diff --git a/src/addrdb.h b/src/addrdb.h
--- a/src/addrdb.h
+++ b/src/addrdb.h
@@ -33,7 +33,7 @@
 
     CBanEntry() { SetNull(); }
 
-    CBanEntry(int64_t nCreateTimeIn) {
+    explicit CBanEntry(int64_t nCreateTimeIn) {
         SetNull();
         nCreateTime = nCreateTimeIn;
     }
diff --git a/src/base58.cpp b/src/base58.cpp
--- a/src/base58.cpp
+++ b/src/base58.cpp
@@ -214,7 +214,8 @@
     const CChainParams &m_params;
 
 public:
-    DestinationEncoder(const CChainParams &params) : m_params(params) {}
+    explicit DestinationEncoder(const CChainParams &params)
+        : m_params(params) {}
 
     std::string operator()(const CKeyID &id) const {
         std::vector<uint8_t> data =
diff --git a/src/bench/checkqueue.cpp b/src/bench/checkqueue.cpp
--- a/src/bench/checkqueue.cpp
+++ b/src/bench/checkqueue.cpp
@@ -26,7 +26,7 @@
     struct PrevectorJob {
         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()() { return true; }
diff --git a/src/blockencodings.h b/src/blockencodings.h
--- a/src/blockencodings.h
+++ b/src/blockencodings.h
@@ -18,7 +18,7 @@
     CTransactionRef &tx;
 
 public:
-    TransactionCompressor(CTransactionRef &txIn) : tx(txIn) {}
+    explicit TransactionCompressor(CTransactionRef &txIn) : tx(txIn) {}
 
     ADD_SERIALIZE_METHODS;
 
@@ -84,7 +84,7 @@
     std::vector<CTransactionRef> txn;
 
     BlockTransactions() {}
-    BlockTransactions(const BlockTransactionsRequest &req)
+    explicit BlockTransactions(const BlockTransactionsRequest &req)
         : blockhash(req.blockhash), txn(req.indices.size()) {}
 
     ADD_SERIALIZE_METHODS;
diff --git a/src/chain.h b/src/chain.h
--- a/src/chain.h
+++ b/src/chain.h
@@ -124,7 +124,7 @@
 
     CBlockIndex() { SetNull(); }
 
-    CBlockIndex(const CBlockHeader &block) {
+    explicit CBlockIndex(const CBlockHeader &block) {
         SetNull();
 
         nVersion = block.nVersion;
diff --git a/src/compressor.h b/src/compressor.h
--- a/src/compressor.h
+++ b/src/compressor.h
@@ -55,7 +55,7 @@
     bool Decompress(unsigned int nSize, const std::vector<uint8_t> &out);
 
 public:
-    CScriptCompressor(CScript &scriptIn) : script(scriptIn) {}
+    explicit CScriptCompressor(CScript &scriptIn) : script(scriptIn) {}
 
     template <typename Stream> void Serialize(Stream &s) const {
         std::vector<uint8_t> compr;
@@ -98,7 +98,7 @@
     static uint64_t CompressAmount(Amount nAmount);
     static Amount DecompressAmount(uint64_t nAmount);
 
-    CTxOutCompressor(CTxOut &txoutIn) : txout(txoutIn) {}
+    explicit CTxOutCompressor(CTxOut &txoutIn) : txout(txoutIn) {}
 
     ADD_SERIALIZE_METHODS;
 
diff --git a/src/cuckoocache.h b/src/cuckoocache.h
--- a/src/cuckoocache.h
+++ b/src/cuckoocache.h
@@ -55,7 +55,7 @@
      * @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;
         mem.reset(new std::atomic<uint8_t>[size]);
diff --git a/src/dbwrapper.h b/src/dbwrapper.h
--- a/src/dbwrapper.h
+++ b/src/dbwrapper.h
@@ -21,7 +21,8 @@
 
