Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

68 files changed, 105 insertions, 105 deletions

diff --git a/src/addrdb.h b/src/addrdb.h
index 8105ebc9b3..d930de204d 100644
--- a/src/addrdb.h
+++ b/src/addrdb.h
@@ -37,7 +37,7 @@ public:
         SetNull();
     }
 
-    CBanEntry(int64_t nCreateTimeIn)
+    explicit CBanEntry(int64_t nCreateTimeIn)
     {
         SetNull();
         nCreateTime = nCreateTimeIn;
diff --git a/src/base58.cpp b/src/base58.cpp
index c70e358ebb..3802f953f9 100644
--- a/src/base58.cpp
+++ b/src/base58.cpp
@@ -218,7 +218,7 @@ private:
     CBitcoinAddress* addr;
 
 public:
-    CBitcoinAddressVisitor(CBitcoinAddress* addrIn) : addr(addrIn) {}
+    explicit CBitcoinAddressVisitor(CBitcoinAddress* addrIn) : addr(addrIn) {}
 
     bool operator()(const CKeyID& id) const { return addr->Set(id); }
     bool operator()(const CScriptID& id) const { return addr->Set(id); }
diff --git a/src/bench/checkqueue.cpp b/src/bench/checkqueue.cpp
index 88a2a570f9..b7ae5c2d57 100644
--- a/src/bench/checkqueue.cpp
+++ b/src/bench/checkqueue.cpp
@@ -67,7 +67,7 @@ static void CCheckQueueSpeedPrevectorJob(benchmark::State& state)
         prevector<PREVECTOR_SIZE, uint8_t> p;
         PrevectorJob(){
         }
-        PrevectorJob(FastRandomContext& insecure_rand){
+        explicit PrevectorJob(FastRandomContext& insecure_rand){
             p.resize(insecure_rand.randrange(PREVECTOR_SIZE*2));
         }
         bool operator()()
diff --git a/src/blockencodings.h b/src/blockencodings.h
index 5a1d80d421..50478f9f32 100644
--- a/src/blockencodings.h
+++ b/src/blockencodings.h
@@ -16,7 +16,7 @@ struct TransactionCompressor {
 private:
     CTransactionRef& tx;
 public:
-    TransactionCompressor(CTransactionRef& txIn) : tx(txIn) {}
+    explicit TransactionCompressor(CTransactionRef& txIn) : tx(txIn) {}
 
     ADD_SERIALIZE_METHODS;
 
@@ -75,7 +75,7 @@ public:
     std::vector<CTransactionRef> txn;
 
     BlockTransactions() {}
-    BlockTransactions(const BlockTransactionsRequest& req) :
+    explicit BlockTransactions(const BlockTransactionsRequest& req) :
         blockhash(req.blockhash), txn(req.indexes.size()) {}
 
     ADD_SERIALIZE_METHODS;
@@ -198,7 +198,7 @@ protected:
     CTxMemPool* pool;
 public:
     CBlockHeader header;
-    PartiallyDownloadedBlock(CTxMemPool* poolIn) : pool(poolIn) {}
+    explicit PartiallyDownloadedBlock(CTxMemPool* poolIn) : pool(poolIn) {}
 
     // extra_txn is a list of extra transactions to look at, in <witness hash, reference> form
     ReadStatus InitData(const CBlockHeaderAndShortTxIDs& cmpctblock, const std::vector<std::pair<uint256, CTransactionRef>>& extra_txn);
diff --git a/src/chain.h b/src/chain.h
index 02db9ec771..ef7e6f9554 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -247,7 +247,7 @@ public:
         SetNull();
     }
 
-    CBlockIndex(const CBlockHeader& block)
+    explicit CBlockIndex(const CBlockHeader& block)
     {
         SetNull();
 
diff --git a/src/checkqueue.h b/src/checkqueue.h
index 4bc6be45f8..6377fbe942 100644
--- a/src/checkqueue.h
+++ b/src/checkqueue.h
@@ -131,7 +131,7 @@ public:
     boost::mutex ControlMutex;
 
     //! Create a new check queue
-    CCheckQueue(unsigned int nBatchSizeIn) : nIdle(0), nTotal(0), fAllOk(true), nTodo(0), fQuit(false), nBatchSize(nBatchSizeIn) {}
+    explicit CCheckQueue(unsigned int nBatchSizeIn) : nIdle(0), nTotal(0), fAllOk(true), nTodo(0), fQuit(false), nBatchSize(nBatchSizeIn) {}
 
     //! Worker thread
     void Thread()
diff --git a/src/compressor.h b/src/compressor.h
index 015911484a..094c1bcfe1 100644
--- a/src/compressor.h
+++ b/src/compressor.h
@@ -53,7 +53,7 @@ protected:
     unsigned int GetSpecialSize(unsigned int nSize) const;
     bool Decompress(unsigned int nSize, const std::vector<unsigned char> &out);
 public:
-    CScriptCompressor(CScript &scriptIn) : script(scriptIn) { }
+    explicit CScriptCompressor(CScript &scriptIn) : script(scriptIn) { }
 
     template<typename Stream>
     void Serialize(Stream &s) const {
@@ -99,7 +99,7 @@ public:
     static uint64_t CompressAmount(uint64_t nAmount);
     static uint64_t DecompressAmount(uint64_t nAmount);
 
-    CTxOutCompressor(CTxOut &txoutIn) : txout(txoutIn) { }
+    explicit CTxOutCompressor(CTxOut &txoutIn) : txout(txoutIn) { }
 
     ADD_SERIALIZE_METHODS;
 
diff --git a/src/crypto/aes.h b/src/crypto/aes.h
index e9f1b52e71..a7b63b19df 100644
--- a/src/crypto/aes.h
+++ b/src/crypto/aes.h
@@ -22,7 +22,7 @@ private:
     AES128_ctx ctx;
 
 public:
-    AES128Encrypt(const unsigned char key[16]);
+    explicit AES128Encrypt(const unsigned char key[16]);
     ~AES128Encrypt();
     void Encrypt(unsigned char ciphertext[16], const unsigned char plaintext[16]) const;
 };
@@ -34,7 +34,7 @@ private:
     AES128_ctx ctx;
 
 public:
-    AES128Decrypt(const unsigned char key[16]);
+    explicit AES128Decrypt(const unsigned char key[16]);
     ~AES128Decrypt();
     void Decrypt(unsigned char plaintext[16], const unsigned char ciphertext[16]) const;
 };
@@ -46,7 +46,7 @@ private:
     AES256_ctx ctx;
 
