Make objects in range declarations immutable by default. Avoid unnecessary copying of objects in range declarations.

36 files changed, 77 insertions, 77 deletions

diff --git a/src/bech32.cpp b/src/bech32.cpp
index c55f22b9b7..d6b29391a9 100644
--- a/src/bech32.cpp
+++ b/src/bech32.cpp
@@ -62,7 +62,7 @@ uint32_t PolyMod(const data& v)
     // v, it corresponds to x^2 + v0*x + v1 mod g(x). As 1 mod g(x) = 1, that is the starting value
     // for `c`.
     uint32_t c = 1;
-    for (auto v_i : v) {
+    for (const auto v_i : v) {
         // We want to update `c` to correspond to a polynomial with one extra term. If the initial
         // value of `c` consists of the coefficients of c(x) = f(x) mod g(x), we modify it to
         // correspond to c'(x) = (f(x) * x + v_i) mod g(x), where v_i is the next input to
@@ -149,7 +149,7 @@ std::string Encode(const std::string& hrp, const data& values) {
     data combined = Cat(values, checksum);
     std::string ret = hrp + '1';
     ret.reserve(ret.size() + combined.size());
-    for (auto c : combined) {
+    for (const auto c : combined) {
         ret += CHARSET[c];
     }
     return ret;
diff --git a/src/bitcoin-tx.cpp b/src/bitcoin-tx.cpp
index 20f0218ac1..9ee3e8eee7 100644
--- a/src/bitcoin-tx.cpp
+++ b/src/bitcoin-tx.cpp
@@ -385,7 +385,7 @@ static void MutateTxAddOutMultiSig(CMutableTransaction& tx, const std::string& s
     CScript scriptPubKey = GetScriptForMultisig(required, pubkeys);
 
     if (bSegWit) {
-        for (CPubKey& pubkey : pubkeys) {
+        for (const CPubKey& pubkey : pubkeys) {
             if (!pubkey.IsCompressed()) {
                 throw std::runtime_error("Uncompressed pubkeys are not useable for SegWit outputs");
             }
diff --git a/src/cuckoocache.h b/src/cuckoocache.h
index 15f6873961..4d0b094fa2 100644
--- a/src/cuckoocache.h
+++ b/src/cuckoocache.h
@@ -397,7 +397,7 @@ public:
         std::array<uint32_t, 8> locs = compute_hashes(e);
         // Make sure we have not already inserted this element
         // If we have, make sure that it does not get deleted
-        for (uint32_t loc : locs)
+        for (const uint32_t loc : locs)
             if (table[loc] == e) {
                 please_keep(loc);
                 epoch_flags[loc] = last_epoch;
@@ -405,7 +405,7 @@ public:
             }
         for (uint8_t depth = 0; depth < depth_limit; ++depth) {
             // First try to insert to an empty slot, if one exists
-            for (uint32_t loc : locs) {
+            for (const uint32_t loc : locs) {
                 if (!collection_flags.bit_is_set(loc))
                     continue;
                 table[loc] = std::move(e);
@@ -467,7 +467,7 @@ public:
     inline bool contains(const Element& e, const bool erase) const
     {
         std::array<uint32_t, 8> locs = compute_hashes(e);
-        for (uint32_t loc : locs)
+        for (const uint32_t loc : locs)
             if (table[loc] == e) {
                 if (erase)
                     allow_erase(loc);
diff --git a/src/net.cpp b/src/net.cpp
index df2cb54b7a..84294a65ad 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -344,7 +344,7 @@ CNode* CConnman::FindNode(const CService& addr)
 bool CConnman::CheckIncomingNonce(uint64_t nonce)
 {
     LOCK(cs_vNodes);
-    for (CNode* pnode : vNodes) {
+    for (const CNode* pnode : vNodes) {
         if (!pnode->fSuccessfullyConnected && !pnode->fInbound && pnode->GetLocalNonce() == nonce)
             return false;
     }
@@ -1614,7 +1614,7 @@ void CConnman::ThreadDNSAddressSeed()
 
