nullptr;
 
     // Return existing
     BlockMap::iterator mi = mapBlockIndex.find(hash);
@@ -3470,13 +3470,13 @@ bool static LoadBlockIndexDB(const CChainParams& chainparams)
                 pindex->nChainTx = pindex->nTx;
             }
         }
-        if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS) && (pindex->nChainTx || pindex->pprev == NULL))
+        if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS) && (pindex->nChainTx || pindex->pprev == nullptr))
             setBlockIndexCandidates.insert(pindex);
         if (pindex->nStatus & BLOCK_FAILED_MASK && (!pindexBestInvalid || pindex->nChainWork > pindexBestInvalid->nChainWork))
             pindexBestInvalid = pindex;
         if (pindex->pprev)
             pindex->BuildSkip();
-        if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == NULL || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
+        if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == nullptr || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
             pindexBestHeader = pindex;
     }
 
@@ -3574,7 +3574,7 @@ CVerifyDB::~CVerifyDB()
 bool CVerifyDB::VerifyDB(const CChainParams& chainparams, CCoinsView *coinsview, int nCheckLevel, int nCheckDepth)
 {
     LOCK(cs_main);
-    if (chainActive.Tip() == NULL || chainActive.Tip()->pprev == NULL)
+    if (chainActive.Tip() == nullptr || chainActive.Tip()->pprev == nullptr)
         return true;
 
     // Verify blocks in the best chain
@@ -3584,7 +3584,7 @@ bool CVerifyDB::VerifyDB(const CChainParams& chainparams, CCoinsView *coinsview,
     LogPrintf("Verifying last %i blocks at level %i\n", nCheckDepth, nCheckLevel);
     CCoinsViewCache coins(coinsview);
     CBlockIndex* pindexState = chainActive.Tip();
-    CBlockIndex* pindexFailure = NULL;
+    CBlockIndex* pindexFailure = nullptr;
     int nGoodTransactions = 0;
     CValidationState state;
     int reportDone = 0;
@@ -3777,7 +3777,7 @@ bool RewindBlockIndex(const CChainParams& params)
             // of the blockchain).
             break;
         }
-        if (!DisconnectTip(state, params, NULL)) {
+        if (!DisconnectTip(state, params, nullptr)) {
             return error("RewindBlockIndex: unable to disconnect block at height %i", pindex->nHeight);
         }
         // Occasionally flush state to disk.
@@ -3826,7 +3826,7 @@ bool RewindBlockIndex(const CChainParams& params)
         }
     }
 
-    if (chainActive.Tip() != NULL) {
+    if (chainActive.Tip() != nullptr) {
         // We can't prune block index candidates based on our tip if we have
         // no tip due to chainActive being empty!
         PruneBlockIndexCandidates();
@@ -3851,9 +3851,9 @@ void UnloadBlockIndex()
 {
     LOCK(cs_main);
     setBlockIndexCandidates.clear();
-    chainActive.SetTip(NULL);
-    pindexBestInvalid = NULL;
-    pindexBestHeader = NULL;
+    chainActive.SetTip(nullptr);
+    pindexBestInvalid = nullptr;
+    pindexBestHeader = nullptr;
     mempool.clear();
     mapBlocksUnlinked.clear();
     vinfoBlockFile.clear();
@@ -3989,7 +3989,7 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                 if (mapBlockIndex.count(hash) == 0 || (mapBlockIndex[hash]->nStatus & BLOCK_HAVE_DATA) == 0) {
                     LOCK(cs_main);
                     CValidationState state;
-                    if (AcceptBlock(pblock, state, chainparams, NULL, true, dbp, NULL))
+                    if (AcceptBlock(pblock, state, chainparams, nullptr, true, dbp, nullptr))
                         nLoaded++;
                     if (state.IsError())
                         break;
@@ -4023,7 +4023,7 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                                     head.ToString());
                             LOCK(cs_main);
                             CValidationState dummy;
-                            if (AcceptBlock(pblockrecursive, dummy, chainparams, NULL, true, &it->second, NULL))
+                            if (AcceptBlock(pblockrecursive, dummy, chainparams, nullptr, true, &it->second, nullptr))
                             {
                                 nLoaded++;
                                 queue.push_back(pblockrecursive->GetHash());
@@ -4070,35 +4070,35 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
 
     assert(forward.size() == mapBlockIndex.size());
 
-    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(NULL);
+    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(nullptr);
     CBlockIndex *pindex = rangeGenesis.first->second;
     rangeGenesis.first++;
-    assert(rangeGenesis.first == rangeGenesis.second); // There is only one index entry with parent NULL.
+    assert(rangeGenesis.first == rangeGenesis.second); // There is only one index entry with parent nullptr.
 
