diff --git a/src/kernel.cpp b/src/kernel.cpp
index 243ad1a00cecb..23441cc20cffb 100644
--- a/src/kernel.cpp
+++ b/src/kernel.cpp
@@ -221,7 +221,7 @@ bool ComputeNextStakeModifier(const CBlockIndex* pindexPrev, uint64_t& nStakeMod
                 strSelectionMap.replace(pindex->nHeight - nHeightFirstCandidate, 1, "=");
             pindex = pindex->pprev;
         }
-        for (const PAIRTYPE(uint256, const CBlockIndex*) & item : mapSelectedBlocks) {
+        for (const PAIRTYPE(const uint256, const CBlockIndex*) & item : mapSelectedBlocks) {
             // 'S' indicates selected proof-of-stake blocks
             // 'W' indicates selected proof-of-work blocks
             strSelectionMap.replace(item.second->nHeight - nHeightFirstCandidate, 1, item.second->IsProofOfStake() ? "S" : "W");
diff --git a/src/main.cpp b/src/main.cpp
index ff79eb0880d70..4133a2f53c18e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1227,7 +1227,7 @@ std::list<libzerocoin::CoinDenomination> ZerocoinSpendListFromBlock(const CBlock
         if (!tx.IsZerocoinSpend())
             continue;
 
-        for (const CTxIn txin : tx.vin) {
+        for (const CTxIn & txin : tx.vin) {
             if (!txin.scriptSig.IsZerocoinSpend())
                 continue;
 
@@ -1424,7 +1424,7 @@ bool CheckTransaction(const CTransaction& tx, bool fZerocoinActive, bool fReject
 
         if (tx.IsZerocoinSpend()) {
             //require that a zerocoinspend only has inputs that are zerocoins
-            for (const CTxIn in : tx.vin) {
+            for (const CTxIn & in : tx.vin) {
                 if (!in.scriptSig.IsZerocoinSpend())
                     return state.DoS(100,
                                      error("CheckTransaction() : zerocoinspend contains inputs that are not zerocoins"));
@@ -1627,7 +1627,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState& state, const CTransa
             // do all inputs exist?
             // Note that this does not check for the presence of actual outputs (see the next check for that),
             // only helps filling in pfMissingInputs (to determine missing vs spent).
-            for (const CTxIn txin : tx.vin) {
+            for (const CTxIn & txin : tx.vin) {
                 if (!view.HaveCoins(txin.prevout.hash)) {
                     if (pfMissingInputs)
                         *pfMissingInputs = true;
@@ -1904,7 +1904,7 @@ bool AcceptableInputs(CTxMemPool& pool, CValidationState& state, const CTransact
             // do all inputs exist?
             // Note that this does not check for the presence of actual outputs (see the next check for that),
             // only helps filling in pfMissingInputs (to determine missing vs spent).
-            for (const CTxIn txin : tx.vin) {
+            for (const CTxIn & txin : tx.vin) {
                 if (!view.HaveCoins(txin.prevout.hash)) {
                     if (pfMissingInputs)
                         *pfMissingInputs = true;
@@ -2536,7 +2536,7 @@ bool DisconnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex
         if (tx.ContainsZerocoins()) {
             if (tx.IsZerocoinSpend()) {
                 //erase all zerocoinspends in this transaction
-                for (const CTxIn txin : tx.vin) {
+                for (const CTxIn & txin : tx.vin) {
                     if (txin.scriptSig.IsZerocoinSpend()) {
                         CoinSpend spend = TxInToZerocoinSpend(txin);
                         if (!zerocoinDB->EraseCoinSpend(spend.getCoinSerialNumber()))
@@ -3267,7 +3267,7 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
     for (const CTransaction& tx: block.vtx) {
         if (tx.IsCoinBase() || tx.IsZerocoinSpend())
             continue;
-        for (const CTxIn in: tx.vin) {
+        for (const CTxIn & in: tx.vin) {
             LogPrint("map", "mapStakeSpent: Insert %s | %u\n", in.prevout.ToString(), pindex->nHeight);
             mapStakeSpent.insert(std::make_pair(in.prevout, pindex->nHeight));
         }
@@ -4424,7 +4424,7 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
 
