diff --git a/src/qt/addressbookpage.cpp b/src/qt/addressbookpage.cpp
--- a/src/qt/addressbookpage.cpp
+++ b/src/qt/addressbookpage.cpp
@@ -55,7 +55,7 @@
 
 AddressBookPage::AddressBookPage(const PlatformStyle *platformStyle, Mode _mode,
                                  Tabs _tab, QWidget *parent)
-    : QDialog(parent), ui(new Ui::AddressBookPage), model(0), mode(_mode),
+    : QDialog(parent), ui(new Ui::AddressBookPage), model(nullptr), mode(_mode),
       tab(_tab) {
     ui->setupUi(this);
 
diff --git a/src/qt/addresstablemodel.h b/src/qt/addresstablemodel.h
--- a/src/qt/addresstablemodel.h
+++ b/src/qt/addresstablemodel.h
@@ -25,7 +25,7 @@
     Q_OBJECT
 
 public:
-    explicit AddressTableModel(WalletModel *parent = 0);
+    explicit AddressTableModel(WalletModel *parent = nullptr);
     ~AddressTableModel();
 
     enum ColumnIndex {
diff --git a/src/qt/addresstablemodel.cpp b/src/qt/addresstablemodel.cpp
--- a/src/qt/addresstablemodel.cpp
+++ b/src/qt/addresstablemodel.cpp
@@ -286,7 +286,7 @@
 
 Qt::ItemFlags AddressTableModel::flags(const QModelIndex &index) const {
     if (!index.isValid()) {
-        return 0;
+        return Qt::NoItemFlags;
     }
     AddressTableEntry *rec =
         static_cast<AddressTableEntry *>(index.internalPointer());
diff --git a/src/qt/askpassphrasedialog.cpp b/src/qt/askpassphrasedialog.cpp
--- a/src/qt/askpassphrasedialog.cpp
+++ b/src/qt/askpassphrasedialog.cpp
@@ -18,8 +18,8 @@
 #include <QPushButton>
 
 AskPassphraseDialog::AskPassphraseDialog(Mode _mode, QWidget *parent)
-    : QDialog(parent), ui(new Ui::AskPassphraseDialog), mode(_mode), model(0),
-      fCapsLock(false) {
+    : QDialog(parent), ui(new Ui::AskPassphraseDialog), mode(_mode),
+      model(nullptr), fCapsLock(false) {
     ui->setupUi(this);
 
     ui->passEdit1->setMinimumSize(ui->passEdit1->sizeHint());
diff --git a/src/qt/bantablemodel.h b/src/qt/bantablemodel.h
--- a/src/qt/bantablemodel.h
+++ b/src/qt/bantablemodel.h
@@ -43,7 +43,8 @@
     Q_OBJECT
 
 public:
-    explicit BanTableModel(interfaces::Node &node, ClientModel *parent = 0);
+    explicit BanTableModel(interfaces::Node &node,
+                           ClientModel *parent = nullptr);
     ~BanTableModel();
     void startAutoRefresh();
     void stopAutoRefresh();
diff --git a/src/qt/bantablemodel.cpp b/src/qt/bantablemodel.cpp
--- a/src/qt/bantablemodel.cpp
+++ b/src/qt/bantablemodel.cpp
@@ -76,7 +76,7 @@
             return &cachedBanlist[idx];
         }
 
-        return 0;
+        return nullptr;
     }
 };
 
@@ -136,7 +136,7 @@
 
 Qt::ItemFlags BanTableModel::flags(const QModelIndex &index) const {
     if (!index.isValid()) {
-        return 0;
+        return Qt::NoItemFlags;
     }
 
     Qt::ItemFlags retval = Qt::ItemIsSelectable | Qt::ItemIsEnabled;
diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -189,9 +189,9 @@
 
 BitcoinApplication::BitcoinApplication(interfaces::Node &node, int &argc,
                                        char **argv)
-    : QApplication(argc, argv), coreThread(0), m_node(node), optionsModel(0),
-      clientModel(0), window(0), pollShutdownTimer(0), returnValue(0),
-      platformStyle(0) {
+    : QApplication(argc, argv), coreThread(nullptr), m_node(node),
+      optionsModel(nullptr), clientModel(nullptr), window(nullptr),
+      pollShutdownTimer(nullptr), returnValue(0), platformStyle(nullptr) {
     setQuitOnLastWindowClosed(false);
 }
 