     // Iterate over the entire block tree, using depth-first search.
     // Along the way, remember whether there are blocks on the path from genesis
     // block being explored which are the first to have certain properties.
     size_t nNodes = 0;
     int nHeight = 0;
-    CBlockIndex* pindexFirstInvalid = NULL; // Oldest ancestor of pindex which is invalid.
-    CBlockIndex* pindexFirstMissing = NULL; // Oldest ancestor of pindex which does not have BLOCK_HAVE_DATA.
-    CBlockIndex* pindexFirstNeverProcessed = NULL; // Oldest ancestor of pindex for which nTx == 0.
-    CBlockIndex* pindexFirstNotTreeValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_TREE (regardless of being valid or not).
-    CBlockIndex* pindexFirstNotTransactionsValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_TRANSACTIONS (regardless of being valid or not).
-    CBlockIndex* pindexFirstNotChainValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_CHAIN (regardless of being valid or not).
-    CBlockIndex* pindexFirstNotScriptsValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_SCRIPTS (regardless of being valid or not).
-    while (pindex != NULL) {
+    CBlockIndex* pindexFirstInvalid = nullptr; // Oldest ancestor of pindex which is invalid.
+    CBlockIndex* pindexFirstMissing = nullptr; // Oldest ancestor of pindex which does not have BLOCK_HAVE_DATA.
+    CBlockIndex* pindexFirstNeverProcessed = nullptr; // Oldest ancestor of pindex for which nTx == 0.
+    CBlockIndex* pindexFirstNotTreeValid = nullptr; // Oldest ancestor of pindex which does not have BLOCK_VALID_TREE (regardless of being valid or not).
+    CBlockIndex* pindexFirstNotTransactionsValid = nullptr; // Oldest ancestor of pindex which does not have BLOCK_VALID_TRANSACTIONS (regardless of being valid or not).
+    CBlockIndex* pindexFirstNotChainValid = nullptr; // Oldest ancestor of pindex which does not have BLOCK_VALID_CHAIN (regardless of being valid or not).
+    CBlockIndex* pindexFirstNotScriptsValid = nullptr; // Oldest ancestor of pindex which does not have BLOCK_VALID_SCRIPTS (regardless of being valid or not).
+    while (pindex != nullptr) {
         nNodes++;
-        if (pindexFirstInvalid == NULL && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;
-        if (pindexFirstMissing == NULL && !(pindex->nStatus & BLOCK_HAVE_DATA)) pindexFirstMissing = pindex;
-        if (pindexFirstNeverProcessed == NULL && pindex->nTx == 0) pindexFirstNeverProcessed = pindex;
-        if (pindex->pprev != NULL && pindexFirstNotTreeValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TREE) pindexFirstNotTreeValid = pindex;
-        if (pindex->pprev != NULL && pindexFirstNotTransactionsValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TRANSACTIONS) pindexFirstNotTransactionsValid = pindex;
-        if (pindex->pprev != NULL && pindexFirstNotChainValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_CHAIN) pindexFirstNotChainValid = pindex;
-        if (pindex->pprev != NULL && pindexFirstNotScriptsValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_SCRIPTS) pindexFirstNotScriptsValid = pindex;
+        if (pindexFirstInvalid == nullptr && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;
+        if (pindexFirstMissing == nullptr && !(pindex->nStatus & BLOCK_HAVE_DATA)) pindexFirstMissing = pindex;
+        if (pindexFirstNeverProcessed == nullptr && pindex->nTx == 0) pindexFirstNeverProcessed = pindex;
+        if (pindex->pprev != nullptr && pindexFirstNotTreeValid == nullptr && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TREE) pindexFirstNotTreeValid = pindex;
+        if (pindex->pprev != nullptr && pindexFirstNotTransactionsValid == nullptr && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TRANSACTIONS) pindexFirstNotTransactionsValid = pindex;
+        if (pindex->pprev != nullptr && pindexFirstNotChainValid == nullptr && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_CHAIN) pindexFirstNotChainValid = pindex;
+        if (pindex->pprev != nullptr && pindexFirstNotScriptsValid == nullptr && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_SCRIPTS) pindexFirstNotScriptsValid = pindex;
 
         // Begin: actual consistency checks.
-        if (pindex->pprev == NULL) {
+        if (pindex->pprev == nullptr) {
             // Genesis block checks.
             assert(pindex->GetBlockHash() == consensusParams.hashGenesisBlock); // Genesis block's hash must match.
             assert(pindex == chainActive.Genesis()); // The current active chain's genesis block must be this block.
@@ -4117,26 +4117,26 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
         if (pindex->nStatus & BLOCK_HAVE_UNDO) assert(pindex->nStatus & BLOCK_HAVE_DATA);
         assert(((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_TRANSACTIONS) == (pindex->nTx > 0)); // This is pruning-independent.
         // All parents having had data (at some point) is equivalent to all parents being VALID_TRANSACTIONS, which is equivalent to nChainTx being set.
-        assert((pindexFirstNeverProcessed != NULL) == (pindex->nChainTx == 0)); // nChainTx != 0 is used to signal that all parent blocks have been processed (but may have been pruned).
-        assert((pindexFirstNotTransactionsValid != NULL) == (pindex->nChainTx == 0));
+        assert((pindexFirstNeverProcessed != nullptr) == (pindex->nChainTx == 0)); // nChainTx != 0 is used to signal that all parent blocks have been processed (but may have been pruned).
+        assert((pindexFirstNotTransactionsValid != nullptr) == (pindex->nChainTx == 0));
         assert(pindex->nHeight == nHeight); // nHeight must be consistent.
-        assert(pindex->pprev == NULL || pindex->nChainWork >= pindex->pprev->nChainWork); // For every block except the genesis block, the chainwork must be larger than the parent's.
+        assert(pindex->pprev == nullptr || pindex->nChainWork >= pindex->pprev->nChainWork); // For every block except the genesis block, the chainwork must be larger than the parent's.
         assert(nHeight < 2 || (pindex->pskip && (pindex->pskip->nHeight < nHeight))); // The pskip pointer must point back for all but the first 2 blocks.
-        assert(pindexFirstNotTreeValid == NULL); // All mapBlockIndex entries must at least be TREE valid
-        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_TREE) assert(pindexFirstNotTreeValid == NULL); // TREE valid implies all parents are TREE valid
-        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_CHAIN) assert(pindexFirstNotChainValid == NULL); // CHAIN valid implies all parents are CHAIN valid
-        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_SCRIPTS) assert(pindexFirstNotScriptsValid == NULL); // SCRIPTS valid implies all parents are SCRIPTS valid
-        if (pindexFirstInvalid == NULL) {
+        assert(pindexFirstNotTreeValid == nullptr); // All mapBlockIndex entries must at least be TREE valid
+        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_TREE) assert(pindexFirstNotTreeValid == nullptr); // TREE valid implies all parents are TREE valid
+        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_CHAIN) assert(pindexFirstNotChainValid == nullptr); // CHAIN valid implies all parents are CHAIN valid
+        if ((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_SCRIPTS) assert(pindexFirstNotScriptsValid == nullptr); // SCRIPTS valid implies all parents are SCRIPTS valid
+        if (pindexFirstInvalid == nullptr) {
             // Checks for not-invalid blocks.
             assert((pindex->nStatus & BLOCK_FAILED_MASK) == 0); // The failed mask cannot be set for blocks without invalid parents.
         }
-        if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && pindexFirstNeverProcessed == NULL) {
-            if (pindexFirstInvalid == NULL) {
+        if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && pindexFirstNeverProcessed == nullptr) {
+            if (pindexFirstInvalid == nullptr) {
                 // If this block sorts at least as good as the current tip and
                 // is valid and we have all data for its parents, it must be in
                 // setBlockIndexCandidates.  chainActive.Tip() must also be there
                 // even if some data has been pruned.
-                if (pindexFirstMissing == NULL || pindex == chainActive.Tip()) {
+                if (pindexFirstMissing == nullptr || pindex == chainActive.Tip()) {
                     assert(setBlockIndexCandidates.count(pindex));
                 }
                 // If some parent is missing, then it could be that this block was in
@@ -4157,13 +4157,13 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
             }
             rangeUnlinked.first++;
         }
-        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed != NULL && pindexFirstInvalid == NULL) {
+        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed != nullptr && pindexFirstInvalid == nullptr) {
             // If this block has block data available, some parent was never received, and has no invalid parents, it must be in mapBlocksUnlinked.
             assert(foundInUnlinked);
         }
         if (!(pindex->nStatus & BLOCK_HAVE_DATA)) assert(!foundInUnlinked); // Can't be in mapBlocksUnlinked if we don't HAVE_DATA
-        if (pindexFirstMissing == NULL) assert(!foundInUnlinked); // We aren't missing data for any parent -- cannot be in mapBlocksUnlinked.
-        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed == NULL && pindexFirstMissing != NULL) {
+        if (pindexFirstMissing == nullptr) assert(!foundInUnlinked); // We aren't missing data for any parent -- cannot be in mapBlocksUnlinked.
+        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed == nullptr && pindexFirstMissing != nullptr) {
             // We HAVE_DATA for this block, have received data for all parents at some point, but we're currently missing data for some parent.
             assert(fHavePruned); // We must have pruned.
             // This block may have entered mapBlocksUnlinked if:
@@ -4175,7 +4175,7 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
             // So if this block is itself better than chainActive.Tip() and it wasn't in
             // setBlockIndexCandidates, then it must be in mapBlocksUnlinked.
             if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && setBlockIndexCandidates.count(pindex) == 0) {
-                if (pindexFirstInvalid == NULL) {
+                if (pindexFirstInvalid == nullptr) {
                     assert(foundInUnlinked);
                 }
             }
@@ -4196,13 +4196,13 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
         while (pindex) {
             // We are going to either move to a parent or a sibling of pindex.
             // If pindex was the first with a certain property, unset the corresponding variable.
-            if (pindex == pindexFirstInvalid) pindexFirstInvalid = NULL;
-            if (pindex == pindexFirstMissing) pindexFirstMissing = NULL;
-            if (pindex == pindexFirstNeverProcessed) pindexFirstNeverProcessed = NULL;
-            if (pindex == pindexFirstNotTreeValid) pindexFirstNotTreeValid = NULL;
-            if (pindex == pindexFirstNotTransactionsValid) pindexFirstNotTransactionsValid = NULL;
-            if (pindex == pindexFirstNotChainValid) pindexFirstNotChainValid = NULL;
-            if (pindex == pindexFirstNotScriptsValid) pindexFirstNotScriptsValid = NULL;
+            if (pindex == pindexFirstInvalid) pindexFirstInvalid = nullptr;
+            if (pindex == pindexFirstMissing) pindexFirstMissing = nullptr;
+            if (pindex == pindexFirstNeverProcessed) pindexFirstNeverProcessed = nullptr;
+            if (pindex == pindexFirstNotTreeValid) pindexFirstNotTreeValid = nullptr;
+            if (pindex == pindexFirstNotTransactionsValid) pindexFirstNotTransactionsValid = nullptr;
+            if (pindex == pindexFirstNotChainValid) pindexFirstNotChainValid = nullptr;
+            if (pindex == pindexFirstNotScriptsValid) pindexFirstNotScriptsValid = nullptr;
             // Find our parent.
             CBlockIndex* pindexPar = pindex->pprev;
             // Find which child we just visited.
@@ -4299,7 +4299,7 @@ bool LoadMempool(void)
             CValidationState state;
             if (nTime + nExpiryTimeout > nNow) {
                 LOCK(cs_main);
-                AcceptToMemoryPoolWithTime(chainparams, mempool, state, tx, true, NULL, nTime, NULL, false, 0);
+                AcceptToMemoryPoolWithTime(chainparams, mempool, state, tx, true, nullptr, nTime, nullptr, false, 0);
                 if (state.IsValid()) {
                     ++count;
                 } else {
@@ -4375,10 +4375,10 @@ void DumpMempool(void)
 