             // double check that there are no double spent zPHR spends in this block
             if (tx.IsZerocoinSpend()) {
-                for (const CTxIn txIn : tx.vin) {
+                for (const CTxIn & txIn : tx.vin) {
                     if (txIn.scriptSig.IsZerocoinSpend()) {
                         libzerocoin::CoinSpend spend = TxInToZerocoinSpend(txIn);
                         if (count(vBlockSerials.begin(), vBlockSerials.end(), spend.getCoinSerialNumber()))
@@ -6176,7 +6176,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             GetMainSignals().Inventory(inv.hash);
 
             if (pfrom->nSendSize > (SendBufferSize() * 2)) {
-                LOCK(cs_main);
                 return error("send buffer size() = %u", pfrom->nSendSize);
             }
         }
@@ -6477,7 +6476,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         for (const CBlockHeader& header : headers) {
             CValidationState state;
             if (pindexLast != NULL && header.hashPrevBlock != pindexLast->GetBlockHash()) {
-                LOCK(cs_main);
                 Misbehaving(pfrom->GetId(), 20);
                 return error("non-continuous headers sequence");
             }
diff --git a/src/masternode-budget.cpp b/src/masternode-budget.cpp
index 787d3abd0e0bc..a0ddb5e5533f4 100644
--- a/src/masternode-budget.cpp
+++ b/src/masternode-budget.cpp
@@ -59,7 +59,7 @@ bool IsBudgetCollateralValid(uint256 nTxCollateralHash, uint256 nExpectedHash, s
     findScript << OP_RETURN << ToByteVector(nExpectedHash);
 
     bool foundOpReturn = false;
-    for (const CTxOut o : txCollateral.vout) {
+    for (const CTxOut & o : txCollateral.vout) {
         if (!o.scriptPubKey.IsNormalPaymentScript() && !o.scriptPubKey.IsUnspendable()) {
             strError = strprintf("Invalid Script %s", txCollateral.ToString());
             LogPrint("mnbudget","CBudgetProposalBroadcast::IsBudgetCollateralValid - %s\n", strError);
diff --git a/src/obfuscation.cpp b/src/obfuscation.cpp
index f060b308247f2..12d57f402232b 100644
--- a/src/obfuscation.cpp
+++ b/src/obfuscation.cpp
@@ -204,7 +204,7 @@ void CObfuscationPool::ProcessMessageObfuscation(CNode* pfrom, std::string& strC
             CValidationState state;
             CMutableTransaction tx;
 
-            for (const CTxOut o : out) {
+            for (const CTxOut & o : out) {
                 nValueOut += o.nValue;
                 tx.vout.push_back(o);
 
@@ -222,7 +222,7 @@ void CObfuscationPool::ProcessMessageObfuscation(CNode* pfrom, std::string& strC
                 }
             }
 
-            for (const CTxIn i : in) {
+            for (const CTxIn & i : in) {
                 tx.vin.push_back(i);
 
                 LogPrint("obfuscation", "dsi -- tx in %s\n", i.ToString());
@@ -318,7 +318,7 @@ void CObfuscationPool::ProcessMessageObfuscation(CNode* pfrom, std::string& strC
         bool success = false;
         int count = 0;
 
-        for (const CTxIn item : sigs) {
+        for (const CTxIn & item : sigs) {
             if (AddScriptSig(item)) success = true;
             LogPrint("obfuscation", " -- sigs count %d %d\n", (int)sigs.size(), count);
             count++;
@@ -690,8 +690,8 @@ void CObfuscationPool::ChargeFees()
 