@@ -219,15 +219,15 @@
     }
 
     delete window;
-    window = 0;
+    window = nullptr;
 #ifdef ENABLE_WALLET
     delete paymentServer;
-    paymentServer = 0;
+    paymentServer = nullptr;
 #endif
     delete optionsModel;
-    optionsModel = 0;
+    optionsModel = nullptr;
     delete platformStyle;
-    platformStyle = 0;
+    platformStyle = nullptr;
 }
 
 #ifdef ENABLE_WALLET
@@ -242,7 +242,8 @@
 
 void BitcoinApplication::createWindow(const Config *config,
                                       const NetworkStyle *networkStyle) {
-    window = new BitcoinGUI(m_node, config, platformStyle, networkStyle, 0);
+    window =
+        new BitcoinGUI(m_node, config, platformStyle, networkStyle, nullptr);
 
     pollShutdownTimer = new QTimer(window);
     connect(pollShutdownTimer, &QTimer::timeout, window,
@@ -250,7 +251,7 @@
 }
 
 void BitcoinApplication::createSplashScreen(const NetworkStyle *networkStyle) {
-    SplashScreen *splash = new SplashScreen(m_node, 0, networkStyle);
+    SplashScreen *splash = new SplashScreen(m_node, nullptr, networkStyle);
     // We don't hold a direct pointer to the splash screen after creation, but
     // the splash screen will take care of deleting itself when slotFinish
     // happens.
@@ -336,7 +337,7 @@
     qDebug() << __func__ << ": Requesting shutdown";
     startThread();
     window->hide();
-    window->setClientModel(0);
+    window->setClientModel(nullptr);
     pollShutdownTimer->stop();
 
 #ifdef ENABLE_WALLET
@@ -344,7 +345,7 @@
     m_wallet_controller = nullptr;
 #endif
     delete clientModel;
-    clientModel = 0;
+    clientModel = nullptr;
 
     m_node.startShutdown();
 
@@ -422,7 +423,7 @@
 
 void BitcoinApplication::handleRunawayException(const QString &message) {
     QMessageBox::critical(
-        0, "Runaway exception",
+        nullptr, "Runaway exception",
         BitcoinGUI::tr("A fatal error occurred. Bitcoin can no longer continue "
                        "safely and will quit.") +
             QString("\n\n") + message);
@@ -561,7 +562,7 @@
     std::string error;
     if (!node->parseParameters(argc, argv, error)) {
         QMessageBox::critical(
-            0, QObject::tr(PACKAGE_NAME),
+            nullptr, QObject::tr(PACKAGE_NAME),
             QObject::tr("Error parsing command line arguments: %1.")
                 .arg(QString::fromStdString(error)));
         return EXIT_FAILURE;
@@ -611,7 +612,7 @@
     /// - Do not call GetDataDir(true) before this step finishes.
     if (!fs::is_directory(GetDataDir(false))) {
         QMessageBox::critical(
-            0, QObject::tr(PACKAGE_NAME),
+            nullptr, QObject::tr(PACKAGE_NAME),
             QObject::tr(
                 "Error: Specified data directory \"%1\" does not exist.")
                 .arg(QString::fromStdString(gArgs.GetArg("-datadir", ""))));
@@ -619,7 +620,7 @@
     }
     if (!node->readConfigFiles(error)) {
         QMessageBox::critical(
-            0, QObject::tr(PACKAGE_NAME),
+            nullptr, QObject::tr(PACKAGE_NAME),
             QObject::tr("Error: Cannot parse configuration file: %1.")
                 .arg(QString::fromStdString(error)));
         return EXIT_FAILURE;
@@ -638,7 +639,7 @@
     try {
         node->selectParams(gArgs.GetChainName());
     } catch (std::exception &e) {
-        QMessageBox::critical(0, QObject::tr(PACKAGE_NAME),
+        QMessageBox::critical(nullptr, QObject::tr(PACKAGE_NAME),
                               QObject::tr("Error: %1").arg(e.what()));
         return EXIT_FAILURE;
     }
diff --git a/src/qt/bitcoinamountfield.h b/src/qt/bitcoinamountfield.h
--- a/src/qt/bitcoinamountfield.h
+++ b/src/qt/bitcoinamountfield.h
@@ -24,9 +24,9 @@
         Amount value READ value WRITE setValue NOTIFY valueChanged USER true)
 
 public:
-    explicit BitcoinAmountField(QWidget *parent = 0);
+    explicit BitcoinAmountField(QWidget *parent = nullptr);
 