 public:
-    AES256Encrypt(const unsigned char key[32]);
+    explicit AES256Encrypt(const unsigned char key[32]);
     ~AES256Encrypt();
     void Encrypt(unsigned char ciphertext[16], const unsigned char plaintext[16]) const;
 };
@@ -58,7 +58,7 @@ private:
     AES256_ctx ctx;
 
 public:
-    AES256Decrypt(const unsigned char key[32]);
+    explicit AES256Decrypt(const unsigned char key[32]);
     ~AES256Decrypt();
     void Decrypt(unsigned char plaintext[16], const unsigned char ciphertext[16]) const;
 };
diff --git a/src/cuckoocache.h b/src/cuckoocache.h
index fd24d05ee7..9246a3924e 100644
--- a/src/cuckoocache.h
+++ b/src/cuckoocache.h
@@ -58,7 +58,7 @@ public:
      * @post All calls to bit_is_set (without subsequent bit_unset) will return
      * true.
      */
-    bit_packed_atomic_flags(uint32_t size)
+    explicit bit_packed_atomic_flags(uint32_t size)
     {
         // pad out the size if needed
         size = (size + 7) / 8;
diff --git a/src/dbwrapper.h b/src/dbwrapper.h
index 54092f9f59..e19fde51c1 100644
--- a/src/dbwrapper.h
+++ b/src/dbwrapper.h
@@ -22,7 +22,7 @@ static const size_t DBWRAPPER_PREALLOC_VALUE_SIZE = 1024;
 class dbwrapper_error : public std::runtime_error
 {
 public:
-    dbwrapper_error(const std::string& msg) : std::runtime_error(msg) {}
+    explicit dbwrapper_error(const std::string& msg) : std::runtime_error(msg) {}
 };
 
 class CDBWrapper;
@@ -61,7 +61,7 @@ public:
     /**
      * @param[in] _parent   CDBWrapper that this batch is to be submitted to
      */
-    CDBBatch(const CDBWrapper &_parent) : parent(_parent), ssKey(SER_DISK, CLIENT_VERSION), ssValue(SER_DISK, CLIENT_VERSION), size_estimate(0) { };
+    explicit CDBBatch(const CDBWrapper &_parent) : parent(_parent), ssKey(SER_DISK, CLIENT_VERSION), ssValue(SER_DISK, CLIENT_VERSION), size_estimate(0) { };
 
     void Clear()
     {
diff --git a/src/hash.h b/src/hash.h
index b9952d39fc..ad59bb1817 100644
--- a/src/hash.h
+++ b/src/hash.h
@@ -168,7 +168,7 @@ private:
     Source* source;
 
 public:
-    CHashVerifier(Source* source_) : CHashWriter(source_->GetType(), source_->GetVersion()), source(source_) {}
+    explicit CHashVerifier(Source* source_) : CHashWriter(source_->GetType(), source_->GetVersion()), source(source_) {}
 
     void read(char* pch, size_t nSize)
     {
diff --git a/src/httprpc.cpp b/src/httprpc.cpp
index 8c1a74677a..91f96ef207 100644
--- a/src/httprpc.cpp
+++ b/src/httprpc.cpp
@@ -43,7 +43,7 @@ private:
 class HTTPRPCTimerInterface : public RPCTimerInterface
 {
 public:
-    HTTPRPCTimerInterface(struct event_base* _base) : base(_base)
+    explicit HTTPRPCTimerInterface(struct event_base* _base) : base(_base)
     {
     }
     const char* Name() override
diff --git a/src/httpserver.cpp b/src/httpserver.cpp
index 815d660288..ed1ee53904 100644
--- a/src/httpserver.cpp
+++ b/src/httpserver.cpp
@@ -79,7 +79,7 @@ private:
     {
     public:
         WorkQueue &wq;
-        ThreadCounter(WorkQueue &w): wq(w)
+        explicit ThreadCounter(WorkQueue &w): wq(w)
         {
             std::lock_guard<std::mutex> lock(wq.cs);
             wq.numThreads += 1;
@@ -93,7 +93,7 @@ private:
     };
 
 public:
-    WorkQueue(size_t _maxDepth) : running(true),
+    explicit WorkQueue(size_t _maxDepth) : running(true),
                                  maxDepth(_maxDepth),
                                  numThreads(0)
     {
diff --git a/src/httpserver.h b/src/httpserver.h
index 3e434bf0a0..91ce5b4e00 100644
--- a/src/httpserver.h
+++ b/src/httpserver.h
@@ -61,7 +61,7 @@ private:
     bool replySent;
 
 public:
-    HTTPRequest(struct evhttp_request* req);
+    explicit HTTPRequest(struct evhttp_request* req);
     ~HTTPRequest();
 
     enum RequestMethod {
diff --git a/src/init.cpp b/src/init.cpp
index f9a63348fd..fbce718626 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -136,7 +136,7 @@ bool ShutdownRequested()
 class CCoinsViewErrorCatcher : public CCoinsViewBacked
 {
 public:
-    CCoinsViewErrorCatcher(CCoinsView* view) : CCoinsViewBacked(view) {}
+    explicit CCoinsViewErrorCatcher(CCoinsView* view) : CCoinsViewBacked(view) {}
     bool GetCoin(const COutPoint &outpoint, Coin &coin) const override {
         try {
             return CCoinsViewBacked::GetCoin(outpoint, coin);
diff --git a/src/limitedmap.h b/src/limitedmap.h
index e9dcb6defd..7afc8b458d 100644
--- a/src/limitedmap.h
+++ b/src/limitedmap.h
@@ -27,7 +27,7 @@ protected:
     size_type nMaxSize;
 
 public:
-    limitedmap(size_type nMaxSizeIn)
+    explicit limitedmap(size_type nMaxSizeIn)
     {
         assert(nMaxSizeIn > 0);
         nMaxSize = nMaxSizeIn;
diff --git a/src/miner.h b/src/miner.h
index 685b4e0cc8..6e5fe761db 100644
--- a/src/miner.h
+++ b/src/miner.h
@@ -33,7 +33,7 @@ struct CBlockTemplate
 // Container for tracking updates to ancestor feerate as we include (parent)
 // transactions in a block
 struct CTxMemPoolModifiedEntry {
-    CTxMemPoolModifiedEntry(CTxMemPool::txiter entry)
+    explicit CTxMemPoolModifiedEntry(CTxMemPool::txiter entry)
     {
         iter = entry;
         nSizeWithAncestors = entry->GetSizeWithAncestors();
@@ -116,7 +116,7 @@ typedef indexed_modified_transaction_set::index<ancestor_score>::type::iterator
 
 struct update_for_parent_inclusion
 {
-    update_for_parent_inclusion(CTxMemPool::txiter it) : iter(it) {}
+    explicit update_for_parent_inclusion(CTxMemPool::txiter it) : iter(it) {}
 
     void operator() (CTxMemPoolModifiedEntry &e)
     {
@@ -164,7 +164,7 @@ public:
         CFeeRate blockMinFeeRate;
     };
 