 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 +62,7 @@
     /**
      * @param[in] _parent   CDBWrapper that this batch is to be submitted to
      */
-    CDBBatch(const CDBWrapper &_parent)
+    explicit CDBBatch(const CDBWrapper &_parent)
         : parent(_parent), ssKey(SER_DISK, CLIENT_VERSION),
           ssValue(SER_DISK, CLIENT_VERSION), size_estimate(0){};
 
diff --git a/src/hash.h b/src/hash.h
--- a/src/hash.h
+++ b/src/hash.h
@@ -173,7 +173,7 @@
     Source *source;
 
 public:
-    CHashVerifier(Source *source_)
+    explicit CHashVerifier(Source *source_)
         : CHashWriter(source_->GetType(), source_->GetVersion()),
           source(source_) {}
 
diff --git a/src/httprpc.cpp b/src/httprpc.cpp
--- a/src/httprpc.cpp
+++ b/src/httprpc.cpp
@@ -48,7 +48,7 @@
 
 class HTTPRPCTimerInterface : public RPCTimerInterface {
 public:
-    HTTPRPCTimerInterface(struct event_base *_base) : base(_base) {}
+    explicit HTTPRPCTimerInterface(struct event_base *_base) : base(_base) {}
 
     const char *Name() override { return "HTTP"; }
 
diff --git a/src/httpserver.h b/src/httpserver.h
--- a/src/httpserver.h
+++ b/src/httpserver.h
@@ -72,7 +72,7 @@
     bool replySent;
 
 public:
-    HTTPRequest(struct evhttp_request *req);
+    explicit HTTPRequest(struct evhttp_request *req);
     ~HTTPRequest();
 
     enum RequestMethod { UNKNOWN, GET, POST, HEAD, PUT, OPTIONS };
diff --git a/src/httpserver.cpp b/src/httpserver.cpp
--- a/src/httpserver.cpp
+++ b/src/httpserver.cpp
@@ -82,7 +82,7 @@
     class ThreadCounter {
     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;
         }
@@ -94,7 +94,7 @@
     };
 
 public:
-    WorkQueue(size_t _maxDepth)
+    explicit WorkQueue(size_t _maxDepth)
         : running(true), maxDepth(_maxDepth), numThreads(0) {}
     /** Precondition: worker threads have all stopped (call WaitExit) */
     ~WorkQueue() {}
diff --git a/src/init.cpp b/src/init.cpp
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -134,7 +134,8 @@
  */
 class CCoinsViewErrorCatcher final : 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
--- a/src/limitedmap.h
+++ b/src/limitedmap.h
@@ -27,7 +27,7 @@
     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
--- a/src/miner.h
+++ b/src/miner.h
@@ -42,7 +42,7 @@
 // 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();
         nBillableSizeWithAncestors = entry->GetBillableSizeWithAncestors();
@@ -124,7 +124,7 @@
     modtxscoreiter;
 
 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) {
         e.nModFeesWithAncestors -= iter->GetFee();
diff --git a/src/netaddress.h b/src/netaddress.h
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -36,7 +36,7 @@
 
 public:
     CNetAddr();
-    CNetAddr(const struct in_addr &ipv4Addr);
+    explicit CNetAddr(const struct in_addr &ipv4Addr);
     void SetIP(const CNetAddr &ip);
 
 private:
@@ -100,7 +100,8 @@
     std::vector<uint8_t> 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);
@@ -167,7 +168,7 @@
     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);
     unsigned short GetPort() const;
     bool GetSockAddr(struct sockaddr *paddr, socklen_t *addrlen) const;
     bool SetSockAddr(const struct sockaddr *paddr);
@@ -182,7 +183,7 @@
     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/netmessagemaker.h b/src/netmessagemaker.h
--- a/src/netmessagemaker.h
+++ b/src/netmessagemaker.h
@@ -11,7 +11,7 @@
 
 class CNetMsgMaker {
 public:
-    CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn) {}
+    explicit CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn) {}
 
     template <typename... Args>
     CSerializedNetMsg Make(int nFlags, std::string sCommand,
diff --git a/src/policy/fees.h b/src/policy/fees.h
--- a/src/policy/fees.h
+++ b/src/policy/fees.h
@@ -275,7 +275,7 @@
 class FeeFilterRounder {
 public:
     /** Create new FeeFilterRounder */
-    FeeFilterRounder(const CFeeRate &minIncrementalFee);
+    explicit FeeFilterRounder(const CFeeRate &minIncrementalFee);
 
     /** Quantize a minimum fee for privacy purpose before broadcast **/
     Amount round(const Amount currentMinFee);
diff --git a/src/primitives/block.h b/src/primitives/block.h
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -111,7 +111,8 @@
 
