Merge #13249: Make objects in range declarations immutable by default. Avoid unnecessary copying of objects in range declarations.

f34c8c466a0e514edac2e8683127b4176ad5d321 Make objects in range declarations immutable by default. Avoid unnecessary copying of objects in range declarations. (practicalswift)

Pull request description:

  Make objects in range declarations immutable by default.

  Rationale:
  * Immutable objects are easier to reason about.
  * Prevents accidental or hard-to-notice change of value.

Tree-SHA512: cad69d35f0cf8a938b848e65dd537c621d96fe3369be306b65ef0cd1baf6cc0a9f28bc230e1e383d810c555a6743d08cb6b2b0bd51856d4611f537a12e5abb8b

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 e9e9cf4c92..c51a1b4a74 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 9f950831c4..b48a3bd221 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 86fdebdbef..632ce0750a 100644
--- a/src/qt/bitcoingui.cpp
+++ b/src/qt/bitcoingui.cpp
@@ -1276,7 +1276,7 @@ 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 f0d767bbfc..165c278ae7 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 9cc4e77768..621b86eec8 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 57f4f7529f..6f4b5ecd26 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 ffedead0e3..9957ac074b 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 8e2f5abe66..c74eb7531a 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 05217149e3..39434e4bb6 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -193,7 +193,7 @@ 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);
@@ -454,7 +454,7 @@ 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);
             }
@@ -899,7 +899,7 @@ 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 2326383586..4940267bae 100644
--- a/src/utilstrencodings.cpp
+++ b/src/utilstrencodings.cpp
@@ -72,7 +72,7 @@ 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 a9eea5edd9..7ec0c6a961 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 31eb1e902f..e419f8d164 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2337,7 +2337,7 @@ 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 6cbfbc1f90..e41f829f02 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -1484,7 +1484,7 @@ 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;
@@ -1723,7 +1723,7 @@ 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);
@@ -1969,7 +1969,7 @@ 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))
@@ -2260,7 +2260,7 @@ std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins() const
 
     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)) {
@@ -2270,7 +2270,7 @@ 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();
@@ -3146,17 +3146,17 @@ 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();
@@ -3433,7 +3433,7 @@ 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 */
@@ -3447,7 +3447,7 @@ 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;
@@ -3483,7 +3483,7 @@ 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);
 
@@ -3498,12 +3498,12 @@ 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 5ce4d080e5..5e85151358 100644
--- a/src/wallet/walletdb.cpp
+++ b/src/wallet/walletdb.cpp
@@ -520,7 +520,7 @@ 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:
@@ -611,7 +611,7 @@ 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++;
         }
@@ -642,7 +642,7 @@ 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;
     }