 //! Guess how far we are in the verification process at the given block index
 double GuessVerificationProgress(const ChainTxData& data, CBlockIndex *pindex) {
-    if (pindex == NULL)
+    if (pindex == nullptr)
         return 0.0;
 
-    int64_t nNow = time(NULL);
+    int64_t nNow = time(nullptr);
 
     double fTxTotal;
 
diff --git a/src/validation.h b/src/validation.h
index b44b72d2d4..edb8eab894 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -246,7 +246,7 @@ bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<cons
  * @param[in]  chainparams The params for the chain we want to connect to
  * @param[out] ppindex If set, the pointer will be set to point to the last new block index object for the given headers
  */
-bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, CValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex=NULL);
+bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, CValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex=nullptr);
 
 /** Check whether enough disk space is available for an incoming block */
 bool CheckDiskSpace(uint64_t nAdditionalBytes = 0);
@@ -255,7 +255,7 @@ FILE* OpenBlockFile(const CDiskBlockPos &pos, bool fReadOnly = false);
 /** Translation to a filesystem path */
 fs::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix);
 /** Import blocks from an external file */
-bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp = NULL);
+bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp = nullptr);
 /** Ensures we have a genesis block in the block tree, possibly writing one to disk. */
 bool LoadGenesisBlock(const CChainParams& chainparams);
 /** Load the block tree and coins database from disk,
@@ -300,7 +300,7 @@ void PruneBlockFilesManual(int nManualPruneHeight);
 /** (try to) add transaction to memory pool
  * plTxnReplaced will be appended to with all transactions replaced from mempool **/
 bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransactionRef &tx, bool fLimitFree,
-                        bool* pfMissingInputs, std::list<CTransactionRef>* plTxnReplaced = NULL,
+                        bool* pfMissingInputs, std::list<CTransactionRef>* plTxnReplaced = nullptr,
                         bool fOverrideMempoolLimit=false, const CAmount nAbsurdFee=0);
 
 /** Convert CValidationState to a human-readable message for logging */
@@ -346,7 +346,7 @@ bool TestLockPointValidity(const LockPoints* lp);
  *
  * See consensus/consensus.h for flag definitions.
  */
-bool CheckSequenceLocks(const CTransaction &tx, int flags, LockPoints* lp = NULL, bool useExistingLockPoints = false);
+bool CheckSequenceLocks(const CTransaction &tx, int flags, LockPoints* lp = nullptr, bool useExistingLockPoints = false);
 
 /**
  * Closure representing one script verification
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
index 8047e17aa8..04a692d826 100644
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -28,14 +28,14 @@ ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex*
     int64_t nTimeTimeout = EndTime(params);
 