         LOCK(cs_vNodes);
         int nRelevant = 0;
-        for (auto pnode : vNodes) {
+        for (const CNode* pnode : vNodes) {
             nRelevant += pnode->fSuccessfullyConnected && !pnode->fFeeler && !pnode->fOneShot && !pnode->m_manual_connection && !pnode->fInbound;
         }
         if (nRelevant >= 2) {
@@ -1733,7 +1733,7 @@ int CConnman::GetExtraOutboundCount()
     int nOutbound = 0;
     {
         LOCK(cs_vNodes);
-        for (CNode* pnode : vNodes) {
+        for (const CNode* pnode : vNodes) {
             if (!pnode->fInbound && !pnode->m_manual_connection && !pnode->fFeeler && !pnode->fDisconnect && !pnode->fOneShot && pnode->fSuccessfullyConnected) {
                 ++nOutbound;
             }
@@ -1803,7 +1803,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         std::set<std::vector<unsigned char> > setConnected;
         {
             LOCK(cs_vNodes);
-            for (CNode* pnode : vNodes) {
+            for (const CNode* pnode : vNodes) {
                 if (!pnode->fInbound && !pnode->m_manual_connection) {
                     // Netgroups for inbound and addnode peers are not excluded because our goal here
                     // is to not use multiple of our limited outbound slots on a single netgroup
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index a6a8814dff..acef179a4f 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -882,7 +882,7 @@ void PeerLogicValidation::BlockConnected(const std::shared_ptr<const CBlock>& pb
     // Erase orphan transactions included or precluded by this block
     if (vOrphanErase.size()) {
         int nErased = 0;
-        for (uint256 &orphanHash : vOrphanErase) {
+        for (const uint256& orphanHash : vOrphanErase) {
             nErased += EraseOrphanTx(orphanHash);
         }
         LogPrint(BCLog::MEMPOOL, "Erased %d orphan tx included or conflicted by block\n", nErased);
@@ -2288,7 +2288,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                 }
             }
 
-            for (uint256 hash : vEraseQueue)
+            for (const uint256& hash : vEraseQueue)
                 EraseOrphanTx(hash);
         }
         else if (fMissingInputs)
diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
index 712d20b71e..f1d3074c2f 100644
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -416,7 +416,7 @@ void BitcoinApplication::requestShutdown()
 
 #ifdef ENABLE_WALLET
     window->removeAllWallets();
-    for (WalletModel *walletModel : m_wallet_models) {
+    for (const WalletModel* walletModel : m_wallet_models) {
         delete walletModel;
     }
     m_wallet_models.clear();
diff --git a/src/qt/bitcoingui.cpp b/src/qt/bitcoingui.cpp
index 5321cd94fd..d87882ea7d 100644
--- a/src/qt/bitcoingui.cpp
+++ b/src/qt/bitcoingui.cpp
@@ -1273,7 +1273,7 @@ void UnitDisplayStatusBarControl::mousePressEvent(QMouseEvent *event)
 void UnitDisplayStatusBarControl::createContextMenu()
 {
     menu = new QMenu(this);
-    for (BitcoinUnits::Unit u : BitcoinUnits::availableUnits())
+    for (const BitcoinUnits::Unit u : BitcoinUnits::availableUnits())
     {
         QAction *menuAction = new QAction(QString(BitcoinUnits::longName(u)), this);
         menuAction->setData(QVariant(u));
diff --git a/src/qt/peertablemodel.cpp b/src/qt/peertablemodel.cpp
index 59a751fb12..8cfedca57f 100644
--- a/src/qt/peertablemodel.cpp
+++ b/src/qt/peertablemodel.cpp
@@ -65,7 +65,7 @@ public:
             interfaces::Node::NodesStats nodes_stats;
             node.getNodesStats(nodes_stats);
             cachedNodeStats.reserve(nodes_stats.size());
-            for (auto& node_stats : nodes_stats)
+            for (const auto& node_stats : nodes_stats)
             {
                 CNodeCombinedStats stats;
                 stats.nodeStats = std::get<0>(node_stats);
diff --git a/src/qt/sendcoinsdialog.cpp b/src/qt/sendcoinsdialog.cpp
index eed71397ab..6f66bc19e1 100644
--- a/src/qt/sendcoinsdialog.cpp
+++ b/src/qt/sendcoinsdialog.cpp
@@ -348,7 +348,7 @@ void SendCoinsDialog::on_sendButton_clicked()
     questionString.append("<hr />");
     CAmount totalAmount = currentTransaction.getTotalTransactionAmount() + txFee;
     QStringList alternativeUnits;
-    for (BitcoinUnits::Unit u : BitcoinUnits::availableUnits())
+    for (const BitcoinUnits::Unit u : BitcoinUnits::availableUnits())
     {
         if(u != model->getOptionsModel()->getDisplayUnit())
             alternativeUnits.append(BitcoinUnits::formatHtmlWithUnit(u, totalAmount));
diff --git a/src/qt/splashscreen.cpp b/src/qt/splashscreen.cpp
index 0b111cc6d7..1eff4f6b65 100644
--- a/src/qt/splashscreen.cpp
+++ b/src/qt/splashscreen.cpp
@@ -200,7 +200,7 @@ void SplashScreen::unsubscribeFromCoreSignals()
     // Disconnect signals from client
     m_handler_init_message->disconnect();
     m_handler_show_progress->disconnect();
-    for (auto& handler : m_connected_wallet_handlers) {
+    for (const auto& handler : m_connected_wallet_handlers) {
         handler->disconnect();
     }
     m_connected_wallet_handlers.clear();
diff --git a/src/qt/trafficgraphwidget.cpp b/src/qt/trafficgraphwidget.cpp
index f087e7569c..3aed3f2b97 100644
--- a/src/qt/trafficgraphwidget.cpp
+++ b/src/qt/trafficgraphwidget.cpp
@@ -141,10 +141,10 @@ void TrafficGraphWidget::updateRates()
     }
 