-    Amount value(bool *value = 0) const;
+    Amount value(bool *value = nullptr) const;
     void setValue(const Amount value);
 
     /** Set single step in satoshis **/
diff --git a/src/qt/bitcoinamountfield.cpp b/src/qt/bitcoinamountfield.cpp
--- a/src/qt/bitcoinamountfield.cpp
+++ b/src/qt/bitcoinamountfield.cpp
@@ -52,7 +52,9 @@
         }
     }
 
-    Amount value(bool *valid_out = 0) const { return parse(text(), valid_out); }
+    Amount value(bool *valid_out = nullptr) const {
+        return parse(text(), valid_out);
+    }
 
     void setValue(const Amount value) {
         lineEdit()->setText(BitcoinUnits::format(
@@ -134,7 +136,7 @@
      * return validity.
      * @note Must return 0 if !valid.
      */
-    Amount parse(const QString &text, bool *valid_out = 0) const {
+    Amount parse(const QString &text, bool *valid_out = nullptr) const {
         Amount val = Amount::zero();
         bool valid = BitcoinUnits::parse(currentUnit, text, &val);
         if (valid) {
@@ -174,7 +176,7 @@
             return StepUpEnabled;
         }
 
-        StepEnabled rv = 0;
+        StepEnabled rv = StepNone;
         bool valid = false;
         Amount val = value(&valid);
         if (valid) {
@@ -195,7 +197,7 @@
 #include <qt/bitcoinamountfield.moc>
 
 BitcoinAmountField::BitcoinAmountField(QWidget *parent)
-    : QWidget(parent), amount(0) {
+    : QWidget(parent), amount(nullptr) {
     amount = new AmountSpinBox(this);
     amount->setLocale(QLocale::c());
     amount->installEventFilter(this);
diff --git a/src/qt/bitcoingui.h b/src/qt/bitcoingui.h
--- a/src/qt/bitcoingui.h
+++ b/src/qt/bitcoingui.h
@@ -68,7 +68,8 @@
 
     explicit BitcoinGUI(interfaces::Node &node, const Config *,
                         const PlatformStyle *platformStyle,
-                        const NetworkStyle *networkStyle, QWidget *parent = 0);
+                        const NetworkStyle *networkStyle,
+                        QWidget *parent = nullptr);
     ~BitcoinGUI();
 
     /**
diff --git a/src/qt/bitcoingui.cpp b/src/qt/bitcoingui.cpp
--- a/src/qt/bitcoingui.cpp
+++ b/src/qt/bitcoingui.cpp
@@ -89,7 +89,7 @@
     setWindowIcon(m_network_style->getTrayAndWindowIcon());
     updateWindowTitle();
 
-    rpcConsole = new RPCConsole(node, _platformStyle, 0);
+    rpcConsole = new RPCConsole(node, _platformStyle, nullptr);
     helpMessageDialog = new HelpMessageDialog(node, this, false);
 #ifdef ENABLE_WALLET
     if (enableWallet) {
@@ -1286,7 +1286,7 @@
     bool proxy_enabled = clientModel->getProxyInfo(ip_port);
 
     if (proxy_enabled) {
-        if (labelProxyIcon->pixmap() == 0) {
+        if (labelProxyIcon->pixmap() == nullptr) {
             QString ip_port_q = QString::fromStdString(ip_port);
             labelProxyIcon->setPixmap(
                 platformStyle->SingleColorIcon(":/icons/proxy")
@@ -1346,7 +1346,7 @@
         progressDialog = new QProgressDialog(title, "", 0, 100);
         progressDialog->setWindowModality(Qt::ApplicationModal);
         progressDialog->setMinimumDuration(0);
-        progressDialog->setCancelButton(0);
+        progressDialog->setCancelButton(nullptr);
         progressDialog->setAutoClose(false);
         progressDialog->setValue(0);
     } else if (progressDialog) {
@@ -1412,7 +1412,7 @@
 