     if (state == POOL_STATUS_SIGNING) {
         // who didn't sign?
-        for (const CObfuScationEntry v : entries) {
-            for (const CTxDSIn s : v.sev) {
+        for (const CObfuScationEntry & v : entries) {
+            for (const CTxDSIn & s : v.sev) {
                 if (!s.fHasSig) {
                     LogPrintf("CObfuscationPool::ChargeFees -- found uncooperative node (didn't sign). Found offence\n");
                     offences++;
@@ -743,8 +743,8 @@ void CObfuscationPool::ChargeFees()
 
     if (state == POOL_STATUS_SIGNING) {
         // who didn't sign?
-        for (const CObfuScationEntry v : entries) {
-            for (const CTxDSIn s : v.sev) {
+        for (const CObfuScationEntry & v : entries) {
+            for (const CTxDSIn & s : v.sev) {
                 if (!s.fHasSig && r > target) {
                     LogPrintf("CObfuscationPool::ChargeFees -- found uncooperative node (didn't sign). charging fees.\n");
 
@@ -960,7 +960,7 @@ bool CObfuscationPool::IsCollateralValid(const CTransaction& txCollateral)
     int64_t nValueOut = 0;
     bool missingTx = false;
 
-    for (const CTxOut o : txCollateral.vout) {
+    for (const CTxOut & o : txCollateral.vout) {
         nValueOut += o.nValue;
 
         if (!o.scriptPubKey.IsNormalPaymentScript()) {
@@ -969,7 +969,7 @@ bool CObfuscationPool::IsCollateralValid(const CTransaction& txCollateral)
         }
     }
 
-    for (const CTxIn i : txCollateral.vin) {
+    for (const CTxIn & i : txCollateral.vin) {
         CTransaction tx2;
         uint256 hash;
         if (GetTransaction(i.prevout.hash, tx2, hash, true)) {
@@ -1268,8 +1268,8 @@ bool CObfuscationPool::SignFinalTransaction(CTransaction& finalTransactionNew, C
     vector<CTxIn> sigs;
 
     //make sure my inputs/outputs are present, otherwise refuse to sign
-    for (const CObfuScationEntry e : entries) {
-        for (const CTxDSIn s : e.sev) {
+    for (const CObfuScationEntry & e : entries) {
+        for (const CTxDSIn & s : e.sev) {
             /* Sign my transaction and all outputs */
             int mine = -1;
             CScript prevPubKey = CScript();
@@ -1298,7 +1298,7 @@ bool CObfuscationPool::SignFinalTransaction(CTransaction& finalTransactionNew, C
                     }
                 }
 
-                for (const CTxOut o : e.vout)
+                for (const CTxOut & o : e.vout)
                     nValue2 += o.nValue;
 
                 int targetOuputs = e.vout.size();
@@ -1845,7 +1845,7 @@ bool CObfuscationPool::IsCompatibleWithEntries(std::vector<CTxOut>& vout)
 {
     if (GetDenominations(vout) == 0) return false;
 
-    for (const CObfuScationEntry v : entries) {
+    for (const CObfuScationEntry & v : entries) {
         LogPrintf(" IsCompatibleWithEntries %d %d\n", GetDenominations(vout), GetDenominations(v.vout));
         /*
         for (CTxOut o1 : vout)
diff --git a/src/qt/clientmodel.cpp b/src/qt/clientmodel.cpp
index 17aa3b4284224..f54c7935e09c4 100644
--- a/src/qt/clientmodel.cpp
+++ b/src/qt/clientmodel.cpp
@@ -307,7 +307,7 @@ bool ClientModel::getTorInfo(std::string& ip_port) const
     if (GetProxy((Network) 3, onion) && IsReachable((Network) 3)) {
         {
             LOCK(cs_mapLocalHost);
-            for (const std::pair<CNetAddr, LocalServiceInfo> &item : mapLocalHost) {
+            for (const std::pair<const CNetAddr, LocalServiceInfo> &item : mapLocalHost) {
                 if (item.first.IsTor()) {
                      CService addrOnion = CService(item.first.ToString(), item.second.nPort);
                      ip_port = addrOnion.ToStringIPPort();
@@ -317,4 +317,4 @@ bool ClientModel::getTorInfo(std::string& ip_port) const
         }
     }
     return false;
-}
\ No newline at end of file
+}
diff --git a/src/rpc/net.cpp b/src/rpc/net.cpp
index a5d45748bf834..593242617501f 100644
--- a/src/rpc/net.cpp
+++ b/src/rpc/net.cpp
@@ -423,7 +423,7 @@ UniValue getnetworkinfo(const UniValue& params, bool fHelp)
     UniValue localAddresses(UniValue::VARR);
     {
         LOCK(cs_mapLocalHost);
-        for (const PAIRTYPE(CNetAddr, LocalServiceInfo) & item : mapLocalHost) {
+        for (const PAIRTYPE(const CNetAddr, LocalServiceInfo) & item : mapLocalHost) {
             UniValue rec(UniValue::VOBJ);
             rec.push_back(Pair("address", item.first.ToString()));
             rec.push_back(Pair("port", item.second.nPort));
diff --git a/src/swifttx.cpp b/src/swifttx.cpp
index 7e36b9ac214ef..22794d1ce9cc8 100644
--- a/src/swifttx.cpp
+++ b/src/swifttx.cpp
@@ -62,7 +62,7 @@ void ProcessMessageSwiftTX(CNode* pfrom, std::string& strCommand, CDataStream& v
             return;
         }
 