     float tmax = 0.0f;
-    for (float f : vSamplesIn) {
+    for (const float f : vSamplesIn) {
         if(f > tmax) tmax = f;
     }
-    for (float f : vSamplesOut) {
+    for (const float f : vSamplesOut) {
         if(f > tmax) tmax = f;
     }
     fMax = tmax;
diff --git a/src/qt/transactiondesc.cpp b/src/qt/transactiondesc.cpp
index 6286ddc0e0..59332be754 100644
--- a/src/qt/transactiondesc.cpp
+++ b/src/qt/transactiondesc.cpp
@@ -152,13 +152,13 @@ QString TransactionDesc::toHTML(interfaces::Node& node, interfaces::Wallet& wall
     else
     {
         isminetype fAllFromMe = ISMINE_SPENDABLE;
-        for (isminetype mine : wtx.txin_is_mine)
+        for (const isminetype mine : wtx.txin_is_mine)
         {
             if(fAllFromMe > mine) fAllFromMe = mine;
         }
 
         isminetype fAllToMe = ISMINE_SPENDABLE;
-        for (isminetype mine : wtx.txout_is_mine)
+        for (const isminetype mine : wtx.txout_is_mine)
         {
             if(fAllToMe > mine) fAllToMe = mine;
         }
diff --git a/src/qt/transactionrecord.cpp b/src/qt/transactionrecord.cpp
index c767de5eda..d1a7527ac7 100644
--- a/src/qt/transactionrecord.cpp
+++ b/src/qt/transactionrecord.cpp
@@ -77,14 +77,14 @@ QList<TransactionRecord> TransactionRecord::decomposeTransaction(const interface
     {
         bool involvesWatchAddress = false;
         isminetype fAllFromMe = ISMINE_SPENDABLE;
-        for (isminetype mine : wtx.txin_is_mine)
+        for (const isminetype mine : wtx.txin_is_mine)
         {
             if(mine & ISMINE_WATCH_ONLY) involvesWatchAddress = true;
             if(fAllFromMe > mine) fAllFromMe = mine;
         }
 
         isminetype fAllToMe = ISMINE_SPENDABLE;
-        for (isminetype mine : wtx.txout_is_mine)
+        for (const isminetype mine : wtx.txout_is_mine)
         {
             if(mine & ISMINE_WATCH_ONLY) involvesWatchAddress = true;
             if(fAllToMe > mine) fAllToMe = mine;
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index a77fea8ea8..ee895bdbe8 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -426,7 +426,7 @@ static void entryToJSON(UniValue &info, const CTxMemPoolEntry &e) EXCLUSIVE_LOCK
     UniValue spent(UniValue::VARR);
     const CTxMemPool::txiter &it = mempool.mapTx.find(tx.GetHash());
     const CTxMemPool::setEntries &setChildren = mempool.GetMemPoolChildren(it);
-    for (const CTxMemPool::txiter &childiter : setChildren) {
+    for (CTxMemPool::txiter childiter : setChildren) {
         spent.push_back(childiter->GetTx().GetHash().ToString());
     }
 
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index 623b0bd86a..e7de256078 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -922,7 +922,7 @@ static UniValue estimaterawfee(const JSONRPCRequest& request)
 
     UniValue result(UniValue::VOBJ);
 
-    for (FeeEstimateHorizon horizon : {FeeEstimateHorizon::SHORT_HALFLIFE, FeeEstimateHorizon::MED_HALFLIFE, FeeEstimateHorizon::LONG_HALFLIFE}) {
+    for (const FeeEstimateHorizon horizon : {FeeEstimateHorizon::SHORT_HALFLIFE, FeeEstimateHorizon::MED_HALFLIFE, FeeEstimateHorizon::LONG_HALFLIFE}) {
         CFeeRate feeRate;
         EstimationResult buckets;
 
diff --git a/src/rpc/net.cpp b/src/rpc/net.cpp
index 343b7d3b05..169a452659 100644
--- a/src/rpc/net.cpp
+++ b/src/rpc/net.cpp
@@ -164,7 +164,7 @@ static UniValue getpeerinfo(const JSONRPCRequest& request)
             obj.pushKV("synced_headers", statestats.nSyncHeight);
             obj.pushKV("synced_blocks", statestats.nCommonHeight);
             UniValue heights(UniValue::VARR);
-            for (int height : statestats.vHeightInFlight) {
+            for (const int height : statestats.vHeightInFlight) {
                 heights.push_back(height);
             }
             obj.pushKV("inflight", heights);
diff --git a/src/test/checkqueue_tests.cpp b/src/test/checkqueue_tests.cpp
index 39d0061c1c..2732598b4f 100644
--- a/src/test/checkqueue_tests.cpp
+++ b/src/test/checkqueue_tests.cpp
@@ -155,7 +155,7 @@ static void Correct_Queue_range(std::vector<size_t> range)
     }
     // Make vChecks here to save on malloc (this test can be slow...)
     std::vector<FakeCheckCheckCompletion> vChecks;
-    for (auto i : range) {
+    for (const size_t i : range) {
         size_t total = i;
         FakeCheckCheckCompletion::n_calls = 0;
         CCheckQueueControl<FakeCheckCheckCompletion> control(small_queue.get());
@@ -253,7 +253,7 @@ BOOST_AUTO_TEST_CASE(test_CheckQueue_Recovers_From_Failure)
     }
 