-    BlockAssembler(const CChainParams& params);
+    explicit BlockAssembler(const CChainParams& params);
     BlockAssembler(const CChainParams& params, const Options& options);
 
     /** Construct a new block template with coinbase to scriptPubKeyIn */
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index e09b56a631..596ae1139b 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -2783,7 +2783,7 @@ class CompareInvMempoolOrder
 {
     CTxMemPool *mp;
 public:
-    CompareInvMempoolOrder(CTxMemPool *_mempool)
+    explicit CompareInvMempoolOrder(CTxMemPool *_mempool)
     {
         mp = _mempool;
     }
diff --git a/src/net_processing.h b/src/net_processing.h
index db6d81e6b6..f4a43980a5 100644
--- a/src/net_processing.h
+++ b/src/net_processing.h
@@ -32,7 +32,7 @@ private:
     CConnman* connman;
 
 public:
-    PeerLogicValidation(CConnman* connmanIn);
+    explicit PeerLogicValidation(CConnman* connmanIn);
 
     void BlockConnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindexConnected, const std::vector<CTransactionRef>& vtxConflicted) override;
     void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;
diff --git a/src/netaddress.h b/src/netaddress.h
index 57ce897db5..6ca99b36b5 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -36,7 +36,7 @@ class CNetAddr
 
     public:
         CNetAddr();
-        CNetAddr(const struct in_addr& ipv4Addr);
+        explicit CNetAddr(const struct in_addr& ipv4Addr);
         void Init();
         void SetIP(const CNetAddr& ip);
 
@@ -82,7 +82,7 @@ class CNetAddr
         std::vector<unsigned char> GetGroup() const;
         int GetReachabilityFrom(const CNetAddr *paddrPartner = nullptr) const;
 
-        CNetAddr(const struct in6_addr& pipv6Addr, const uint32_t scope = 0);
+        explicit CNetAddr(const struct in6_addr& pipv6Addr, const uint32_t scope = 0);
         bool GetIn6Addr(struct in6_addr* pipv6Addr) const;
 
         friend bool operator==(const CNetAddr& a, const CNetAddr& b);
@@ -146,7 +146,7 @@ class CService : public CNetAddr
         CService();
         CService(const CNetAddr& ip, unsigned short port);
         CService(const struct in_addr& ipv4Addr, unsigned short port);
-        CService(const struct sockaddr_in& addr);
+        explicit CService(const struct sockaddr_in& addr);
         void Init();
         unsigned short GetPort() const;
         bool GetSockAddr(struct sockaddr* paddr, socklen_t *addrlen) const;
@@ -160,7 +160,7 @@ class CService : public CNetAddr
         std::string ToStringIPPort() const;
 
         CService(const struct in6_addr& ipv6Addr, unsigned short port);
-        CService(const struct sockaddr_in6& addr);
+        explicit CService(const struct sockaddr_in6& addr);
 
         ADD_SERIALIZE_METHODS;
 
diff --git a/src/netbase.h b/src/netbase.h
index f02c5e0b05..6572f0a12e 100644
--- a/src/netbase.h
+++ b/src/netbase.h
@@ -29,7 +29,7 @@ class proxyType
 {
 public:
     proxyType(): randomize_credentials(false) {}
-    proxyType(const CService &_proxy, bool _randomize_credentials=false): proxy(_proxy), randomize_credentials(_randomize_credentials) {}
+    explicit proxyType(const CService &_proxy, bool _randomize_credentials=false): proxy(_proxy), randomize_credentials(_randomize_credentials) {}
 
     bool IsValid() const { return proxy.IsValid(); }
 
diff --git a/src/netmessagemaker.h b/src/netmessagemaker.h
index 8e8a6e4a02..79b2501c5d 100644
--- a/src/netmessagemaker.h
+++ b/src/netmessagemaker.h
@@ -12,7 +12,7 @@
 class CNetMsgMaker
 {
 public:
-    CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn){}
+    explicit CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn){}
 
     template <typename... Args>
     CSerializedNetMsg Make(int nFlags, std::string sCommand, Args&&... args) const
diff --git a/src/policy/fees.h b/src/policy/fees.h
index f4ef793643..6edaf28714 100644
--- a/src/policy/fees.h
+++ b/src/policy/fees.h
@@ -284,7 +284,7 @@ private:
 
 public:
     /** Create new FeeFilterRounder */
-    FeeFilterRounder(const CFeeRate& minIncrementalFee);
+    explicit FeeFilterRounder(const CFeeRate& minIncrementalFee);
 
     /** Quantize a minimum fee for privacy purpose before broadcast **/
     CAmount round(CAmount currentMinFee);
diff --git a/src/primitives/block.h b/src/primitives/block.h
index c90a1dfa64..292df40896 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -129,7 +129,7 @@ struct CBlockLocator
 
     CBlockLocator() {}
 
-    CBlockLocator(const std::vector<uint256>& vHaveIn) : vHave(vHaveIn) {}
+    explicit CBlockLocator(const std::vector<uint256>& vHaveIn) : vHave(vHaveIn) {}
 
     ADD_SERIALIZE_METHODS;
 
diff --git a/src/protocol.h b/src/protocol.h
index 7890bb627d..67e01d9606 100644
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -39,7 +39,7 @@ public:
     };
     typedef unsigned char MessageStartChars[MESSAGE_START_SIZE];
 