     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/primitives/transaction.h b/src/primitives/transaction.h
--- a/src/primitives/transaction.h
+++ b/src/primitives/transaction.h
@@ -385,7 +385,7 @@
         : hashPrevouts(txdata.hashPrevouts), hashSequence(txdata.hashSequence),
           hashOutputs(txdata.hashOutputs) {}
 
-    PrecomputedTransactionData(const CTransaction &tx);
+    explicit PrecomputedTransactionData(const CTransaction &tx);
 };
 
 #endif // BITCOIN_PRIMITIVES_TRANSACTION_H
diff --git a/src/protocol.h b/src/protocol.h
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -49,7 +49,7 @@
     };
     typedef std::array<uint8_t, MESSAGE_START_SIZE> MessageMagic;
 
-    CMessageHeader(const MessageMagic &pchMessageStartIn);
+    explicit CMessageHeader(const MessageMagic &pchMessageStartIn);
     CMessageHeader(const MessageMagic &pchMessageStartIn,
                    const char *pszCommand, unsigned int nMessageSizeIn);
 
diff --git a/src/pubkey.h b/src/pubkey.h
--- a/src/pubkey.h
+++ b/src/pubkey.h
@@ -31,7 +31,7 @@
 class CKeyID : public uint160 {
 public:
     CKeyID() : uint160() {}
-    CKeyID(const uint160 &in) : uint160(in) {}
+    explicit CKeyID(const uint160 &in) : uint160(in) {}
 };
 
 typedef uint256 ChainCode;
@@ -74,7 +74,9 @@
     }
 
     //! Construct a public key from a byte vector.
-    CPubKey(const std::vector<uint8_t> &_vch) { Set(_vch.begin(), _vch.end()); }
+    explicit CPubKey(const std::vector<uint8_t> &_vch) {
+        Set(_vch.begin(), _vch.end());
+    }
 
     //! Simple read-only vector-like interface to the pubkey data.
     unsigned int size() const { return GetLen(vch[0]); }
diff --git a/src/qt/coincontroldialog.h b/src/qt/coincontroldialog.h
--- a/src/qt/coincontroldialog.h
+++ b/src/qt/coincontroldialog.h
@@ -30,10 +30,10 @@
 
 class CCoinControlWidgetItem : public QTreeWidgetItem {
 public:
-    CCoinControlWidgetItem(QTreeWidget *parent, int type = Type)
+    explicit CCoinControlWidgetItem(QTreeWidget *parent, int type = Type)
         : QTreeWidgetItem(parent, type) {}
-    CCoinControlWidgetItem(int type = Type) : QTreeWidgetItem(type) {}
-    CCoinControlWidgetItem(QTreeWidgetItem *parent, int type = Type)
+    explicit CCoinControlWidgetItem(int type = Type) : QTreeWidgetItem(type) {}
+    explicit CCoinControlWidgetItem(QTreeWidgetItem *parent, int type = Type)
         : QTreeWidgetItem(parent, type) {}
 
     bool operator<(const QTreeWidgetItem &other) const override;
diff --git a/src/qt/intro.cpp b/src/qt/intro.cpp
--- a/src/qt/intro.cpp
+++ b/src/qt/intro.cpp
@@ -50,7 +50,7 @@
     Q_OBJECT
 
 public:
-    FreespaceChecker(Intro *intro);
+    explicit FreespaceChecker(Intro *intro);
 
     enum Status { ST_OK, ST_ERROR };
 
diff --git a/src/qt/notificator.cpp b/src/qt/notificator.cpp
--- a/src/qt/notificator.cpp
+++ b/src/qt/notificator.cpp
@@ -74,7 +74,7 @@
 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
--- a/src/qt/overviewpage.cpp
+++ b/src/qt/overviewpage.cpp
@@ -24,8 +24,8 @@
 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::BCH),
           platformStyle(_platformStyle) {}
 
diff --git a/src/qt/paymentrequestplus.cpp b/src/qt/paymentrequestplus.cpp
--- a/src/qt/paymentrequestplus.cpp
+++ b/src/qt/paymentrequestplus.cpp
@@ -20,7 +20,7 @@
 