     for (auto times = 0; times < 10; ++times) {
-        for (bool end_fails : {true, false}) {
+        for (const bool end_fails : {true, false}) {
             CCheckQueueControl<FailingCheck> control(fail_queue.get());
             {
                 std::vector<FailingCheck> vChecks;
diff --git a/src/test/coins_tests.cpp b/src/test/coins_tests.cpp
index 63b91b74c0..56b50c9bdb 100644
--- a/src/test/coins_tests.cpp
+++ b/src/test/coins_tests.cpp
@@ -736,7 +736,7 @@ static void CheckAddCoinBase(CAmount base_value, CAmount cache_value, CAmount mo
 template <typename... Args>
 static void CheckAddCoin(Args&&... args)
 {
-    for (CAmount base_value : {ABSENT, PRUNED, VALUE1})
+    for (const CAmount base_value : {ABSENT, PRUNED, VALUE1})
         CheckAddCoinBase(base_value, std::forward<Args>(args)...);
 }
 
@@ -848,10 +848,10 @@ BOOST_AUTO_TEST_CASE(ccoins_write)
     // they would be too repetitive (the parent cache is never updated in these
     // cases). The loop below covers these cases and makes sure the parent cache
     // is always left unchanged.
-    for (CAmount parent_value : {ABSENT, PRUNED, VALUE1})
-        for (CAmount child_value : {ABSENT, PRUNED, VALUE2})
-            for (char parent_flags : parent_value == ABSENT ? ABSENT_FLAGS : FLAGS)
-                for (char child_flags : child_value == ABSENT ? ABSENT_FLAGS : CLEAN_FLAGS)
+    for (const CAmount parent_value : {ABSENT, PRUNED, VALUE1})
+        for (const CAmount child_value : {ABSENT, PRUNED, VALUE2})
+            for (const char parent_flags : parent_value == ABSENT ? ABSENT_FLAGS : FLAGS)
+                for (const char child_flags : child_value == ABSENT ? ABSENT_FLAGS : CLEAN_FLAGS)
                     CheckWriteCoins(parent_value, child_value, parent_value, parent_flags, child_flags, parent_flags);
 }
 
diff --git a/src/test/cuckoocache_tests.cpp b/src/test/cuckoocache_tests.cpp
index ec7aba34ca..dbceb9d2e0 100644
--- a/src/test/cuckoocache_tests.cpp
+++ b/src/test/cuckoocache_tests.cpp
@@ -82,11 +82,11 @@ static double test_cache(size_t megabytes, double load)
      */
     std::vector<uint256> hashes_insert_copy = hashes;
     /** Do the insert */
-    for (uint256& h : hashes_insert_copy)
+    for (const uint256& h : hashes_insert_copy)
         set.insert(h);
     /** Count the hits */
     uint32_t count = 0;
-    for (uint256& h : hashes)
+    for (const uint256& h : hashes)
         count += set.contains(h, false);
     double hit_rate = ((double)count) / ((double)n_insert);
     return hit_rate;
@@ -323,7 +323,7 @@ static void test_cache_generations()
                 reads.push_back(inserts[i]);
             for (uint32_t i = n_insert - (n_insert / 4); i < n_insert; ++i)
                 reads.push_back(inserts[i]);
-            for (auto h : inserts)
+            for (const auto& h : inserts)
                 c.insert(h);
         }
     };
diff --git a/src/test/dbwrapper_tests.cpp b/src/test/dbwrapper_tests.cpp
index 4dff1e5c72..fd0a4fda4b 100644
--- a/src/test/dbwrapper_tests.cpp
+++ b/src/test/dbwrapper_tests.cpp
@@ -26,7 +26,7 @@ BOOST_FIXTURE_TEST_SUITE(dbwrapper_tests, BasicTestingSetup)
 BOOST_AUTO_TEST_CASE(dbwrapper)
 {
     // Perform tests both obfuscated and non-obfuscated.
-    for (bool obfuscate : {false, true}) {
+    for (const bool obfuscate : {false, true}) {
         fs::path ph = SetDataDir(std::string("dbwrapper").append(obfuscate ? "_true" : "_false"));
         CDBWrapper dbw(ph, (1 << 20), true, false, obfuscate);
         char key = 'k';
@@ -46,7 +46,7 @@ BOOST_AUTO_TEST_CASE(dbwrapper)
 BOOST_AUTO_TEST_CASE(dbwrapper_batch)
 {
     // Perform tests both obfuscated and non-obfuscated.
-    for (bool obfuscate : {false, true}) {
+    for (const bool obfuscate : {false, true}) {
         fs::path ph = SetDataDir(std::string("dbwrapper_batch").append(obfuscate ? "_true" : "_false"));
         CDBWrapper dbw(ph, (1 << 20), true, false, obfuscate);
 