-    CMessageHeader(const MessageStartChars& pchMessageStartIn);
+    explicit CMessageHeader(const MessageStartChars& pchMessageStartIn);
     CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn);
 
     std::string GetCommand() const;
diff --git a/src/pubkey.h b/src/pubkey.h
index dbf0e23f20..65738d8fe2 100644
--- a/src/pubkey.h
+++ b/src/pubkey.h
@@ -30,7 +30,7 @@ class CKeyID : public uint160
 {
 public:
     CKeyID() : uint160() {}
-    CKeyID(const uint160& in) : uint160(in) {}
+    explicit CKeyID(const uint160& in) : uint160(in) {}
 };
 
 typedef uint256 ChainCode;
@@ -88,7 +88,7 @@ public:
     }
 
     //! Construct a public key from a byte vector.
-    CPubKey(const std::vector<unsigned char>& _vch)
+    explicit CPubKey(const std::vector<unsigned char>& _vch)
     {
         Set(_vch.begin(), _vch.end());
     }
diff --git a/src/qt/callback.h b/src/qt/callback.h
index a8b593a652..da6b0c4c2e 100644
--- a/src/qt/callback.h
+++ b/src/qt/callback.h
@@ -16,7 +16,7 @@ class FunctionCallback : public Callback
     F f;
 
 public:
-    FunctionCallback(F f_) : f(std::move(f_)) {}
+    explicit FunctionCallback(F f_) : f(std::move(f_)) {}
     ~FunctionCallback() override {}
     void call() override { f(this); }
 };
diff --git a/src/qt/coincontroldialog.h b/src/qt/coincontroldialog.h
index 99a9f893ff..4949c91771 100644
--- a/src/qt/coincontroldialog.h
+++ b/src/qt/coincontroldialog.h
@@ -30,9 +30,9 @@ namespace Ui {
 class CCoinControlWidgetItem : public QTreeWidgetItem
 {
 public:
-    CCoinControlWidgetItem(QTreeWidget *parent, int type = Type) : QTreeWidgetItem(parent, type) {}
-    CCoinControlWidgetItem(int type = Type) : QTreeWidgetItem(type) {}
-    CCoinControlWidgetItem(QTreeWidgetItem *parent, int type = Type) : QTreeWidgetItem(parent, type) {}
+    explicit CCoinControlWidgetItem(QTreeWidget *parent, int type = Type) : QTreeWidgetItem(parent, type) {}
+    explicit CCoinControlWidgetItem(int type = Type) : QTreeWidgetItem(type) {}
+    explicit CCoinControlWidgetItem(QTreeWidgetItem *parent, int type = Type) : QTreeWidgetItem(parent, type) {}
 
     bool operator<(const QTreeWidgetItem &other) const;
 };
diff --git a/src/qt/intro.cpp b/src/qt/intro.cpp
index 72809942f1..0ff95d8502 100644
--- a/src/qt/intro.cpp
+++ b/src/qt/intro.cpp
@@ -43,7 +43,7 @@ class FreespaceChecker : public QObject
     Q_OBJECT
 
 public:
-    FreespaceChecker(Intro *intro);
+    explicit FreespaceChecker(Intro *intro);
 
     enum Status {
         ST_OK,
diff --git a/src/qt/notificator.cpp b/src/qt/notificator.cpp
index 8718929c6a..a7a7a4ce11 100644
--- a/src/qt/notificator.cpp
+++ b/src/qt/notificator.cpp
@@ -93,7 +93,7 @@ class FreedesktopImage
 {
 public:
     FreedesktopImage() {}
-    FreedesktopImage(const QImage &img);
+    explicit FreedesktopImage(const QImage &img);
 
     static int metaType();
 
diff --git a/src/qt/overviewpage.cpp b/src/qt/overviewpage.cpp
index ba344f4dbf..ba1839e7b4 100644
--- a/src/qt/overviewpage.cpp
+++ b/src/qt/overviewpage.cpp
@@ -25,7 +25,7 @@ class TxViewDelegate : public QAbstractItemDelegate
 {
     Q_OBJECT
 public:
-    TxViewDelegate(const PlatformStyle *_platformStyle, QObject *parent=nullptr):
+    explicit TxViewDelegate(const PlatformStyle *_platformStyle, QObject *parent=nullptr):
         QAbstractItemDelegate(parent), unit(BitcoinUnits::BTC),
         platformStyle(_platformStyle)
     {
diff --git a/src/qt/paymentrequestplus.cpp b/src/qt/paymentrequestplus.cpp
index 0a5cb98668..d3799f59ab 100644
--- a/src/qt/paymentrequestplus.cpp
+++ b/src/qt/paymentrequestplus.cpp
@@ -22,7 +22,7 @@
 class SSLVerifyError : public std::runtime_error
 {
 public:
-    SSLVerifyError(std::string err) : std::runtime_error(err) { }
+    explicit SSLVerifyError(std::string err) : std::runtime_error(err) { }
 };
 
 bool PaymentRequestPlus::parse(const QByteArray& data)
diff --git a/src/qt/paymentserver.h b/src/qt/paymentserver.h
index f845c4c7c8..98b2364b92 100644
--- a/src/qt/paymentserver.h
+++ b/src/qt/paymentserver.h
@@ -72,7 +72,7 @@ public:
     static bool ipcSendCommandLine();
 
     // parent should be QApplication object
-    PaymentServer(QObject* parent, bool startLocalServer = true);
+    explicit PaymentServer(QObject* parent, bool startLocalServer = true);
     ~PaymentServer();
 
     // Load root certificate authorities. Pass nullptr (default)
diff --git a/src/qt/utilitydialog.h b/src/qt/utilitydialog.h
index acaa864148..738eeed136 100644
--- a/src/qt/utilitydialog.h
+++ b/src/qt/utilitydialog.h
@@ -41,7 +41,7 @@ class ShutdownWindow : public QWidget
     Q_OBJECT
 
 public:
-    ShutdownWindow(QWidget *parent=0, Qt::WindowFlags f=0);
+    explicit ShutdownWindow(QWidget *parent=0, Qt::WindowFlags f=0);
     static QWidget *showShutdownWindow(BitcoinGUI *window);
 
 protected:
diff --git a/src/rest.cpp b/src/rest.cpp
index 6a4b005f90..154ee04eed 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -48,7 +48,7 @@ struct CCoin {
     ADD_SERIALIZE_METHODS;
 
     CCoin() : nHeight(0) {}
-    CCoin(Coin&& in) : nHeight(in.nHeight), out(std::move(in.out)) {}
+    explicit CCoin(Coin&& in) : nHeight(in.nHeight), out(std::move(in.out)) {}
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action)
diff --git a/src/reverse_iterator.h b/src/reverse_iterator.h
index 46789e5417..ab467f07c9 100644
--- a/src/reverse_iterator.h
+++ b/src/reverse_iterator.h
@@ -17,7 +17,7 @@ class reverse_range
     T &m_x;
     
 public:
-    reverse_range(T &x) : m_x(x) {}
+    explicit reverse_range(T &x) : m_x(x) {}
     
     auto begin() const -> decltype(this->m_x.rbegin())
     {
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index f498b5c8ea..652d886c7e 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -686,7 +686,7 @@ public:
     bool found;
     CValidationState state;
 