 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
--- a/src/qt/paymentserver.h
+++ b/src/qt/paymentserver.h
@@ -66,7 +66,7 @@
     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) to read from
diff --git a/src/qt/utilitydialog.h b/src/qt/utilitydialog.h
--- a/src/qt/utilitydialog.h
+++ b/src/qt/utilitydialog.h
@@ -39,7 +39,7 @@
     Q_OBJECT
 
 public:
-    ShutdownWindow(QWidget *parent = 0, Qt::WindowFlags f = 0);
+    explicit ShutdownWindow(QWidget *parent = nullptr, Qt::WindowFlags f = 0);
     static QWidget *showShutdownWindow(BitcoinGUI *window);
 
 protected:
diff --git a/src/rest.cpp b/src/rest.cpp
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -45,7 +45,8 @@
     CTxOut out;
 
     CCoin() : nHeight(0) {}
-    CCoin(Coin in) : nHeight(in.GetHeight()), out(std::move(in.GetTxOut())) {}
+    explicit CCoin(Coin in)
+        : nHeight(in.GetHeight()), out(std::move(in.GetTxOut())) {}
 
     ADD_SERIALIZE_METHODS;
 
diff --git a/src/reverse_iterator.h b/src/reverse_iterator.h
--- a/src/reverse_iterator.h
+++ b/src/reverse_iterator.h
@@ -15,7 +15,7 @@
     T &x;
 
 public:
-    reverse_range(T &xin) : x(xin) {}
+    explicit reverse_range(T &xin) : x(xin) {}
 
     auto begin() const -> decltype(this->x.rbegin()) { return x.rbegin(); }
 
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -685,7 +685,7 @@
     bool found;
     CValidationState state;
 
-    submitblock_StateCatcher(const uint256 &hashIn)
+    explicit submitblock_StateCatcher(const uint256 &hashIn)
         : hash(hashIn), found(false), state() {}
 
 protected:
diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -140,7 +140,7 @@
 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
--- a/src/rpc/server.h
+++ b/src/rpc/server.h
@@ -40,7 +40,8 @@
  * 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.h b/src/scheduler.h
--- a/src/scheduler.h
+++ b/src/scheduler.h
@@ -107,7 +107,7 @@
     void ProcessQueue();
 
 public:
-    SingleThreadedSchedulerClient(CScheduler *pschedulerIn)
+    explicit SingleThreadedSchedulerClient(CScheduler *pschedulerIn)
         : m_pscheduler(pschedulerIn) {}
     void AddToProcessQueue(std::function<void(void)> func);
 
diff --git a/src/scheduler.cpp b/src/scheduler.cpp
--- a/src/scheduler.cpp
+++ b/src/scheduler.cpp
@@ -181,7 +181,7 @@
     // to ensure both happen safely even if callback() throws.
     struct RAIICallbacksRunning {
         SingleThreadedSchedulerClient *instance;
-        RAIICallbacksRunning(SingleThreadedSchedulerClient *_instance)
+        explicit RAIICallbacksRunning(SingleThreadedSchedulerClient *_instance)
             : instance(_instance) {}
         ~RAIICallbacksRunning() {
             {
diff --git a/src/script/sign.h b/src/script/sign.h
--- a/src/script/sign.h
+++ b/src/script/sign.h
@@ -21,7 +21,8 @@
     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;
@@ -66,7 +67,7 @@
 /** A signature creator that just produces 72-byte empty signatures. */
 class DummySignatureCreator : public BaseSignatureCreator {
 public:
-    DummySignatureCreator(const CKeyStore *keystoreIn)
+    explicit DummySignatureCreator(const CKeyStore *keystoreIn)
         : BaseSignatureCreator(keystoreIn) {}
     const BaseSignatureChecker &Checker() const override;
     bool CreateSig(std::vector<uint8_t> &vchSig, const CKeyID &keyid,
diff --git a/src/script/standard.cpp b/src/script/standard.cpp
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -241,7 +241,7 @@
     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
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -412,7 +412,7 @@
     I &n;
 
 public:
-    CVarInt(I &nIn) : n(nIn) {}
+    explicit CVarInt(I &nIn) : n(nIn) {}
 
     template <typename Stream> void Serialize(Stream &s) const {
         WriteVarInt<Stream, I>(s, n);
@@ -428,7 +428,7 @@
     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 {
         WriteCompactSize<Stream>(s, n);
@@ -444,7 +444,7 @@
     std::string &string;
 
 public:
-    LimitedString(std::string &_string) : string(_string) {}
+    explicit LimitedString(std::string &_string) : string(_string) {}
 
     template <typename Stream> void Unserialize(Stream &s) {
         size_t size = ReadCompactSize(s);
diff --git a/src/support/lockedpool.h b/src/support/lockedpool.h
--- a/src/support/lockedpool.h
+++ b/src/support/lockedpool.h
@@ -161,8 +161,8 @@
      * 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();
 