-        for (const CTxOut o : tx.vout) {
+        for (const CTxOut & o : tx.vout) {
             // IX supports normal scripts and unspendable scripts (used in DS collateral and Budget collateral).
             // TODO: Look into other script types that are normal and can be included
             if (!o.scriptPubKey.IsNormalPaymentScript() && !o.scriptPubKey.IsUnspendable()) {
@@ -186,10 +186,10 @@ bool IsIXTXValid(const CTransaction& txCollateral)
     CAmount nValueOut = 0;
     bool missingTx = false;
 
-    for (const CTxOut o : txCollateral.vout)
+    for (const CTxOut & o : txCollateral.vout)
         nValueOut += o.nValue;
 
-    for (const CTxIn i : txCollateral.vin) {
+    for (const CTxIn & i : txCollateral.vin) {
         CTransaction tx2;
         uint256 hash;
         if (GetTransaction(i.prevout.hash, tx2, hash, true)) {
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 100ebf1f0a2ed..5f584ce14b9d8 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -592,7 +592,7 @@ void CTxMemPool::check(const CCoinsViewCache* pcoins) const
         else {
             CValidationState state;
             CTxUndo undo;
-            assert(CheckInputs(tx, state, mempoolDuplicate, false, 0, false, NULL));
+            assert(CheckInputs(tx, state, mempoolDuplicate, false, 0, false, false));
             UpdateCoins(tx, state, mempoolDuplicate, undo, 1000000);
         }
     }
@@ -606,7 +606,7 @@ void CTxMemPool::check(const CCoinsViewCache* pcoins) const
             stepsSinceLastRemove++;
             assert(stepsSinceLastRemove < waitingOnDependants.size());
         } else {
-            assert(CheckInputs(entry->GetTx(), state, mempoolDuplicate, false, 0, false, NULL));
+            assert(CheckInputs(entry->GetTx(), state, mempoolDuplicate, false, 0, false, false));
             CTxUndo undo;
             UpdateCoins(entry->GetTx(), state, mempoolDuplicate, undo, 1000000);
             stepsSinceLastRemove = 0;
diff --git a/src/wallet/crypter.cpp b/src/wallet/crypter.cpp
index 00862ef0a634f..0cfe7ae8aa13a 100644
--- a/src/wallet/crypter.cpp
+++ b/src/wallet/crypter.cpp
@@ -167,7 +167,7 @@ bool EncryptAES256(const SecureString& sKey, const SecureString& sPlaintext, con
 
     // Perform the encryption
 #if OPENSSL_VERSION_NUMBER >= 0x10100000L
-    EVP_CIPHER_CTX* ctx;
+    EVP_CIPHER_CTX* ctx = nullptr;
 #else
     EVP_CIPHER_CTX ctx;
 #endif