     // A block's state is always the same as that of the first of its period, so it is computed based on a pindexPrev whose height equals a multiple of nPeriod - 1.
-    if (pindexPrev != NULL) {
+    if (pindexPrev != nullptr) {
         pindexPrev = pindexPrev->GetAncestor(pindexPrev->nHeight - ((pindexPrev->nHeight + 1) % nPeriod));
     }
 
     // Walk backwards in steps of nPeriod to find a pindexPrev whose information is known
     std::vector<const CBlockIndex*> vToCompute;
     while (cache.count(pindexPrev) == 0) {
-        if (pindexPrev == NULL) {
+        if (pindexPrev == nullptr) {
             // The genesis block is by definition defined.
             cache[pindexPrev] = THRESHOLD_DEFINED;
             break;
@@ -112,7 +112,7 @@ BIP9Stats AbstractThresholdConditionChecker::GetStateStatisticsFor(const CBlockI
     stats.period = Period(params);
     stats.threshold = Threshold(params);
 
-    if (pindex == NULL)
+    if (pindex == nullptr)
         return stats;
 
     // Find beginning of period
@@ -150,12 +150,12 @@ int AbstractThresholdConditionChecker::GetStateSinceHeightFor(const CBlockIndex*
     // right now pindexPrev points to the block prior to the block that we are computing for, thus:
     // if we are computing for the last block of a period, then pindexPrev points to the second to last block of the period, and
     // if we are computing for the first block of a period, then pindexPrev points to the last block of the previous period.
-    // The parent of the genesis block is represented by NULL.
+    // The parent of the genesis block is represented by nullptr.
     pindexPrev = pindexPrev->GetAncestor(pindexPrev->nHeight - ((pindexPrev->nHeight + 1) % nPeriod));
 
     const CBlockIndex* previousPeriodParent = pindexPrev->GetAncestor(pindexPrev->nHeight - nPeriod);
 
-    while (previousPeriodParent != NULL && GetStateFor(previousPeriodParent, params, cache) == initialState) {
+    while (previousPeriodParent != nullptr && GetStateFor(previousPeriodParent, params, cache) == initialState) {
         pindexPrev = previousPeriodParent;
         previousPeriodParent = pindexPrev->GetAncestor(pindexPrev->nHeight - nPeriod);
     }
diff --git a/src/versionbits.h b/src/versionbits.h
index f4dfb71515..71dc0c8500 100644
--- a/src/versionbits.h
+++ b/src/versionbits.h
@@ -27,7 +27,7 @@ enum ThresholdState {
 
 // A map that gives the state for blocks whose height is a multiple of Period().
 // The map is indexed by the block's parent, however, so all keys in the map
-// will either be NULL or a block with (height + 1) % Period() == 0.
+// will either be nullptr or a block with (height + 1) % Period() == 0.
 typedef std::map<const CBlockIndex*, ThresholdState> ThresholdConditionCache;
 
 struct VBDeploymentInfo {
diff --git a/src/wallet/db.cpp b/src/wallet/db.cpp
index da2d180756..4c06c2c1f9 100644
--- a/src/wallet/db.cpp
+++ b/src/wallet/db.cpp
@@ -47,7 +47,7 @@ void CDBEnv::Reset()
     fMockDb = false;
 }
 
-CDBEnv::CDBEnv() : dbenv(NULL)
+CDBEnv::CDBEnv() : dbenv(nullptr)
 {
     Reset();
 }
@@ -56,7 +56,7 @@ CDBEnv::~CDBEnv()
 {
     EnvShutdown();
     delete dbenv;
-    dbenv = NULL;
+    dbenv = nullptr;
 }
 
 void CDBEnv::Close()
@@ -125,7 +125,7 @@ void CDBEnv::MakeMock()
     dbenv->set_lk_max_objects(10000);
     dbenv->set_flags(DB_AUTO_COMMIT, 1);
     dbenv->log_set_config(DB_LOG_IN_MEMORY, 1);
-    int ret = dbenv->open(NULL,
+    int ret = dbenv->open(nullptr,
                          DB_CREATE |
                              DB_INIT_LOCK |
                              DB_INIT_LOG |
@@ -147,10 +147,10 @@ CDBEnv::VerifyResult CDBEnv::Verify(const std::string& strFile, recoverFunc_type
     assert(mapFileUseCount.count(strFile) == 0);
 
     Db db(dbenv, 0);
-    int result = db.verify(strFile.c_str(), NULL, NULL, 0);
+    int result = db.verify(strFile.c_str(), nullptr, nullptr, 0);
     if (result == 0)
         return VERIFY_OK;
-    else if (recoverFunc == NULL)
+    else if (recoverFunc == nullptr)
         return RECOVER_FAIL;
 
     // Try to recover:
@@ -170,7 +170,7 @@ bool CDB::Recover(const std::string& filename, void *callbackDataIn, bool (*reco
     int64_t now = GetTime();
     newFilename = strprintf("%s.%d.bak", filename, now);
 
-    int result = bitdb.dbenv->dbrename(NULL, filename.c_str(), NULL,
+    int result = bitdb.dbenv->dbrename(nullptr, filename.c_str(), nullptr,
                                        newFilename.c_str(), DB_AUTO_COMMIT);
     if (result == 0)
         LogPrintf("Renamed %s to %s\n", filename, newFilename);
@@ -190,7 +190,7 @@ bool CDB::Recover(const std::string& filename, void *callbackDataIn, bool (*reco
     LogPrintf("Salvage(aggressive) found %u records\n", salvagedData.size());
 
     std::unique_ptr<Db> pdbCopy(new Db(bitdb.dbenv, 0));
-    int ret = pdbCopy->open(NULL,               // Txn pointer
+    int ret = pdbCopy->open(nullptr,               // Txn pointer
                             filename.c_str(),   // Filename
                             "main",             // Logical db name
                             DB_BTREE,           // Database type
@@ -299,7 +299,7 @@ bool CDBEnv::Salvage(const std::string& strFile, bool fAggressive, std::vector<C
     std::stringstream strDump;
 
     Db db(dbenv, 0);
-    int result = db.verify(strFile.c_str(), NULL, &strDump, flags);
+    int result = db.verify(strFile.c_str(), nullptr, &strDump, flags);
     if (result == DB_VERIFY_BAD) {
         LogPrintf("CDBEnv::Salvage: Database salvage found errors, all data may not be recoverable.\n");
         if (!fAggressive) {
@@ -357,7 +357,7 @@ void CDBEnv::CheckpointLSN(const std::string& strFile)
 }
 