     /**
@@ -235,7 +235,7 @@
     }
 
 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/test/base58_tests.cpp b/src/test/base58_tests.cpp
--- a/src/test/base58_tests.cpp
+++ b/src/test/base58_tests.cpp
@@ -87,7 +87,7 @@
     std::string exp_addrType;
 
 public:
-    TestAddrTypeVisitor(const std::string &_exp_addrType)
+    explicit TestAddrTypeVisitor(const std::string &_exp_addrType)
         : exp_addrType(_exp_addrType) {}
     bool operator()(const CKeyID &id) const {
         return (exp_addrType == "pubkey");
@@ -106,7 +106,7 @@
     std::vector<uint8_t> exp_payload;
 
 public:
-    TestPayloadVisitor(std::vector<uint8_t> &_exp_payload)
+    explicit TestPayloadVisitor(std::vector<uint8_t> &_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
--- a/src/test/bip32_tests.cpp
+++ b/src/test/bip32_tests.cpp
@@ -24,7 +24,8 @@
     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) {
diff --git a/src/test/blockencodings_tests.cpp b/src/test/blockencodings_tests.cpp
--- a/src/test/blockencodings_tests.cpp
+++ b/src/test/blockencodings_tests.cpp
@@ -133,12 +133,12 @@
     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)) {}
 
     uint64_t GetShortID(const uint256 &txhash) const {
diff --git a/src/test/coins_tests.cpp b/src/test/coins_tests.cpp
--- a/src/test/coins_tests.cpp
+++ b/src/test/coins_tests.cpp
@@ -76,7 +76,7 @@
 
 class CCoinsViewCacheTest : public CCoinsViewCache {
 public:
-    CCoinsViewCacheTest(CCoinsView *_base) : CCoinsViewCache(_base) {}
+    explicit CCoinsViewCacheTest(CCoinsView *_base) : CCoinsViewCache(_base) {}
 
     void SelfTest() const {
         // Manually recompute the dynamic usage of the whole data, and compare
diff --git a/src/test/dbwrapper_tests.cpp b/src/test/dbwrapper_tests.cpp
--- a/src/test/dbwrapper_tests.cpp
+++ b/src/test/dbwrapper_tests.cpp
@@ -229,7 +229,7 @@
     // different lengths. 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
--- a/src/test/test_bitcoin.h
+++ b/src/test/test_bitcoin.h
@@ -54,7 +54,8 @@
 struct BasicTestingSetup {
     ECCVerifyHandle globalVerifyHandle;
 
-    BasicTestingSetup(const std::string &chainName = CBaseChainParams::MAIN);
+    explicit BasicTestingSetup(
+        const std::string &chainName = CBaseChainParams::MAIN);
     ~BasicTestingSetup();
 };
 
@@ -76,7 +77,8 @@
     CScheduler scheduler;
     std::unique_ptr<PeerLogicValidation> peerLogic;
 
-    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
--- a/src/tinyformat.h
+++ b/src/tinyformat.h
@@ -554,7 +554,7 @@
         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>) {}
 
@@ -905,7 +905,7 @@
     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");
@@ -915,7 +915,8 @@
 #define TINYFORMAT_MAKE_FORMATLIST_CONSTRUCTOR(n)                              \
                                                                                \
     template <TINYFORMAT_ARGTYPES(n)>                                          \
-    FormatListN(TINYFORMAT_VARARGS(n)) : FormatList(&m_formatterStore[0], 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
--- a/src/torcontrol.cpp
+++ b/src/torcontrol.cpp
@@ -79,7 +79,7 @@
 
     /** Create a new TorControlConnection.
      */
-    TorControlConnection(struct event_base *base);
+    explicit TorControlConnection(struct event_base *base);
     ~TorControlConnection();
 