-    submitblock_StateCatcher(const uint256 &hashIn) : hash(hashIn), found(false), state() {}
+    explicit submitblock_StateCatcher(const uint256 &hashIn) : hash(hashIn), found(false), state() {}
 
 protected:
     void BlockChecked(const CBlock& block, const CValidationState& stateIn) override {
diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
index efff4a99ae..1dd660eb8f 100644
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -125,7 +125,7 @@ class DescribeAddressVisitor : public boost::static_visitor<UniValue>
 public:
     CWallet * const pwallet;
 
-    DescribeAddressVisitor(CWallet *_pwallet) : pwallet(_pwallet) {}
+    explicit DescribeAddressVisitor(CWallet *_pwallet) : pwallet(_pwallet) {}
 
     UniValue operator()(const CNoDestination &dest) const { return UniValue(UniValue::VOBJ); }
 
diff --git a/src/rpc/server.h b/src/rpc/server.h
index dd6f763245..89b1d169d5 100644
--- a/src/rpc/server.h
+++ b/src/rpc/server.h
@@ -31,7 +31,7 @@ namespace RPCServer
 /** Wrapper for UniValue::VType, which includes typeAny:
  * Used to denote don't care type. Only used by RPCTypeCheckObj */
 struct UniValueType {
-    UniValueType(UniValue::VType _type) : typeAny(false), type(_type) {}
+    explicit UniValueType(UniValue::VType _type) : typeAny(false), type(_type) {}
     UniValueType() : typeAny(true) {}
     bool typeAny;
     UniValue::VType type;
diff --git a/src/scheduler.cpp b/src/scheduler.cpp
index 1d3fb1f6ea..4edb2c6d9b 100644
--- a/src/scheduler.cpp
+++ b/src/scheduler.cpp
@@ -174,7 +174,7 @@ void SingleThreadedSchedulerClient::ProcessQueue() {
     // to ensure both happen safely even if callback() throws.
     struct RAIICallbacksRunning {
         SingleThreadedSchedulerClient* instance;
-        RAIICallbacksRunning(SingleThreadedSchedulerClient* _instance) : instance(_instance) {}
+        explicit RAIICallbacksRunning(SingleThreadedSchedulerClient* _instance) : instance(_instance) {}
         ~RAIICallbacksRunning() {
             {
                 LOCK(instance->m_cs_callbacks_pending);
diff --git a/src/scheduler.h b/src/scheduler.h
index cc2f01f2ff..db93bcb21e 100644
--- a/src/scheduler.h
+++ b/src/scheduler.h
@@ -102,7 +102,7 @@ private:
     void ProcessQueue();
 
 public:
-    SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
+    explicit SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
     void AddToProcessQueue(std::function<void (void)> func);
 
     // Processes all remaining queue members on the calling thread, blocking until queue is empty
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index 437826b5dd..f845e1943b 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -114,7 +114,7 @@ struct PrecomputedTransactionData
 {
     uint256 hashPrevouts, hashSequence, hashOutputs;
 
-    PrecomputedTransactionData(const CTransaction& tx);
+    explicit PrecomputedTransactionData(const CTransaction& tx);
 };
 
 enum SigVersion
diff --git a/src/script/sign.h b/src/script/sign.h
index bd45862892..a0d8ee4ff9 100644
--- a/src/script/sign.h
+++ b/src/script/sign.h
@@ -21,7 +21,7 @@ protected:
     const CKeyStore* keystore;
 
 public:
-    BaseSignatureCreator(const CKeyStore* keystoreIn) : keystore(keystoreIn) {}
+    explicit BaseSignatureCreator(const CKeyStore* keystoreIn) : keystore(keystoreIn) {}
     const CKeyStore& KeyStore() const { return *keystore; };
     virtual ~BaseSignatureCreator() {}
     virtual const BaseSignatureChecker& Checker() const =0;
@@ -54,7 +54,7 @@ public:
 /** A signature creator that just produces 72-byte empty signatures. */
 class DummySignatureCreator : public BaseSignatureCreator {
 public:
-    DummySignatureCreator(const CKeyStore* keystoreIn) : BaseSignatureCreator(keystoreIn) {}
+    explicit DummySignatureCreator(const CKeyStore* keystoreIn) : BaseSignatureCreator(keystoreIn) {}
     const BaseSignatureChecker& Checker() const override;
     bool CreateSig(std::vector<unsigned char>& vchSig, const CKeyID& keyid, const CScript& scriptCode, SigVersion sigversion) const override;
 };
diff --git a/src/script/standard.cpp b/src/script/standard.cpp
index 9570b8c8d9..0f720d9e76 100644
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -253,7 +253,7 @@ class CScriptVisitor : public boost::static_visitor<bool>
 private:
     CScript *script;
 public:
-    CScriptVisitor(CScript *scriptin) { script = scriptin; }
+    explicit CScriptVisitor(CScript *scriptin) { script = scriptin; }
 
     bool operator()(const CNoDestination &dest) const {
         script->clear();
diff --git a/src/serialize.h b/src/serialize.h
index 8b86a07a76..eeb05fa76c 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -402,7 +402,7 @@ class CVarInt
 protected:
     I &n;
 public:
-    CVarInt(I& nIn) : n(nIn) { }
+    explicit CVarInt(I& nIn) : n(nIn) { }
 
     template<typename Stream>
     void Serialize(Stream &s) const {
@@ -420,7 +420,7 @@ class CCompactSize
 protected:
     uint64_t &n;
 public:
-    CCompactSize(uint64_t& nIn) : n(nIn) { }
+    explicit CCompactSize(uint64_t& nIn) : n(nIn) { }
 
     template<typename Stream>
     void Serialize(Stream &s) const {
@@ -439,7 +439,7 @@ class LimitedString
 protected:
     std::string& string;
 public:
-    LimitedString(std::string& _string) : string(_string) {}
+    explicit LimitedString(std::string& _string) : string(_string) {}
 
     template<typename Stream>
     void Unserialize(Stream& s)
diff --git a/src/support/lockedpool.h b/src/support/lockedpool.h
index 7641d81471..cecbdec1aa 100644
--- a/src/support/lockedpool.h
+++ b/src/support/lockedpool.h
@@ -150,7 +150,7 @@ public:
      * If this callback is provided and returns false, the allocation fails (hard fail), if
      * it returns true the allocation proceeds, but it could warn.
      */
-    LockedPool(std::unique_ptr<LockedPageAllocator> allocator, LockingFailed_Callback lf_cb_in = nullptr);
+    explicit LockedPool(std::unique_ptr<LockedPageAllocator> allocator, LockingFailed_Callback lf_cb_in = nullptr);
     ~LockedPool();
 