 
-CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb(NULL), activeTxn(NULL)
+CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb(nullptr), activeTxn(nullptr)
 {
     fReadOnly = (!strchr(pszMode, '+') && !strchr(pszMode, 'w'));
     fFlushOnClose = fFlushOnCloseIn;
@@ -367,7 +367,7 @@ CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb
     }
     const std::string &strFilename = dbw.strFile;
 
-    bool fCreate = strchr(pszMode, 'c') != NULL;
+    bool fCreate = strchr(pszMode, 'c') != nullptr;
     unsigned int nFlags = DB_THREAD;
     if (fCreate)
         nFlags |= DB_CREATE;
@@ -380,7 +380,7 @@ CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb
         strFile = strFilename;
         ++env->mapFileUseCount[strFile];
         pdb = env->mapDb[strFile];
-        if (pdb == NULL) {
+        if (pdb == nullptr) {
             int ret;
             pdb = new Db(env->dbenv, 0);
 
@@ -392,8 +392,8 @@ CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb
                     throw std::runtime_error(strprintf("CDB: Failed to configure for no temp file backing for database %s", strFile));
             }
 
-            ret = pdb->open(NULL,                               // Txn pointer
-                            fMockDb ? NULL : strFile.c_str(),   // Filename
+            ret = pdb->open(nullptr,                               // Txn pointer
+                            fMockDb ? nullptr : strFile.c_str(),   // Filename
                             fMockDb ? strFile.c_str() : "main", // Logical db name
                             DB_BTREE,                           // Database type
                             nFlags,                             // Flags
@@ -401,7 +401,7 @@ CDB::CDB(CWalletDBWrapper& dbw, const char* pszMode, bool fFlushOnCloseIn) : pdb
 
             if (ret != 0) {
                 delete pdb;
-                pdb = NULL;
+                pdb = nullptr;
                 --env->mapFileUseCount[strFile];
                 strFile = "";
                 throw std::runtime_error(strprintf("CDB: Error %d, can't open database %s", ret, strFilename));
@@ -443,8 +443,8 @@ void CDB::Close()
         return;
     if (activeTxn)
         activeTxn->abort();
-    activeTxn = NULL;
-    pdb = NULL;
+    activeTxn = nullptr;
+    pdb = nullptr;
 
     if (fFlushOnClose)
         Flush();
@@ -459,12 +459,12 @@ void CDBEnv::CloseDb(const std::string& strFile)
 {
     {
         LOCK(cs_db);
-        if (mapDb[strFile] != NULL) {
+        if (mapDb[strFile] != nullptr) {
             // Close the database handle
             Db* pdb = mapDb[strFile];
             pdb->close(0);
             delete pdb;
-            mapDb[strFile] = NULL;
+            mapDb[strFile] = nullptr;
         }
     }
 }
@@ -492,7 +492,7 @@ bool CDB::Rewrite(CWalletDBWrapper& dbw, const char* pszSkip)
                     CDB db(dbw, "r");
                     Db* pdbCopy = new Db(env->dbenv, 0);
 
-                    int ret = pdbCopy->open(NULL,               // Txn pointer
+                    int ret = pdbCopy->open(nullptr,               // Txn pointer
                                             strFileRes.c_str(), // Filename
                                             "main",             // Logical db name
                                             DB_BTREE,           // Database type
@@ -527,7 +527,7 @@ bool CDB::Rewrite(CWalletDBWrapper& dbw, const char* pszSkip)
                             }
                             Dbt datKey(ssKey.data(), ssKey.size());
                             Dbt datValue(ssValue.data(), ssValue.size());
-                            int ret2 = pdbCopy->put(NULL, &datKey, &datValue, DB_NOOVERWRITE);
+                            int ret2 = pdbCopy->put(nullptr, &datKey, &datValue, DB_NOOVERWRITE);
                             if (ret2 > 0)
                                 fSuccess = false;
                         }
@@ -541,10 +541,10 @@ bool CDB::Rewrite(CWalletDBWrapper& dbw, const char* pszSkip)
                 }
                 if (fSuccess) {
                     Db dbA(env->dbenv, 0);
-                    if (dbA.remove(strFile.c_str(), NULL, 0))
+                    if (dbA.remove(strFile.c_str(), nullptr, 0))
                         fSuccess = false;
                     Db dbB(env->dbenv, 0);
-                    if (dbB.rename(strFileRes.c_str(), NULL, strFile.c_str(), 0))
+                    if (dbB.rename(strFileRes.c_str(), nullptr, strFile.c_str(), 0))
                         fSuccess = false;
                 }
                 if (!fSuccess)
diff --git a/src/wallet/db.h b/src/wallet/db.h
index 4f3ad0c42d..3614e34fbb 100644
--- a/src/wallet/db.h
+++ b/src/wallet/db.h
@@ -77,10 +77,10 @@ public:
 
     DbTxn* TxnBegin(int flags = DB_TXN_WRITE_NOSYNC)
     {
-        DbTxn* ptxn = NULL;
-        int ret = dbenv->txn_begin(NULL, &ptxn, flags);
+        DbTxn* ptxn = nullptr;
+        int ret = dbenv->txn_begin(nullptr, &ptxn, flags);
         if (!ptxn || ret != 0)
-            return NULL;
+            return nullptr;
         return ptxn;
     }
 };
@@ -191,7 +191,7 @@ public:
         int ret = pdb->get(activeTxn, &datKey, &datValue, 0);
         memory_cleanse(datKey.get_data(), datKey.get_size());
         bool success = false;
-        if (datValue.get_data() != NULL) {
+        if (datValue.get_data() != nullptr) {
             // Unserialize value
             try {
                 CDataStream ssValue((char*)datValue.get_data(), (char*)datValue.get_data() + datValue.get_size(), SER_DISK, CLIENT_VERSION);
@@ -282,11 +282,11 @@ public:
     Dbc* GetCursor()
     {
         if (!pdb)
-            return NULL;
-        Dbc* pcursor = NULL;
-        int ret = pdb->cursor(NULL, &pcursor, 0);
+            return nullptr;
+        Dbc* pcursor = nullptr;
+        int ret = pdb->cursor(nullptr, &pcursor, 0);
         if (ret != 0)
-            return NULL;
+            return nullptr;
         return pcursor;
     }
 