 UnitDisplayStatusBarControl::UnitDisplayStatusBarControl(
     const PlatformStyle *platformStyle)
-    : optionsModel(0), menu(0) {
+    : optionsModel(nullptr), menu(nullptr) {
     createContextMenu();
     setToolTip(tr("Unit to show amounts in. Click to select another unit."));
     QList<BitcoinUnits::Unit> units = BitcoinUnits::availableUnits();
diff --git a/src/qt/clientmodel.h b/src/qt/clientmodel.h
--- a/src/qt/clientmodel.h
+++ b/src/qt/clientmodel.h
@@ -41,7 +41,7 @@
     };
 
     explicit ClientModel(interfaces::Node &node, OptionsModel *optionsModel,
-                         QObject *parent = 0);
+                         QObject *parent = nullptr);
     ~ClientModel();
 
     interfaces::Node &node() const { return m_node; }
diff --git a/src/qt/clientmodel.cpp b/src/qt/clientmodel.cpp
--- a/src/qt/clientmodel.cpp
+++ b/src/qt/clientmodel.cpp
@@ -34,7 +34,7 @@
 ClientModel::ClientModel(interfaces::Node &node, OptionsModel *_optionsModel,
                          QObject *parent)
     : QObject(parent), m_node(node), optionsModel(_optionsModel),
-      peerTableModel(0), banTableModel(0), pollTimer(0) {
+      peerTableModel(nullptr), banTableModel(nullptr), pollTimer(nullptr) {
     cachedBestHeaderHeight = -1;
     cachedBestHeaderTime = -1;
     peerTableModel = new PeerTableModel(m_node, this);
diff --git a/src/qt/coincontroldialog.h b/src/qt/coincontroldialog.h
--- a/src/qt/coincontroldialog.h
+++ b/src/qt/coincontroldialog.h
@@ -44,7 +44,7 @@
 
 public:
     explicit CoinControlDialog(const PlatformStyle *platformStyle,
-                               QWidget *parent = 0);
+                               QWidget *parent = nullptr);
     ~CoinControlDialog();
 
     void setModel(WalletModel *model);
diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -51,7 +51,7 @@
 
 CoinControlDialog::CoinControlDialog(const PlatformStyle *_platformStyle,
                                      QWidget *parent)
-    : QDialog(parent), ui(new Ui::CoinControlDialog), model(0),
+    : QDialog(parent), ui(new Ui::CoinControlDialog), model(nullptr),
       platformStyle(_platformStyle) {
     ui->setupUi(this);
 
diff --git a/src/qt/coincontroltreewidget.h b/src/qt/coincontroltreewidget.h
--- a/src/qt/coincontroltreewidget.h
+++ b/src/qt/coincontroltreewidget.h
@@ -12,7 +12,7 @@
     Q_OBJECT
 
 public:
-    explicit CoinControlTreeWidget(QWidget *parent = 0);
+    explicit CoinControlTreeWidget(QWidget *parent = nullptr);
 
 protected:
     virtual void keyPressEvent(QKeyEvent *event) override;
diff --git a/src/qt/csvmodelwriter.h b/src/qt/csvmodelwriter.h
--- a/src/qt/csvmodelwriter.h
+++ b/src/qt/csvmodelwriter.h
@@ -20,7 +20,7 @@
     Q_OBJECT
 
 public:
-    explicit CSVModelWriter(const QString &filename, QObject *parent = 0);
+    explicit CSVModelWriter(const QString &filename, QObject *parent = nullptr);
 
     void setModel(const QAbstractItemModel *model);
     void addColumn(const QString &title, int column, int role = Qt::EditRole);
diff --git a/src/qt/csvmodelwriter.cpp b/src/qt/csvmodelwriter.cpp
--- a/src/qt/csvmodelwriter.cpp
+++ b/src/qt/csvmodelwriter.cpp
@@ -9,7 +9,7 @@
 #include <QTextStream>
 
 CSVModelWriter::CSVModelWriter(const QString &_filename, QObject *parent)
-    : QObject(parent), filename(_filename), model(0) {}
+    : QObject(parent), filename(_filename), model(nullptr) {}
 
 void CSVModelWriter::setModel(const QAbstractItemModel *_model) {
     this->model = _model;
diff --git a/src/qt/editaddressdialog.cpp b/src/qt/editaddressdialog.cpp
--- a/src/qt/editaddressdialog.cpp
+++ b/src/qt/editaddressdialog.cpp
@@ -12,8 +12,8 @@
 #include <QMessageBox>
 