@@ -82,7 +82,7 @@ BOOST_AUTO_TEST_CASE(dbwrapper_batch)
 BOOST_AUTO_TEST_CASE(dbwrapper_iterator)
 {
     // Perform tests both obfuscated and non-obfuscated.
-    for (bool obfuscate : {false, true}) {
+    for (const bool obfuscate : {false, true}) {
         fs::path ph = SetDataDir(std::string("dbwrapper_iterator").append(obfuscate ? "_true" : "_false"));
         CDBWrapper dbw(ph, (1 << 20), true, false, obfuscate);
 
@@ -216,7 +216,7 @@ BOOST_AUTO_TEST_CASE(iterator_ordering)
         if (x & 1) BOOST_CHECK(dbw.Write(key, value));
     }
 
-    for (int seek_start : {0x00, 0x80}) {
+    for (const int seek_start : {0x00, 0x80}) {
         it->Seek((uint8_t)seek_start);
         for (unsigned int x=seek_start; x<255; ++x) {
             uint8_t key;
@@ -291,7 +291,7 @@ BOOST_AUTO_TEST_CASE(iterator_string_ordering)
     }
 
     std::unique_ptr<CDBIterator> it(const_cast<CDBWrapper&>(dbw).NewIterator());
-    for (int seek_start : {0, 5}) {
+    for (const int seek_start : {0, 5}) {
         snprintf(buf, sizeof(buf), "%d", seek_start);
         StringContentsSerializer seek_key(buf);
         it->Seek(seek_key);
diff --git a/src/test/getarg_tests.cpp b/src/test/getarg_tests.cpp
index eec7b38af7..7592330b10 100644
--- a/src/test/getarg_tests.cpp
+++ b/src/test/getarg_tests.cpp
@@ -24,7 +24,7 @@ static void ResetArgs(const std::string& strArg)
 
     // Convert to char*:
     std::vector<const char*> vecChar;
-    for (std::string& s : vecArg)
+    for (const std::string& s : vecArg)
         vecChar.push_back(s.c_str());
 
     std::string error;
diff --git a/src/test/key_io_tests.cpp b/src/test/key_io_tests.cpp
index 7e475ac610..a0c10d8ddd 100644
--- a/src/test/key_io_tests.cpp
+++ b/src/test/key_io_tests.cpp
@@ -136,7 +136,7 @@ BOOST_AUTO_TEST_CASE(key_io_invalid)
         std::string exp_base58string = test[0].get_str();
 
         // must be invalid as public and as private key
-        for (auto chain : { CBaseChainParams::MAIN, CBaseChainParams::TESTNET, CBaseChainParams::REGTEST }) {
+        for (const auto& chain : { CBaseChainParams::MAIN, CBaseChainParams::TESTNET, CBaseChainParams::REGTEST }) {
             SelectParams(chain);
             destination = DecodeDestination(exp_base58string);
             BOOST_CHECK_MESSAGE(!IsValidDestination(destination), "IsValid pubkey in mainnet:" + strTest);
diff --git a/src/test/skiplist_tests.cpp b/src/test/skiplist_tests.cpp
index bcd2a9c7b6..c0754618fb 100644
--- a/src/test/skiplist_tests.cpp
+++ b/src/test/skiplist_tests.cpp
@@ -146,7 +146,7 @@ BOOST_AUTO_TEST_CASE(findearliestatleast_test)
 BOOST_AUTO_TEST_CASE(findearliestatleast_edge_test)
 {
     std::list<CBlockIndex> blocks;
-    for (unsigned int timeMax : {100, 100, 100, 200, 200, 200, 300, 300, 300}) {
+    for (const unsigned int timeMax : {100, 100, 100, 200, 200, 200, 300, 300, 300}) {
         CBlockIndex* prev = blocks.empty() ? nullptr : &blocks.back();
         blocks.emplace_back();
         blocks.back().nHeight = prev ? prev->nHeight + 1 : 0;
diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
index a89bf785f1..0d7c9f0abf 100644
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -29,7 +29,7 @@ void CConnmanTest::AddNode(CNode& node)
 void CConnmanTest::ClearNodes()
 {
     LOCK(g_connman->cs_vNodes);
-    for (CNode* node : g_connman->vNodes) {
+    for (const CNode* node : g_connman->vNodes) {
         delete node;
     }
     g_connman->vNodes.clear();
diff --git a/src/test/transaction_tests.cpp b/src/test/transaction_tests.cpp
index 33f0a83b7e..c527ad448c 100644
--- a/src/test/transaction_tests.cpp
+++ b/src/test/transaction_tests.cpp
@@ -65,7 +65,7 @@ unsigned int ParseScriptFlags(std::string strFlags)
     std::vector<std::string> words;
     boost::algorithm::split(words, strFlags, boost::algorithm::is_any_of(","));
 