@@ -306,7 +306,7 @@ public:
         int ret = pcursor->get(&datKey, &datValue, fFlags);
         if (ret != 0)
             return ret;
-        else if (datKey.get_data() == NULL || datValue.get_data() == NULL)
+        else if (datKey.get_data() == nullptr || datValue.get_data() == nullptr)
             return 99999;
 
         // Convert to streams
@@ -342,7 +342,7 @@ public:
         if (!pdb || !activeTxn)
             return false;
         int ret = activeTxn->commit(0);
-        activeTxn = NULL;
+        activeTxn = nullptr;
         return (ret == 0);
     }
 
@@ -351,7 +351,7 @@ public:
         if (!pdb || !activeTxn)
             return false;
         int ret = activeTxn->abort();
-        activeTxn = NULL;
+        activeTxn = nullptr;
         return (ret == 0);
     }
 
@@ -366,7 +366,7 @@ public:
         return Write(std::string("version"), nVersion);
     }
 
-    bool static Rewrite(CWalletDBWrapper& dbw, const char* pszSkip = NULL);
+    bool static Rewrite(CWalletDBWrapper& dbw, const char* pszSkip = nullptr);
 };
 
 #endif // BITCOIN_WALLET_DB_H
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 057379d8da..8ef33f248b 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -1818,8 +1818,8 @@ UniValue listsinceblock(const JSONRPCRequest& request)
 
     LOCK2(cs_main, pwallet->cs_wallet);
 
-    const CBlockIndex* pindex = NULL;    // Block index of the specified block or the common ancestor, if the block provided was in a deactivated chain.
-    const CBlockIndex* paltindex = NULL; // Block index of the specified block, even if it's in a deactivated chain.
+    const CBlockIndex* pindex = nullptr;    // Block index of the specified block or the common ancestor, if the block provided was in a deactivated chain.
+    const CBlockIndex* paltindex = nullptr; // Block index of the specified block, even if it's in a deactivated chain.
     int target_confirms = 1;
     isminefilter filter = ISMINE_SPENDABLE;
 
@@ -2725,10 +2725,10 @@ UniValue listunspent(const JSONRPCRequest& request)
 
     UniValue results(UniValue::VARR);
     std::vector<COutput> vecOutputs;
-    assert(pwallet != NULL);
+    assert(pwallet != nullptr);
     LOCK2(cs_main, pwallet->cs_wallet);
 
-    pwallet->AvailableCoins(vecOutputs, !include_unsafe, NULL, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount, nMinDepth, nMaxDepth);
+    pwallet->AvailableCoins(vecOutputs, !include_unsafe, nullptr, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount, nMinDepth, nMaxDepth);
     for (const COutput& out : vecOutputs) {
         CTxDestination address;
         const CScript& scriptPubKey = out.tx->tx->vout[out.i].scriptPubKey;
diff --git a/src/wallet/rpcwallet.h b/src/wallet/rpcwallet.h
index 31e2f6a699..db0808b93b 100644
--- a/src/wallet/rpcwallet.h
+++ b/src/wallet/rpcwallet.h
@@ -14,7 +14,7 @@ void RegisterWalletRPCCommands(CRPCTable &t);
  * Figures out what wallet, if any, to use for a JSONRPCRequest.
  *
  * @param[in] request JSONRPCRequest that wishes to access a wallet
- * @return NULL if no wallet should be used, or a pointer to the CWallet
+ * @return nullptr if no wallet should be used, or a pointer to the CWallet
  */
 CWallet *GetWalletForJSONRPCRequest(const JSONRPCRequest& request);
 
diff --git a/src/wallet/test/crypto_tests.cpp b/src/wallet/test/crypto_tests.cpp
index 744063624c..98d957b322 100644
--- a/src/wallet/test/crypto_tests.cpp
+++ b/src/wallet/test/crypto_tests.cpp
@@ -48,7 +48,7 @@ bool OldEncrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned char>
     bool fOk = true;
 
     EVP_CIPHER_CTX_init(ctx);
-    if (fOk) fOk = EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, chKey, chIV) != 0;
+    if (fOk) fOk = EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), nullptr, chKey, chIV) != 0;
     if (fOk) fOk = EVP_EncryptUpdate(ctx, &vchCiphertext[0], &nCLen, &vchPlaintext[0], nLen) != 0;
     if (fOk) fOk = EVP_EncryptFinal_ex(ctx, (&vchCiphertext[0]) + nCLen, &nFLen) != 0;
     EVP_CIPHER_CTX_cleanup(ctx);
@@ -76,7 +76,7 @@ bool OldDecrypt(const std::vector<unsigned char>& vchCiphertext, CKeyingMaterial
     bool fOk = true;
 
     EVP_CIPHER_CTX_init(ctx);
-    if (fOk) fOk = EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, chKey, chIV) != 0;
+    if (fOk) fOk = EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), nullptr, chKey, chIV) != 0;
     if (fOk) fOk = EVP_DecryptUpdate(ctx, &vchPlaintext[0], &nPLen, &vchCiphertext[0], nLen) != 0;
     if (fOk) fOk = EVP_DecryptFinal_ex(ctx, (&vchPlaintext[0]) + nPLen, &nFLen) != 0;
     EVP_CIPHER_CTX_cleanup(ctx);
diff --git a/src/wallet/test/wallet_test_fixture.cpp b/src/wallet/test/wallet_test_fixture.cpp
index 922fcc8e89..e2f48c45ab 100644
--- a/src/wallet/test/wallet_test_fixture.cpp
+++ b/src/wallet/test/wallet_test_fixture.cpp
@@ -28,7 +28,7 @@ WalletTestingSetup::~WalletTestingSetup()
 {
     UnregisterValidationInterface(pwalletMain);
     delete pwalletMain;
-    pwalletMain = NULL;
+    pwalletMain = nullptr;
 
     bitdb.Flush(true);
     bitdb.Reset();
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 8b03421691..c152b9d697 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -118,7 +118,7 @@ const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
     LOCK(cs_wallet);
     std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(hash);
     if (it == mapWallet.end())
-        return NULL;
+        return nullptr;
     return &(it->second);
 }
 
@@ -215,10 +215,10 @@ bool CWallet::AddKeyPubKeyWithDB(CWalletDB &walletdb, const CKey& secret, const
         pwalletdbEncryption = &walletdb;
     }
     if (!CCryptoKeyStore::AddKeyPubKey(secret, pubkey)) {
-        if (needsDB) pwalletdbEncryption = NULL;
+        if (needsDB) pwalletdbEncryption = nullptr;
         return false;
     }
-    if (needsDB) pwalletdbEncryption = NULL;
+    if (needsDB) pwalletdbEncryption = nullptr;
 