 EditAddressDialog::EditAddressDialog(Mode _mode, QWidget *parent)
-    : QDialog(parent), ui(new Ui::EditAddressDialog), mapper(0), mode(_mode),
-      model(0) {
+    : QDialog(parent), ui(new Ui::EditAddressDialog), mapper(nullptr),
+      mode(_mode), model(nullptr) {
     ui->setupUi(this);
 
     GUIUtil::setupAddressWidget(ui->addressEdit, this);
diff --git a/src/qt/intro.h b/src/qt/intro.h
--- a/src/qt/intro.h
+++ b/src/qt/intro.h
@@ -29,7 +29,7 @@
     Q_OBJECT
 
 public:
-    explicit Intro(QWidget *parent = 0);
+    explicit Intro(QWidget *parent = nullptr);
     ~Intro();
 
     QString getDataDirectory();
diff --git a/src/qt/intro.cpp b/src/qt/intro.cpp
--- a/src/qt/intro.cpp
+++ b/src/qt/intro.cpp
@@ -118,7 +118,7 @@
 }
 
 Intro::Intro(QWidget *parent)
-    : QDialog(parent), ui(new Ui::Intro), thread(0), signalled(false) {
+    : QDialog(parent), ui(new Ui::Intro), thread(nullptr), signalled(false) {
     ui->setupUi(this);
     ui->welcomeLabel->setText(ui->welcomeLabel->text().arg(tr(PACKAGE_NAME)));
     ui->storageLabel->setText(ui->storageLabel->text().arg(tr(PACKAGE_NAME)));
@@ -222,7 +222,7 @@
                 }
                 break;
             } catch (const fs::filesystem_error &) {
-                QMessageBox::critical(0, tr(PACKAGE_NAME),
+                QMessageBox::critical(nullptr, tr(PACKAGE_NAME),
                                       tr("Error: Specified data directory "
                                          "\"%1\" cannot be created.")
                                           .arg(dataDir));
@@ -285,7 +285,7 @@
 
 void Intro::on_ellipsisButton_clicked() {
     QString dir = QDir::toNativeSeparators(QFileDialog::getExistingDirectory(
-        0, "Choose data directory", ui->dataDirectory->text()));
+        nullptr, "Choose data directory", ui->dataDirectory->text()));
     if (!dir.isEmpty()) {
         ui->dataDirectory->setText(dir);
     }
diff --git a/src/qt/networkstyle.cpp b/src/qt/networkstyle.cpp
--- a/src/qt/networkstyle.cpp
+++ b/src/qt/networkstyle.cpp
@@ -82,5 +82,5 @@
                 network_styles[x].titleAddText);
         }
     }
-    return 0;
+    return nullptr;
 }
diff --git a/src/qt/optionsdialog.cpp b/src/qt/optionsdialog.cpp
--- a/src/qt/optionsdialog.cpp
+++ b/src/qt/optionsdialog.cpp
@@ -26,7 +26,8 @@
 #include <QTimer>
 
 OptionsDialog::OptionsDialog(QWidget *parent, bool enableWallet)
-    : QDialog(parent), ui(new Ui::OptionsDialog), model(0), mapper(0) {
+    : QDialog(parent), ui(new Ui::OptionsDialog), model(nullptr),
+      mapper(nullptr) {
     ui->setupUi(this);
 
     /* Main elements init */
diff --git a/src/qt/optionsmodel.h b/src/qt/optionsmodel.h
--- a/src/qt/optionsmodel.h
+++ b/src/qt/optionsmodel.h
@@ -30,7 +30,7 @@
     Q_OBJECT
 
 public:
-    explicit OptionsModel(interfaces::Node &node, QObject *parent = 0,
+    explicit OptionsModel(interfaces::Node &node, QObject *parent = nullptr,
                           bool resetSettings = false);
 
     enum OptionID {
diff --git a/src/qt/overviewpage.h b/src/qt/overviewpage.h
--- a/src/qt/overviewpage.h
+++ b/src/qt/overviewpage.h
@@ -30,7 +30,7 @@
 
 public:
     explicit OverviewPage(const PlatformStyle *platformStyle,
-                          QWidget *parent = 0);
+                          QWidget *parent = nullptr);
     ~OverviewPage();
 
     void setClientModel(ClientModel *clientModel);
diff --git a/src/qt/overviewpage.cpp b/src/qt/overviewpage.cpp
--- a/src/qt/overviewpage.cpp
+++ b/src/qt/overviewpage.cpp
@@ -115,7 +115,8 @@
 #include <qt/overviewpage.moc>
 