-    for (std::string word : words)
+    for (const std::string& word : words)
     {
         if (!mapFlagNames.count(word))
             BOOST_ERROR("Bad test: unknown verification flag '" << word << "'");
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index b2960446eb..14810c3f6b 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -245,7 +245,7 @@ BOOST_AUTO_TEST_CASE(util_GetBoolArg)
     testArgs.ParseParameters(7, (char**)argv_test, error);
 
     // Each letter should be set.
-    for (char opt : "abcdef")
+    for (const char opt : "abcdef")
         BOOST_CHECK(testArgs.IsArgSet({'-', opt}) || !opt);
 
     // Nothing else should be in the map
@@ -394,7 +394,7 @@ BOOST_AUTO_TEST_CASE(util_ReadConfigStream)
                 && test_args.GetArg("-iii", "xxx") == "xxx"
                );
 
-    for (bool def : {false, true}) {
+    for (const bool def : {false, true}) {
         BOOST_CHECK(test_args.GetBoolArg("-a", def)
                      && test_args.GetBoolArg("-b", def)
                      && !test_args.GetBoolArg("-ccc", def)
diff --git a/src/timedata.cpp b/src/timedata.cpp
index 1599508306..291111feb2 100644
--- a/src/timedata.cpp
+++ b/src/timedata.cpp
@@ -94,7 +94,7 @@ void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
             {
                 // If nobody has a time different than ours but within 5 minutes of ours, give a warning
                 bool fMatch = false;
-                for (int64_t nOffset : vSorted)
+                for (const int64_t nOffset : vSorted)
                     if (nOffset != 0 && abs64(nOffset) < 5 * 60)
                         fMatch = true;
 
@@ -109,7 +109,7 @@ void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
         }
 
         if (LogAcceptCategory(BCLog::NET)) {
-            for (int64_t n : vSorted) {
+            for (const int64_t n : vSorted) {
                 LogPrint(BCLog::NET, "%+d  ", n); /* Continued */
             }
             LogPrint(BCLog::NET, "|  "); /* Continued */
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 8bada4dce8..33ef9b1012 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -196,7 +196,7 @@ bool CTxMemPool::CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntr
         }
 
         const setEntries & setMemPoolParents = GetMemPoolParents(stageit);
-        for (const txiter &phash : setMemPoolParents) {
+        for (txiter phash : setMemPoolParents) {
             // If this is a new ancestor, add it.
             if (setAncestors.count(phash) == 0) {
                 parentHashes.insert(phash);
@@ -457,7 +457,7 @@ void CTxMemPool::CalculateDescendants(txiter entryit, setEntries& setDescendants
         stage.erase(it);
 
         const setEntries &setChildren = GetMemPoolChildren(it);
-        for (const txiter &childiter : setChildren) {
+        for (txiter childiter : setChildren) {
             if (!setDescendants.count(childiter)) {
                 stage.insert(childiter);
             }
@@ -907,7 +907,7 @@ size_t CTxMemPool::DynamicMemoryUsage() const {
 void CTxMemPool::RemoveStaged(setEntries &stage, bool updateDescendants, MemPoolRemovalReason reason) {
     AssertLockHeld(cs);
     UpdateForRemoveFromMempool(stage, updateDescendants);
-    for (const txiter& it : stage) {
+    for (txiter it : stage) {
         removeUnchecked(it, reason);
     }
 }
diff --git a/src/utilstrencodings.cpp b/src/utilstrencodings.cpp
index a06d88cb19..20ea2d1a53 100644
--- a/src/utilstrencodings.cpp
+++ b/src/utilstrencodings.cpp
@@ -71,7 +71,7 @@ bool IsHexNumber(const std::string& str)
     if (str.size() > 2 && *str.begin() == '0' && *(str.begin()+1) == 'x') {
         starting_location = 2;
     }
-    for (auto c : str.substr(starting_location)) {
+    for (const char c : str.substr(starting_location)) {
         if (HexDigit(c) < 0) return false;
     }
     // Return false for empty string or "0x".
diff --git a/src/validation.cpp b/src/validation.cpp
index 3e51221a5e..2f966d2f56 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -421,7 +421,7 @@ bool CheckSequenceLocks(const CTransaction &tx, int flags, LockPoints* lp, bool
             // lock on a mempool input, so we can use the return value of
             // CheckSequenceLocks to indicate the LockPoints validity
             int maxInputHeight = 0;
-            for (int height : prevheights) {
+            for (const int height : prevheights) {
                 // Can ignore mempool inputs since we'll fail if they had non-zero locks
                 if (height != tip->nHeight+1) {
                     maxInputHeight = std::max(maxInputHeight, height);
@@ -4291,7 +4291,7 @@ void UnloadBlockIndex()
         warningcache[b].clear();
     }
 