     // check if we need to remove from watch-only
     CScript script;
@@ -558,7 +558,7 @@ void CWallet::SyncMetaData(std::pair<TxSpends::iterator, TxSpends::iterator> ran
     // So: find smallest nOrderPos:
 
     int nMinOrderPos = std::numeric_limits<int>::max();
-    const CWalletTx* copyFrom = NULL;
+    const CWalletTx* copyFrom = nullptr;
     for (TxSpends::iterator it = range.first; it != range.second; ++it)
     {
         const uint256& hash = it->second;
@@ -673,7 +673,7 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
         pwalletdbEncryption = new CWalletDB(*dbw);
         if (!pwalletdbEncryption->TxnBegin()) {
             delete pwalletdbEncryption;
-            pwalletdbEncryption = NULL;
+            pwalletdbEncryption = nullptr;
             return false;
         }
         pwalletdbEncryption->WriteMasterKey(nMasterKeyMaxID, kMasterKey);
@@ -698,7 +698,7 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
         }
 
         delete pwalletdbEncryption;
-        pwalletdbEncryption = NULL;
+        pwalletdbEncryption = nullptr;
 
         Lock();
         Unlock(strWalletPassphrase);
@@ -1021,7 +1021,7 @@ bool CWallet::LoadToWallet(const CWalletTx& wtxIn)
 /**
  * Add a transaction to the wallet, or update it.  pIndex and posInBlock should
  * be set when the transaction was known to be included in a block.  When
- * pIndex == NULL, then wallet state is not updated in AddToWallet, but
+ * pIndex == nullptr, then wallet state is not updated in AddToWallet, but
  * notifications happen and cached balances are marked dirty.
  *
  * If fUpdate is true, existing transactions will be updated.
@@ -1037,7 +1037,7 @@ bool CWallet::AddToWalletIfInvolvingMe(const CTransactionRef& ptx, const CBlockI
     {
         AssertLockHeld(cs_wallet);
 
-        if (pIndex != NULL) {
+        if (pIndex != nullptr) {
             for (const CTxIn& txin : tx.vin) {
                 std::pair<TxSpends::const_iterator, TxSpends::const_iterator> range = mapTxSpends.equal_range(txin.prevout);
                 while (range.first != range.second) {
@@ -1081,7 +1081,7 @@ bool CWallet::AddToWalletIfInvolvingMe(const CTransactionRef& ptx, const CBlockI
             CWalletTx wtx(this, ptx);
 
             // Get merkle branch if transaction was found in a block
-            if (pIndex != NULL)
+            if (pIndex != nullptr)
                 wtx.SetMerkleBranch(pIndex, posInBlock);
 
             return AddToWallet(wtx, false);
@@ -1705,7 +1705,7 @@ bool CWalletTx::RelayWalletTransaction(CConnman* connman)
 std::set<uint256> CWalletTx::GetConflicts() const
 {
     std::set<uint256> result;
-    if (pwallet != NULL)
+    if (pwallet != nullptr)
     {
         uint256 myHash = GetHash();
         result = pwallet->GetConflicts(myHash);
@@ -1902,7 +1902,7 @@ bool CWalletTx::IsTrusted() const
     {
         // Transactions not sent by us: not trusted
         const CWalletTx* parent = pwallet->GetWalletTx(txin.prevout.hash);
-        if (parent == NULL)
+        if (parent == nullptr)
             return false;
         const CTxOut& parentOut = parent->tx->vout[txin.prevout.n];
         if (pwallet->IsMine(parentOut) != ISMINE_SPENDABLE)
@@ -3941,11 +3941,11 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
         DBErrors nZapWalletRet = tempWallet->ZapWalletTx(vWtx);
         if (nZapWalletRet != DB_LOAD_OK) {
             InitError(strprintf(_("Error loading %s: Wallet corrupted"), walletFile));
-            return NULL;
+            return nullptr;
         }
 
         delete tempWallet;
-        tempWallet = NULL;
+        tempWallet = nullptr;
     }
 
     uiInterface.InitMessage(_("Loading wallet..."));
@@ -3959,7 +3959,7 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
     {
         if (nLoadWalletRet == DB_CORRUPT) {
             InitError(strprintf(_("Error loading %s: Wallet corrupted"), walletFile));
-            return NULL;
+            return nullptr;
         }
         else if (nLoadWalletRet == DB_NONCRITICAL_ERROR)
         {
@@ -3969,16 +3969,16 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
         }
         else if (nLoadWalletRet == DB_TOO_NEW) {
             InitError(strprintf(_("Error loading %s: Wallet requires newer version of %s"), walletFile, _(PACKAGE_NAME)));
-            return NULL;
+            return nullptr;
         }
         else if (nLoadWalletRet == DB_NEED_REWRITE)
         {
             InitError(strprintf(_("Wallet needed to be rewritten: restart %s to complete"), _(PACKAGE_NAME)));
-            return NULL;
+            return nullptr;
         }
         else {
             InitError(strprintf(_("Error loading %s"), walletFile));
-            return NULL;
+            return nullptr;
         }
     }
 
@@ -3996,7 +3996,7 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
         if (nMaxVersion < walletInstance->GetVersion())
         {
             InitError(_("Cannot downgrade wallet"));
-            return NULL;
+            return nullptr;
         }
         walletInstance->SetMaxVersion(nMaxVersion);
     }
@@ -4019,7 +4019,7 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
             walletInstance->SetDefaultKey(newDefaultKey);
             if (!walletInstance->SetAddressBook(walletInstance->vchDefaultKey.GetID(), "", "receive")) {
                 InitError(_("Cannot write default address") += "\n");
-                return NULL;
+                return nullptr;
             }
         }
 
@@ -4029,11 +4029,11 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
         bool useHD = GetBoolArg("-usehd", DEFAULT_USE_HD_WALLET);
         if (walletInstance->IsHDEnabled() && !useHD) {
             InitError(strprintf(_("Error loading %s: You can't disable HD on an already existing HD wallet"), walletFile));
-            return NULL;
+            return nullptr;
         }
         if (!walletInstance->IsHDEnabled() && useHD) {
             InitError(strprintf(_("Error loading %s: You can't enable HD on an already existing non-HD wallet"), walletFile));
-            return NULL;
+            return nullptr;
         }
     }
 
@@ -4065,7 +4065,7 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
 
             if (pindexRescan != block) {
                 InitError(_("Prune: last wallet synchronisation goes beyond pruned data. You need to -reindex (download the whole blockchain again in case of pruned node)"));
-                return NULL;
+                return nullptr;
             }
         }
 