     /**
diff --git a/src/txdb.h b/src/txdb.h
--- a/src/txdb.h
+++ b/src/txdb.h
@@ -70,7 +70,8 @@
     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 +110,8 @@
 /** Access to the block database (blocks/index/) */
 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 &);
diff --git a/src/txdb.cpp b/src/txdb.cpp
--- a/src/txdb.cpp
+++ b/src/txdb.cpp
@@ -35,7 +35,7 @@
 struct CoinEntry {
     COutPoint *outpoint;
     char key;
-    CoinEntry(const COutPoint *ptr)
+    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/txmempool.h b/src/txmempool.h
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -234,7 +234,7 @@
 };
 
 struct update_fee_delta {
-    update_fee_delta(Amount _feeDelta) : feeDelta(_feeDelta) {}
+    explicit update_fee_delta(Amount _feeDelta) : feeDelta(_feeDelta) {}
 
     void operator()(CTxMemPoolEntry &e) { e.UpdateFeeDelta(feeDelta); }
 
@@ -243,7 +243,7 @@
 };
 
 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); }
 
diff --git a/src/uint256.h b/src/uint256.h
--- a/src/uint256.h
+++ b/src/uint256.h
@@ -111,7 +111,7 @@
 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<uint8_t> &vch) : base_blob<160>(vch) {}
 };
 
@@ -124,7 +124,7 @@
 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<uint8_t> &vch) : base_blob<256>(vch) {}
 
     /**
diff --git a/src/undo.h b/src/undo.h
--- a/src/undo.h
+++ b/src/undo.h
@@ -29,7 +29,7 @@
     const Coin *pcoin;
 
 public:
-    TxInUndoSerializer(const Coin *pcoinIn) : pcoin(pcoinIn) {}
+    explicit TxInUndoSerializer(const Coin *pcoinIn) : pcoin(pcoinIn) {}
 
     template <typename Stream> void Serialize(Stream &s) const {
         ::Serialize(
@@ -46,7 +46,7 @@
     Coin *pcoin;
 
 public:
-    TxInUndoDeserializer(Coin *pcoinIn) : pcoin(pcoinIn) {}
+    explicit TxInUndoDeserializer(Coin *pcoinIn) : pcoin(pcoinIn) {}
 
     template <typename Stream> void Unserialize(Stream &s) {
         uint32_t nCode = 0;
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2276,7 +2276,7 @@
     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
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -41,7 +41,7 @@
     // our own queue here :(
     SingleThreadedSchedulerClient m_schedulerClient;
 
-    MainSignalsInstance(CScheduler *pscheduler)
+    explicit MainSignalsInstance(CScheduler *pscheduler)
         : m_schedulerClient(pscheduler) {}
 };
 
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -173,7 +173,8 @@
     }
 
 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/test/wallet_test_fixture.h b/src/wallet/test/wallet_test_fixture.h
--- a/src/wallet/test/wallet_test_fixture.h
+++ b/src/wallet/test/wallet_test_fixture.h
@@ -11,7 +11,8 @@
  * 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
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -204,7 +204,7 @@
         Init();
     }
 
-    CMerkleTx(CTransactionRef arg) {
+    explicit CMerkleTx(CTransactionRef arg) {
         SetTx(std::move(arg));
         Init();
     }
@@ -503,7 +503,7 @@
     //! 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;
 
@@ -717,7 +717,7 @@
     unsigned int nMasterKeyMaxID;
 
     // Create wallet with dummy database handle
-    CWallet(const CChainParams &chainParamsIn)
+    explicit CWallet(const CChainParams &chainParamsIn)
         : dbw(new CWalletDBWrapper()), chainParams(chainParamsIn) {
         SetNull();
     }
@@ -1143,7 +1143,7 @@
     bool fInternal;
 
 public:
-    CReserveKey(CWallet *pwalletIn) {
+    explicit CReserveKey(CWallet *pwalletIn) {
         nIndex = -1;
         pwallet = pwalletIn;
         fInternal = false;
diff --git a/src/wallet/walletdb.h b/src/wallet/walletdb.h
--- a/src/wallet/walletdb.h
+++ b/src/wallet/walletdb.h
@@ -107,7 +107,7 @@
     CKeyID hdMasterKeyID;
 
     CKeyMetadata() { SetNull(); }
-    CKeyMetadata(int64_t nCreateTime_) {
+    explicit CKeyMetadata(int64_t nCreateTime_) {
         SetNull();
         nCreateTime = nCreateTime_;
     }
@@ -159,8 +159,8 @@
     }
 
 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) {}
 
     bool WriteName(const CTxDestination &address, const std::string &strName);