     /** Allocate size bytes from this arena.
@@ -217,7 +217,7 @@ public:
     }
 
 private:
-    LockedPoolManager(std::unique_ptr<LockedPageAllocator> allocator);
+    explicit LockedPoolManager(std::unique_ptr<LockedPageAllocator> allocator);
 
     /** Create a new LockedPoolManager specialized to the OS */
     static void CreateInstance();
diff --git a/src/sync.h b/src/sync.h
index ddaf62b3b3..0871c5fb4d 100644
--- a/src/sync.h
+++ b/src/sync.h
@@ -196,7 +196,7 @@ private:
     int value;
 
 public:
-    CSemaphore(int init) : value(init) {}
+    explicit CSemaphore(int init) : value(init) {}
 
     void wait()
     {
@@ -267,7 +267,7 @@ public:
 
     CSemaphoreGrant() : sem(nullptr), fHaveGrant(false) {}
 
-    CSemaphoreGrant(CSemaphore& sema, bool fTry = false) : sem(&sema), fHaveGrant(false)
+    explicit CSemaphoreGrant(CSemaphore& sema, bool fTry = false) : sem(&sema), fHaveGrant(false)
     {
         if (fTry)
             TryAcquire();
diff --git a/src/test/base58_tests.cpp b/src/test/base58_tests.cpp
index b33cdb9fe6..ee633249e9 100644
--- a/src/test/base58_tests.cpp
+++ b/src/test/base58_tests.cpp
@@ -78,7 +78,7 @@ class TestAddrTypeVisitor : public boost::static_visitor<bool>
 private:
     std::string exp_addrType;
 public:
-    TestAddrTypeVisitor(const std::string &_exp_addrType) : exp_addrType(_exp_addrType) { }
+    explicit TestAddrTypeVisitor(const std::string &_exp_addrType) : exp_addrType(_exp_addrType) { }
     bool operator()(const CKeyID &id) const
     {
         return (exp_addrType == "pubkey");
@@ -99,7 +99,7 @@ class TestPayloadVisitor : public boost::static_visitor<bool>
 private:
     std::vector<unsigned char> exp_payload;
 public:
-    TestPayloadVisitor(std::vector<unsigned char> &_exp_payload) : exp_payload(_exp_payload) { }
+    explicit TestPayloadVisitor(std::vector<unsigned char> &_exp_payload) : exp_payload(_exp_payload) { }
     bool operator()(const CKeyID &id) const
     {
         uint160 exp_key(exp_payload);
diff --git a/src/test/bip32_tests.cpp b/src/test/bip32_tests.cpp
index 6bcd550d7b..c851ab2849 100644
--- a/src/test/bip32_tests.cpp
+++ b/src/test/bip32_tests.cpp
@@ -24,7 +24,7 @@ struct TestVector {
     std::string strHexMaster;
     std::vector<TestDerivation> vDerive;
 
-    TestVector(std::string strHexMasterIn) : strHexMaster(strHexMasterIn) {}
+    explicit TestVector(std::string strHexMasterIn) : strHexMaster(strHexMasterIn) {}
 
     TestVector& operator()(std::string pub, std::string prv, unsigned int nChild) {
         vDerive.push_back(TestDerivation());
diff --git a/src/test/blockencodings_tests.cpp b/src/test/blockencodings_tests.cpp
index 812e8534cd..f2d5b385d0 100644
--- a/src/test/blockencodings_tests.cpp
+++ b/src/test/blockencodings_tests.cpp
@@ -118,12 +118,12 @@ public:
     std::vector<uint64_t> shorttxids;
     std::vector<PrefilledTransaction> prefilledtxn;
 
-    TestHeaderAndShortIDs(const CBlockHeaderAndShortTxIDs& orig) {
+    explicit TestHeaderAndShortIDs(const CBlockHeaderAndShortTxIDs& orig) {
         CDataStream stream(SER_NETWORK, PROTOCOL_VERSION);
         stream << orig;
         stream >> *this;
     }
-    TestHeaderAndShortIDs(const CBlock& block) :
+    explicit TestHeaderAndShortIDs(const CBlock& block) :
         TestHeaderAndShortIDs(CBlockHeaderAndShortTxIDs(block, true)) {}
 
     uint64_t GetShortID(const uint256& txhash) const {
diff --git a/src/test/coins_tests.cpp b/src/test/coins_tests.cpp
index 37ddf83f5e..dc358bff95 100644
--- a/src/test/coins_tests.cpp
+++ b/src/test/coins_tests.cpp
@@ -74,7 +74,7 @@ public:
 class CCoinsViewCacheTest : public CCoinsViewCache
 {
 public:
-    CCoinsViewCacheTest(CCoinsView* _base) : CCoinsViewCache(_base) {}
+    explicit CCoinsViewCacheTest(CCoinsView* _base) : CCoinsViewCache(_base) {}
 
     void SelfTest() const
     {
diff --git a/src/test/dbwrapper_tests.cpp b/src/test/dbwrapper_tests.cpp
index 6ed6e7744e..251d5a7142 100644
--- a/src/test/dbwrapper_tests.cpp
+++ b/src/test/dbwrapper_tests.cpp
@@ -231,7 +231,7 @@ struct StringContentsSerializer {
     // This is a terrible idea
     std::string str;
     StringContentsSerializer() {}
-    StringContentsSerializer(const std::string& inp) : str(inp) {}
+    explicit StringContentsSerializer(const std::string& inp) : str(inp) {}
 
     StringContentsSerializer& operator+=(const std::string& s) {
         str += s;
diff --git a/src/test/test_bitcoin.h b/src/test/test_bitcoin.h
index dd3b13c8c8..2ddac2f076 100644
--- a/src/test/test_bitcoin.h
+++ b/src/test/test_bitcoin.h
@@ -41,7 +41,7 @@ static inline bool InsecureRandBool() { return insecure_rand_ctx.randbool(); }
 struct BasicTestingSetup {
     ECCVerifyHandle globalVerifyHandle;
 