@@ -4335,5 +4335,5 @@ int CMerkleTx::GetBlocksToMaturity() const
 
 bool CMerkleTx::AcceptToMemoryPool(const CAmount& nAbsurdFee, CValidationState& state)
 {
-    return ::AcceptToMemoryPool(mempool, state, tx, true, NULL, NULL, false, nAbsurdFee);
+    return ::AcceptToMemoryPool(mempool, state, tx, true, nullptr, nullptr, false, nAbsurdFee);
 }
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 0e26d6cc6a..f97a99d82a 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -342,7 +342,7 @@ public:
 
     CWalletTx()
     {
-        Init(NULL);
+        Init(nullptr);
     }
 
     CWalletTx(const CWallet* pwalletIn, CTransactionRef arg) : CMerkleTx(std::move(arg))
@@ -386,7 +386,7 @@ public:
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
         if (ser_action.ForRead())
-            Init(NULL);
+            Init(nullptr);
         char fSpent = false;
 
         if (!ser_action.ForRead())
@@ -663,7 +663,7 @@ private:
      * all coins from coinControl are selected; Never select unconfirmed coins
      * if they are not ours
      */
-    bool SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CCoinControl *coinControl = NULL) const;
+    bool SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CCoinControl *coinControl = nullptr) const;
 
     CWalletDB *pwalletdbEncryption;
 
@@ -694,7 +694,7 @@ private:
 
     /* Used by TransactionAddedToMemorypool/BlockConnected/Disconnected.
      * Should be called with pindexBlock and posInBlock if this is for a transaction that is included in a block. */
-    void SyncTransaction(const CTransactionRef& tx, const CBlockIndex *pindex = NULL, int posInBlock = 0);
+    void SyncTransaction(const CTransactionRef& tx, const CBlockIndex *pindex = nullptr, int posInBlock = 0);
 
     /* the HD chain data model (external chain counters) */
     CHDChain hdChain;
@@ -773,7 +773,7 @@ public:
     ~CWallet()
     {
         delete pwalletdbEncryption;
-        pwalletdbEncryption = NULL;
+        pwalletdbEncryption = nullptr;
     }
 
     void SetNull()
@@ -781,7 +781,7 @@ public:
         nWalletVersion = FEATURE_BASE;
         nWalletMaxVersion = FEATURE_BASE;
         nMasterKeyMaxID = 0;
-        pwalletdbEncryption = NULL;
+        pwalletdbEncryption = nullptr;
         nOrderPosNext = 0;
         nAccountingEntryNumber = 0;
         nNextResend = 0;
@@ -819,7 +819,7 @@ public:
     /**
      * populate vCoins with vector of available COutputs.
      */
-    void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = NULL, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t& nMaximumCount = 0, const int& nMinDepth = 0, const int& nMaxDepth = 9999999) const;
+    void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = nullptr, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t& nMaximumCount = 0, const int& nMinDepth = 0, const int& nMaxDepth = 9999999) const;
 
     /**
      * Return list of available coins and locked coins grouped by non-change output address.
@@ -908,7 +908,7 @@ public:
      * Increment the next transaction order id
      * @return next transaction order id
      */
-    int64_t IncOrderPosNext(CWalletDB *pwalletdb = NULL);
+    int64_t IncOrderPosNext(CWalletDB *pwalletdb = nullptr);
     DBErrors ReorderTransactions();
     bool AccountMove(std::string strFrom, std::string strTo, CAmount nAmount, std::string strComment = "");
     bool GetAccountPubkey(CPubKey &pubKey, std::string strAccount, bool bForceNew = false);
@@ -1043,7 +1043,7 @@ public:
     bool SetDefaultKey(const CPubKey &vchPubKey);
 
     //! signify that a particular wallet feature is now used. this may change nWalletVersion and nWalletMaxVersion if those are lower
-    bool SetMinVersion(enum WalletFeature, CWalletDB* pwalletdbIn = NULL, bool fExplicit = false);
+    bool SetMinVersion(enum WalletFeature, CWalletDB* pwalletdbIn = nullptr, bool fExplicit = false);
 
     //! change which version we're allowed to upgrade to (note that this does not immediately imply upgrading to that format)
     bool SetMaxVersion(int nVersion);
diff --git a/src/wallet/walletdb.cpp b/src/wallet/walletdb.cpp
index 65a28af46d..c8dc526264 100644
--- a/src/wallet/walletdb.cpp
+++ b/src/wallet/walletdb.cpp
@@ -787,7 +787,7 @@ bool CWalletDB::Recover(const std::string& filename, std::string& out_backup_fil
 {
     // recover without a key filter callback
     // results in recovering all record types
-    return CWalletDB::Recover(filename, NULL, NULL, out_backup_filename);
+    return CWalletDB::Recover(filename, nullptr, nullptr, out_backup_filename);
 }
 
 bool CWalletDB::RecoverKeysOnlyFilter(void *callbackData, CDataStream ssKey, CDataStream ssValue)
diff --git a/src/zmq/zmqnotificationinterface.cpp b/src/zmq/zmqnotificationinterface.cpp
index c063898056..b637f26fb2 100644
--- a/src/zmq/zmqnotificationinterface.cpp
+++ b/src/zmq/zmqnotificationinterface.cpp
@@ -15,7 +15,7 @@ void zmqError(const char *str)
     LogPrint(BCLog::ZMQ, "zmq: Error: %s, errno=%s\n", str, zmq_strerror(errno));
 }
 
-CZMQNotificationInterface::CZMQNotificationInterface() : pcontext(NULL)
+CZMQNotificationInterface::CZMQNotificationInterface() : pcontext(nullptr)
 {
 }
 
@@ -31,7 +31,7 @@ CZMQNotificationInterface::~CZMQNotificationInterface()
 
 CZMQNotificationInterface* CZMQNotificationInterface::Create()
 {
-    CZMQNotificationInterface* notificationInterface = NULL;
+    CZMQNotificationInterface* notificationInterface = nullptr;
     std::map<std::string, CZMQNotifierFactory> factories;
     std::list<CZMQAbstractNotifier*> notifiers;
 
@@ -62,7 +62,7 @@ CZMQNotificationInterface* CZMQNotificationInterface::Create()
         if (!notificationInterface->Initialize())
         {
             delete notificationInterface;
-            notificationInterface = NULL;
+            notificationInterface = nullptr;
         }
     }