-    for (BlockMap::value_type& entry : mapBlockIndex) {
+    for (const BlockMap::value_type& entry : mapBlockIndex) {
         delete entry.second;
     }
     mapBlockIndex.clear();
@@ -4492,7 +4492,7 @@ void CChainState::CheckBlockIndex(const Consensus::Params& consensusParams)
 
     // Build forward-pointing map of the entire block tree.
     std::multimap<CBlockIndex*,CBlockIndex*> forward;
-    for (auto& entry : mapBlockIndex) {
+    for (const std::pair<const uint256, CBlockIndex*>& entry : mapBlockIndex) {
         forward.insert(std::make_pair(entry.second->pprev, entry.second));
     }
 
diff --git a/src/wallet/crypter.cpp b/src/wallet/crypter.cpp
index 4fa9603011..729e4e39b0 100644
--- a/src/wallet/crypter.cpp
+++ b/src/wallet/crypter.cpp
@@ -311,7 +311,7 @@ bool CCryptoKeyStore::EncryptKeys(CKeyingMaterial& vMasterKeyIn)
         return false;
 
     fUseCrypto = true;
-    for (KeyMap::value_type& mKey : mapKeys)
+    for (const KeyMap::value_type& mKey : mapKeys)
     {
         const CKey &key = mKey.second;
         CPubKey vchPubKey = key.GetPubKey();
diff --git a/src/wallet/db.cpp b/src/wallet/db.cpp
index d0fe51801e..0e56c85ee3 100644
--- a/src/wallet/db.cpp
+++ b/src/wallet/db.cpp
@@ -503,7 +503,7 @@ BerkeleyBatch::BerkeleyBatch(BerkeleyDatabase& database, const char* pszMode, bo
             // be implemented, so no equality checks are needed at all. (Newer
             // versions of BDB have an set_lk_exclusive method for this
             // purpose, but the older version we use does not.)
-            for (auto& env : g_dbenvs) {
+            for (const auto& env : g_dbenvs) {
                 CheckUniqueFileid(env.second, strFilename, *pdb_temp);
             }
 
diff --git a/src/wallet/rpcdump.cpp b/src/wallet/rpcdump.cpp
index 60f14e5886..af36d321d5 100644
--- a/src/wallet/rpcdump.cpp
+++ b/src/wallet/rpcdump.cpp
@@ -41,7 +41,7 @@ int64_t static DecodeDumpTime(const std::string &str) {
 
 std::string static EncodeDumpString(const std::string &str) {
     std::stringstream ret;
-    for (unsigned char c : str) {
+    for (const unsigned char c : str) {
         if (c <= 32 || c >= 128 || c == '%') {
             ret << '%' << HexStr(&c, &c + 1);
         } else {
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index c82c914e1e..5059568cc1 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2365,7 +2365,7 @@ static UniValue listlockunspent(const JSONRPCRequest& request)
 
     UniValue ret(UniValue::VARR);
 
-    for (COutPoint &outpt : vOutpts) {
+    for (const COutPoint& outpt : vOutpts) {
         UniValue o(UniValue::VOBJ);
 
         o.pushKV("txid", outpt.hash.GetHex());
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index d0857bcb72..9d33940840 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -1579,7 +1579,7 @@ int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wall
     // Look up the inputs.  We should have already checked that this transaction
     // IsAllFromMe(ISMINE_SPENDABLE), so every input should already be in our
     // wallet, with a valid index into the vout array, and the ability to sign.
-    for (auto& input : tx.vin) {
+    for (const CTxIn& input : tx.vin) {
         const auto mi = wallet->mapWallet.find(input.prevout.hash);
         if (mi == wallet->mapWallet.end()) {
             return -1;
@@ -1819,7 +1819,7 @@ void CWallet::ReacceptWalletTransactions()
     }
 
     // Try to add wallet transactions to memory pool
-    for (std::pair<const int64_t, CWalletTx*>& item : mapSorted) {
+    for (const std::pair<const int64_t, CWalletTx*>& item : mapSorted) {
         CWalletTx& wtx = *(item.second);
         CValidationState state;
         wtx.AcceptToMemoryPool(maxTxFee, state);
@@ -2065,7 +2065,7 @@ std::vector<uint256> CWallet::ResendWalletTransactionsBefore(int64_t nTime, CCon
             continue;
         mapSorted.insert(std::make_pair(wtx.nTimeReceived, &wtx));
     }
-    for (std::pair<const unsigned int, CWalletTx*>& item : mapSorted)
+    for (const std::pair<const unsigned int, CWalletTx*>& item : mapSorted)
     {
         CWalletTx& wtx = *item.second;
         if (wtx.RelayWalletTransaction(connman))
@@ -2368,7 +2368,7 @@ std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins() const
     LOCK2(cs_main, cs_wallet);
     AvailableCoins(availableCoins);
 