-    BasicTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
+    explicit BasicTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
     ~BasicTestingSetup();
 };
 
@@ -56,7 +56,7 @@ struct TestingSetup: public BasicTestingSetup {
     CConnman* connman;
     CScheduler scheduler;
 
-    TestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
+    explicit TestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
     ~TestingSetup();
 };
 
diff --git a/src/tinyformat.h b/src/tinyformat.h
index 5022d46809..2e453e56bb 100644
--- a/src/tinyformat.h
+++ b/src/tinyformat.h
@@ -167,7 +167,7 @@ namespace tinyformat {
 class format_error: public std::runtime_error
 {
 public:
-    format_error(const std::string &what): std::runtime_error(what) {
+    explicit format_error(const std::string &what): std::runtime_error(what) {
     }
 };
 
@@ -498,7 +498,7 @@ class FormatArg
         FormatArg() {}
 
         template<typename T>
-        FormatArg(const T& value)
+        explicit FormatArg(const T& value)
             : m_value(static_cast<const void*>(&value)),
             m_formatImpl(&formatImpl<T>),
             m_toIntImpl(&toIntImpl<T>)
@@ -867,7 +867,7 @@ class FormatListN : public FormatList
     public:
 #ifdef TINYFORMAT_USE_VARIADIC_TEMPLATES
         template<typename... Args>
-        FormatListN(const Args&... args)
+        explicit FormatListN(const Args&... args)
             : FormatList(&m_formatterStore[0], N),
             m_formatterStore { FormatArg(args)... }
         { static_assert(sizeof...(args) == N, "Number of args must be N"); }
@@ -876,7 +876,7 @@ class FormatListN : public FormatList
 #       define TINYFORMAT_MAKE_FORMATLIST_CONSTRUCTOR(n)       \
                                                                \
         template<TINYFORMAT_ARGTYPES(n)>                       \
-        FormatListN(TINYFORMAT_VARARGS(n))                     \
+        explicit FormatListN(TINYFORMAT_VARARGS(n))            \
             : FormatList(&m_formatterStore[0], n)              \
         { assert(n == N); init(0, TINYFORMAT_PASSARGS(n)); }   \
                                                                \
diff --git a/src/torcontrol.cpp b/src/torcontrol.cpp
index 6b96c24af2..1cea197666 100644
--- a/src/torcontrol.cpp
+++ b/src/torcontrol.cpp
@@ -76,7 +76,7 @@ public:
 
     /** Create a new TorControlConnection.
      */
-    TorControlConnection(struct event_base *base);
+    explicit TorControlConnection(struct event_base *base);
     ~TorControlConnection();
 
     /**
diff --git a/src/txdb.cpp b/src/txdb.cpp
index 4c1b04cd94..797ae5713f 100644
--- a/src/txdb.cpp
+++ b/src/txdb.cpp
@@ -35,7 +35,7 @@ namespace {
 struct CoinEntry {
     COutPoint* outpoint;
     char key;
-    CoinEntry(const COutPoint* ptr) : outpoint(const_cast<COutPoint*>(ptr)), key(DB_COIN)  {}
+    explicit CoinEntry(const COutPoint* ptr) : outpoint(const_cast<COutPoint*>(ptr)), key(DB_COIN)  {}
 
     template<typename Stream>
     void Serialize(Stream &s) const {
diff --git a/src/txdb.h b/src/txdb.h
index adcbc73380..603eb08a7a 100644
--- a/src/txdb.h
+++ b/src/txdb.h
@@ -69,7 +69,7 @@ class CCoinsViewDB : public CCoinsView
 protected:
     CDBWrapper db;
 public:
-    CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
+    explicit CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 
     bool GetCoin(const COutPoint &outpoint, Coin &coin) const override;
     bool HaveCoin(const COutPoint &outpoint) const override;
@@ -109,7 +109,7 @@ private:
 class CBlockTreeDB : public CDBWrapper
 {
 public:
-    CBlockTreeDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
+    explicit CBlockTreeDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 private:
     CBlockTreeDB(const CBlockTreeDB&);
     void operator=(const CBlockTreeDB&);
diff --git a/src/txmempool.h b/src/txmempool.h
index 0b183cbd90..5b0db5266e 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -167,7 +167,7 @@ struct update_ancestor_state
 
 struct update_fee_delta
 {
-    update_fee_delta(int64_t _feeDelta) : feeDelta(_feeDelta) { }
+    explicit update_fee_delta(int64_t _feeDelta) : feeDelta(_feeDelta) { }
 
     void operator() (CTxMemPoolEntry &e) { e.UpdateFeeDelta(feeDelta); }
 
@@ -177,7 +177,7 @@ private:
 
 struct update_lock_points
 {
-    update_lock_points(const LockPoints& _lp) : lp(_lp) { }
+    explicit update_lock_points(const LockPoints& _lp) : lp(_lp) { }
 
     void operator() (CTxMemPoolEntry &e) { e.UpdateLockPoints(lp); }
 
@@ -501,7 +501,7 @@ public:
 
     /** Create a new CTxMemPool.
      */
-    CTxMemPool(CBlockPolicyEstimator* estimator = nullptr);
+    explicit CTxMemPool(CBlockPolicyEstimator* estimator = nullptr);
 
     /**
      * If sanity-checking is turned on, check makes sure the pool is
diff --git a/src/uint256.h b/src/uint256.h
index a92ce07f11..3ed694d723 100644
--- a/src/uint256.h
+++ b/src/uint256.h
@@ -111,7 +111,7 @@ public:
 class uint160 : public base_blob<160> {
 public:
     uint160() {}
-    uint160(const base_blob<160>& b) : base_blob<160>(b) {}
+    explicit uint160(const base_blob<160>& b) : base_blob<160>(b) {}
     explicit uint160(const std::vector<unsigned char>& vch) : base_blob<160>(vch) {}
 };
 
@@ -123,7 +123,7 @@ public:
 class uint256 : public base_blob<256> {
 public:
     uint256() {}
-    uint256(const base_blob<256>& b) : base_blob<256>(b) {}
+    explicit uint256(const base_blob<256>& b) : base_blob<256>(b) {}
     explicit uint256(const std::vector<unsigned char>& vch) : base_blob<256>(vch) {}
 