 OverviewPage::OverviewPage(const PlatformStyle *platformStyle, QWidget *parent)
-    : QWidget(parent), ui(new Ui::OverviewPage), clientModel(0), walletModel(0),
+    : QWidget(parent), ui(new Ui::OverviewPage), clientModel(nullptr),
+      walletModel(nullptr),
       txdelegate(new TxViewDelegate(platformStyle, this)) {
     ui->setupUi(this);
 
diff --git a/src/qt/paymentserver.cpp b/src/qt/paymentserver.cpp
--- a/src/qt/paymentserver.cpp
+++ b/src/qt/paymentserver.cpp
@@ -227,10 +227,10 @@
 }
 
 PaymentServer::PaymentServer(QObject *parent, bool startLocalServer)
-    : QObject(parent), saveURIs(true), uriServer(0), optionsModel(0)
+    : QObject(parent), saveURIs(true), uriServer(nullptr), optionsModel(nullptr)
 // clang-format off
 #ifdef ENABLE_BIP70
-      ,netManager(0)
+      ,netManager(nullptr)
 #endif
 // clang-format on
 {
@@ -258,7 +258,7 @@
         if (!uriServer->listen(name)) {
             // constructor is called early in init, so don't use "Q_EMIT
             // message()" here
-            QMessageBox::critical(0, tr("Payment request error"),
+            QMessageBox::critical(nullptr, tr("Payment request error"),
                                   tr("Cannot start click-to-pay handler"));
         } else {
             connect(uriServer, &QLocalServer::newConnection, this,
diff --git a/src/qt/peertablemodel.h b/src/qt/peertablemodel.h
--- a/src/qt/peertablemodel.h
+++ b/src/qt/peertablemodel.h
@@ -50,7 +50,8 @@
     Q_OBJECT
 
 public:
-    explicit PeerTableModel(interfaces::Node &node, ClientModel *parent = 0);
+    explicit PeerTableModel(interfaces::Node &node,
+                            ClientModel *parent = nullptr);
     ~PeerTableModel();
     const CNodeCombinedStats *getNodeStats(int idx);
     int getRowByNodeId(NodeId nodeid);
diff --git a/src/qt/peertablemodel.cpp b/src/qt/peertablemodel.cpp
--- a/src/qt/peertablemodel.cpp
+++ b/src/qt/peertablemodel.cpp
@@ -97,12 +97,13 @@
             return &cachedNodeStats[idx];
         }
 
-        return 0;
+        return nullptr;
     }
 };
 
 PeerTableModel::PeerTableModel(interfaces::Node &node, ClientModel *parent)
-    : QAbstractTableModel(parent), m_node(node), clientModel(parent), timer(0) {
+    : QAbstractTableModel(parent), m_node(node), clientModel(parent),
+      timer(nullptr) {
     columns << tr("NodeId") << tr("Node/Service") << tr("Ping") << tr("Sent")
             << tr("Received") << tr("User Agent");
     priv.reset(new PeerTablePriv());
@@ -187,7 +188,7 @@
 
 Qt::ItemFlags PeerTableModel::flags(const QModelIndex &index) const {
     if (!index.isValid()) {
-        return 0;
+        return Qt::NoItemFlags;
     }
 
     Qt::ItemFlags retval = Qt::ItemIsSelectable | Qt::ItemIsEnabled;
diff --git a/src/qt/platformstyle.cpp b/src/qt/platformstyle.cpp
--- a/src/qt/platformstyle.cpp
+++ b/src/qt/platformstyle.cpp
@@ -129,5 +129,5 @@
                                      platform_styles[x].useExtraSpacing);
         }
     }
-    return 0;
+    return nullptr;
 }
diff --git a/src/qt/qvalidatedlineedit.cpp b/src/qt/qvalidatedlineedit.cpp
--- a/src/qt/qvalidatedlineedit.cpp
+++ b/src/qt/qvalidatedlineedit.cpp
@@ -8,7 +8,7 @@
 #include <qt/guiconstants.h>
 