-    for (auto& coin : availableCoins) {
+    for (const COutput& coin : availableCoins) {
         CTxDestination address;
         if (coin.fSpendable &&
             ExtractDestination(FindNonChangeParentOutput(*coin.tx->tx, coin.i).scriptPubKey, address)) {
@@ -2378,7 +2378,7 @@ std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins() const
 
     std::vector<COutPoint> lockedCoins;
     ListLockedCoins(lockedCoins);
-    for (const auto& output : lockedCoins) {
+    for (const COutPoint& output : lockedCoins) {
         auto it = mapWallet.find(output.hash);
         if (it != mapWallet.end()) {
             int depth = it->second.GetDepthInMainChain();
@@ -3280,17 +3280,17 @@ bool CWallet::NewKeyPool()
         LOCK(cs_wallet);
         WalletBatch batch(*database);
 
-        for (int64_t nIndex : setInternalKeyPool) {
+        for (const int64_t nIndex : setInternalKeyPool) {
             batch.ErasePool(nIndex);
         }
         setInternalKeyPool.clear();
 
-        for (int64_t nIndex : setExternalKeyPool) {
+        for (const int64_t nIndex : setExternalKeyPool) {
             batch.ErasePool(nIndex);
         }
         setExternalKeyPool.clear();
 
-        for (int64_t nIndex : set_pre_split_keypool) {
+        for (const int64_t nIndex : set_pre_split_keypool) {
             batch.ErasePool(nIndex);
         }
         set_pre_split_keypool.clear();
@@ -3567,7 +3567,7 @@ std::set< std::set<CTxDestination> > CWallet::GetAddressGroupings()
         {
             bool any_mine = false;
             // group all input addresses with each other
-            for (CTxIn txin : pcoin->tx->vin)
+            for (const CTxIn& txin : pcoin->tx->vin)
             {
                 CTxDestination address;
                 if(!IsMine(txin)) /* If this input isn't mine, ignore it */
@@ -3581,7 +3581,7 @@ std::set< std::set<CTxDestination> > CWallet::GetAddressGroupings()
             // group change with input addresses
             if (any_mine)
             {
-               for (CTxOut txout : pcoin->tx->vout)
+               for (const CTxOut& txout : pcoin->tx->vout)
                    if (IsChange(txout))
                    {
                        CTxDestination txoutAddr;
@@ -3617,7 +3617,7 @@ std::set< std::set<CTxDestination> > CWallet::GetAddressGroupings()
         // make a set of all the groups hit by this new group
         std::set< std::set<CTxDestination>* > hits;
         std::map< CTxDestination, std::set<CTxDestination>* >::iterator it;
-        for (CTxDestination address : _grouping)
+        for (const CTxDestination& address : _grouping)
             if ((it = setmap.find(address)) != setmap.end())
                 hits.insert((*it).second);
 
@@ -3632,12 +3632,12 @@ std::set< std::set<CTxDestination> > CWallet::GetAddressGroupings()
         uniqueGroupings.insert(merged);
 
         // update setmap
-        for (CTxDestination element : *merged)
+        for (const CTxDestination& element : *merged)
             setmap[element] = merged;
     }
 
     std::set< std::set<CTxDestination> > ret;
-    for (std::set<CTxDestination>* uniqueGrouping : uniqueGroupings)
+    for (const std::set<CTxDestination>* uniqueGrouping : uniqueGroupings)
     {
         ret.insert(*uniqueGrouping);
         delete uniqueGrouping;
diff --git a/src/wallet/walletdb.cpp b/src/wallet/walletdb.cpp
index 1b787be198..aa125f0292 100644
--- a/src/wallet/walletdb.cpp
+++ b/src/wallet/walletdb.cpp
@@ -612,7 +612,7 @@ DBErrors WalletBatch::LoadWallet(CWallet* pwallet)
     if ((wss.nKeys + wss.nCKeys + wss.nWatchKeys) != wss.nKeyMeta)
         pwallet->UpdateTimeFirstKey(1);
 
-    for (uint256 hash : wss.vWalletUpgrade)
+    for (const uint256& hash : wss.vWalletUpgrade)
         WriteTx(pwallet->mapWallet.at(hash));
 
     // Rewrite encrypted wallets of versions 0.4.0 and 0.5.0rc:
@@ -709,7 +709,7 @@ DBErrors WalletBatch::ZapSelectTx(std::vector<uint256>& vTxHashIn, std::vector<u
     // erase each matching wallet TX
     bool delerror = false;
     std::vector<uint256>::iterator it = vTxHashIn.begin();
-    for (uint256 hash : vTxHash) {
+    for (const uint256& hash : vTxHash) {
         while (it < vTxHashIn.end() && (*it) < hash) {
             it++;
         }
@@ -740,7 +740,7 @@ DBErrors WalletBatch::ZapWalletTx(std::vector<CWalletTx>& vWtx)
         return err;
 
     // erase each wallet TX
-    for (uint256& hash : vTxHash) {
+    for (const uint256& hash : vTxHash) {
         if (!EraseTx(hash))
             return DBErrors::CORRUPT;
     }