     /** A cheap hash function that just returns 64 bits from the result, it can be
diff --git a/src/undo.h b/src/undo.h
index 0f9d041bbd..a720de4ac5 100644
--- a/src/undo.h
+++ b/src/undo.h
@@ -33,7 +33,7 @@ public:
         ::Serialize(s, CTxOutCompressor(REF(txout->out)));
     }
 
-    TxInUndoSerializer(const Coin* coin) : txout(coin) {}
+    explicit TxInUndoSerializer(const Coin* coin) : txout(coin) {}
 };
 
 class TxInUndoDeserializer
@@ -57,7 +57,7 @@ public:
         ::Unserialize(s, REF(CTxOutCompressor(REF(txout->out))));
     }
 
-    TxInUndoDeserializer(Coin* coin) : txout(coin) {}
+    explicit TxInUndoDeserializer(Coin* coin) : txout(coin) {}
 };
 
 static const size_t MIN_TRANSACTION_INPUT_WEIGHT = WITNESS_SCALE_FACTOR * ::GetSerializeSize(CTxIn(), SER_NETWORK, PROTOCOL_VERSION);
diff --git a/src/univalue/lib/univalue_utffilter.h b/src/univalue/lib/univalue_utffilter.h
index 0e330dce9c..2fb6a492d1 100644
--- a/src/univalue/lib/univalue_utffilter.h
+++ b/src/univalue/lib/univalue_utffilter.h
@@ -13,7 +13,7 @@
 class JSONUTF8StringFilter
 {
 public:
-    JSONUTF8StringFilter(std::string &s):
+    explicit JSONUTF8StringFilter(std::string &s):
         str(s), is_valid(true), codepoint(0), state(0), surpair(0)
     {
     }
diff --git a/src/validation.cpp b/src/validation.cpp
index 8bd23a0f1d..d1a8b8460a 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1563,7 +1563,7 @@ private:
     int bit;
 
 public:
-    WarningBitsConditionChecker(int bitIn) : bit(bitIn) {}
+    explicit WarningBitsConditionChecker(int bitIn) : bit(bitIn) {}
 
     int64_t BeginTime(const Consensus::Params& params) const override { return 0; }
     int64_t EndTime(const Consensus::Params& params) const override { return std::numeric_limits<int64_t>::max(); }
@@ -2135,7 +2135,7 @@ private:
     CTxMemPool &pool;
 
 public:
-    ConnectTrace(CTxMemPool &_pool) : blocksConnected(1), pool(_pool) {
+    explicit ConnectTrace(CTxMemPool &_pool) : blocksConnected(1), pool(_pool) {
         pool.NotifyEntryRemoved.connect(boost::bind(&ConnectTrace::NotifyEntryRemoved, this, _1, _2));
     }
 
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index bf20d606f8..be5029dec3 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -30,7 +30,7 @@ struct MainSignalsInstance {
     // our own queue here :(
     SingleThreadedSchedulerClient m_schedulerClient;
 
-    MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}
+    explicit MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}
 };
 
 static CMainSignals g_signals;
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
index 59588023ad..64ae939672 100644
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -185,7 +185,7 @@ protected:
     }
 
 public:
-    VersionBitsConditionChecker(Consensus::DeploymentPos id_) : id(id_) {}
+    explicit VersionBitsConditionChecker(Consensus::DeploymentPos id_) : id(id_) {}
     uint32_t Mask(const Consensus::Params& params) const { return ((uint32_t)1) << params.vDeployments[id].bit; }
 };
 
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 094357639b..9a53491838 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -1123,7 +1123,7 @@ public:
     CWallet * const pwallet;
     CScriptID result;
 
-    Witnessifier(CWallet *_pwallet) : pwallet(_pwallet) {}
+    explicit Witnessifier(CWallet *_pwallet) : pwallet(_pwallet) {}
 
     bool operator()(const CNoDestination &dest) const { return false; }
 
diff --git a/src/wallet/test/wallet_test_fixture.h b/src/wallet/test/wallet_test_fixture.h
index 97a6d98397..9373b7907c 100644
--- a/src/wallet/test/wallet_test_fixture.h
+++ b/src/wallet/test/wallet_test_fixture.h
@@ -10,7 +10,7 @@
 /** Testing setup and teardown for wallet.
  */
 struct WalletTestingSetup: public TestingSetup {
-    WalletTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
+    explicit WalletTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
     ~WalletTestingSetup();
 };
 
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index f97a99d82a..19e1638d35 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -208,7 +208,7 @@ public:
         Init();
     }
 
-    CMerkleTx(CTransactionRef arg)
+    explicit CMerkleTx(CTransactionRef arg)
     {
         SetTx(std::move(arg));
         Init();
@@ -548,7 +548,7 @@ public:
     //! todo: add something to note what created it (user, getnewaddress, change)
     //!   maybe should have a map<string, string> property map
 
-    CWalletKey(int64_t nExpires=0);
+    explicit CWalletKey(int64_t nExpires=0);
 
     ADD_SERIALIZE_METHODS;
 
@@ -765,7 +765,7 @@ public:
     }
 
     // Create wallet with passed-in database handle
-    CWallet(std::unique_ptr<CWalletDBWrapper> dbw_in) : dbw(std::move(dbw_in))
+    explicit CWallet(std::unique_ptr<CWalletDBWrapper> dbw_in) : dbw(std::move(dbw_in))
     {
         SetNull();
     }
@@ -1145,7 +1145,7 @@ protected:
     CPubKey vchPubKey;
     bool fInternal;
 public:
-    CReserveKey(CWallet* pwalletIn)
+    explicit CReserveKey(CWallet* pwalletIn)
     {
         nIndex = -1;
         pwallet = pwalletIn;
diff --git a/src/wallet/walletdb.h b/src/wallet/walletdb.h
index d78f143ebd..e4857f6cab 100644
--- a/src/wallet/walletdb.h
+++ b/src/wallet/walletdb.h
@@ -105,7 +105,7 @@ public:
     {
         SetNull();
     }
-    CKeyMetadata(int64_t nCreateTime_)
+    explicit CKeyMetadata(int64_t nCreateTime_)
     {
         SetNull();
         nCreateTime = nCreateTime_;
@@ -162,7 +162,7 @@ private:
     }
 
 public:
-    CWalletDB(CWalletDBWrapper& dbw, const char* pszMode = "r+", bool _fFlushOnClose = true) :
+    explicit CWalletDB(CWalletDBWrapper& dbw, const char* pszMode = "r+", bool _fFlushOnClose = true) :
         batch(dbw, pszMode, _fFlushOnClose),
         m_dbw(dbw)
     {