 QValidatedLineEdit::QValidatedLineEdit(QWidget *parent)
-    : QLineEdit(parent), valid(true), checkValidator(0) {
+    : QLineEdit(parent), valid(true), checkValidator(nullptr) {
     connect(this, &QValidatedLineEdit::textChanged, this,
             &QValidatedLineEdit::markValid);
 }
diff --git a/src/qt/qvaluecombobox.h b/src/qt/qvaluecombobox.h
--- a/src/qt/qvaluecombobox.h
+++ b/src/qt/qvaluecombobox.h
@@ -17,7 +17,7 @@
         QVariant value READ value WRITE setValue NOTIFY valueChanged USER true)
 
 public:
-    explicit QValueComboBox(QWidget *parent = 0);
+    explicit QValueComboBox(QWidget *parent = nullptr);
 
     QVariant value() const;
     void setValue(const QVariant &value);
diff --git a/src/qt/receivecoinsdialog.h b/src/qt/receivecoinsdialog.h
--- a/src/qt/receivecoinsdialog.h
+++ b/src/qt/receivecoinsdialog.h
@@ -39,7 +39,7 @@
     };
 
     explicit ReceiveCoinsDialog(const PlatformStyle *platformStyle,
-                                QWidget *parent = 0);
+                                QWidget *parent = nullptr);
     ~ReceiveCoinsDialog();
 
     void setModel(WalletModel *model);
diff --git a/src/qt/receivecoinsdialog.cpp b/src/qt/receivecoinsdialog.cpp
--- a/src/qt/receivecoinsdialog.cpp
+++ b/src/qt/receivecoinsdialog.cpp
@@ -26,8 +26,9 @@
 
 ReceiveCoinsDialog::ReceiveCoinsDialog(const PlatformStyle *_platformStyle,
                                        QWidget *parent)
-    : QDialog(parent), ui(new Ui::ReceiveCoinsDialog), columnResizingFixer(0),
-      model(0), platformStyle(_platformStyle) {
+    : QDialog(parent), ui(new Ui::ReceiveCoinsDialog),
+      columnResizingFixer(nullptr), model(nullptr),
+      platformStyle(_platformStyle) {
     ui->setupUi(this);
 
     if (!_platformStyle->getImagesOnButtons()) {
diff --git a/src/qt/receiverequestdialog.h b/src/qt/receiverequestdialog.h
--- a/src/qt/receiverequestdialog.h
+++ b/src/qt/receiverequestdialog.h
@@ -28,7 +28,7 @@
     Q_OBJECT
 
 public:
-    explicit QRImageWidget(QWidget *parent = 0);
+    explicit QRImageWidget(QWidget *parent = nullptr);
     QImage exportImage();
 
 public Q_SLOTS:
@@ -47,7 +47,7 @@
     Q_OBJECT
 
 public:
-    explicit ReceiveRequestDialog(QWidget *parent = 0);
+    explicit ReceiveRequestDialog(QWidget *parent = nullptr);
     ~ReceiveRequestDialog();
 
     void setModel(WalletModel *model);
diff --git a/src/qt/receiverequestdialog.cpp b/src/qt/receiverequestdialog.cpp
--- a/src/qt/receiverequestdialog.cpp
+++ b/src/qt/receiverequestdialog.cpp
@@ -27,7 +27,8 @@
 #include <qrencode.h>
 #endif
 
-QRImageWidget::QRImageWidget(QWidget *parent) : QLabel(parent), contextMenu(0) {
+QRImageWidget::QRImageWidget(QWidget *parent)
+    : QLabel(parent), contextMenu(nullptr) {
     contextMenu = new QMenu(this);
     QAction *saveImageAction = new QAction(tr("&Save Image..."), this);
     connect(saveImageAction, &QAction::triggered, this,
@@ -86,7 +87,7 @@
 }
 
 ReceiveRequestDialog::ReceiveRequestDialog(QWidget *parent)
-    : QDialog(parent), ui(new Ui::ReceiveRequestDialog), model(0) {
+    : QDialog(parent), ui(new Ui::ReceiveRequestDialog), model(nullptr) {
     ui->setupUi(this);
 
 #ifndef USE_QRCODE
diff --git a/src/qt/rpcconsole.cpp b/src/qt/rpcconsole.cpp
--- a/src/qt/rpcconsole.cpp
+++ b/src/qt/rpcconsole.cpp
@@ -560,7 +560,7 @@
 
 // set library version labels
 #ifdef ENABLE_WALLET
-    ui->berkeleyDBVersion->setText(DbEnv::version(0, 0, 0));
+    ui->berkeleyDBVersion->setText(DbEnv::version(nullptr, nullptr, nullptr));
 #else
     ui->label_berkeleyDBVersion->hide();
     ui->berkeleyDBVersion->hide();
diff --git a/src/qt/sendcoinsdialog.cpp b/src/qt/sendcoinsdialog.cpp
--- a/src/qt/sendcoinsdialog.cpp
+++ b/src/qt/sendcoinsdialog.cpp
@@ -469,7 +469,7 @@
 }
 
 void SendCoinsDialog::updateTabsAndLabels() {
-    setupTabChain(0);
+    setupTabChain(nullptr);
     coinControlUpdateLabels();
 }
 
diff --git a/src/qt/sendcoinsentry.cpp b/src/qt/sendcoinsentry.cpp
--- a/src/qt/sendcoinsentry.cpp
+++ b/src/qt/sendcoinsentry.cpp
@@ -172,7 +172,7 @@
     }
 
     // Sending a zero amount is invalid
-    if (ui->payAmount->value(0) <= Amount::zero()) {
+    if (ui->payAmount->value(nullptr) <= Amount::zero()) {
         ui->payAmount->setValid(false);
         retval = false;
     }
diff --git a/src/qt/signverifymessagedialog.cpp b/src/qt/signverifymessagedialog.cpp
--- a/src/qt/signverifymessagedialog.cpp
+++ b/src/qt/signverifymessagedialog.cpp
@@ -20,7 +20,7 @@
 
 SignVerifyMessageDialog::SignVerifyMessageDialog(
     const PlatformStyle *_platformStyle, QWidget *parent)
-    : QDialog(parent), ui(new Ui::SignVerifyMessageDialog), model(0),
+    : QDialog(parent), ui(new Ui::SignVerifyMessageDialog), model(nullptr),
       platformStyle(_platformStyle) {
     ui->setupUi(this);
 
diff --git a/src/qt/splashscreen.cpp b/src/qt/splashscreen.cpp
--- a/src/qt/splashscreen.cpp
+++ b/src/qt/splashscreen.cpp
@@ -28,7 +28,7 @@
 
 SplashScreen::SplashScreen(interfaces::Node &node, Qt::WindowFlags f,
                            const NetworkStyle *networkStyle)
-    : QWidget(0, f), curAlignment(0), m_node(node) {
+    : QWidget(nullptr, f), curAlignment(0), m_node(node) {
     // set reference point, paddings
     int paddingRight = 50;
     int paddingTop = 50;
diff --git a/src/qt/trafficgraphwidget.h b/src/qt/trafficgraphwidget.h
--- a/src/qt/trafficgraphwidget.h
+++ b/src/qt/trafficgraphwidget.h
@@ -19,7 +19,7 @@
     Q_OBJECT
 
 public:
-    explicit TrafficGraphWidget(QWidget *parent = 0);
+    explicit TrafficGraphWidget(QWidget *parent = nullptr);
     void setClientModel(ClientModel *model);
     int getGraphRangeMins() const;
 
diff --git a/src/qt/trafficgraphwidget.cpp b/src/qt/trafficgraphwidget.cpp
--- a/src/qt/trafficgraphwidget.cpp
+++ b/src/qt/trafficgraphwidget.cpp
@@ -19,8 +19,8 @@
 #define YMARGIN 10
 
 TrafficGraphWidget::TrafficGraphWidget(QWidget *parent)
-    : QWidget(parent), timer(0), fMax(0.0f), nMins(0), vSamplesIn(),
-      vSamplesOut(), nLastBytesIn(0), nLastBytesOut(0), clientModel(0) {
+    : QWidget(parent), timer(nullptr), fMax(0.0f), nMins(0), vSamplesIn(),
+      vSamplesOut(), nLastBytesIn(0), nLastBytesOut(0), clientModel(nullptr) {
     timer = new QTimer(this);
     connect(timer, &QTimer::timeout, this, &TrafficGraphWidget::updateRates);
 }
diff --git a/src/qt/transactiondescdialog.h b/src/qt/transactiondescdialog.h
--- a/src/qt/transactiondescdialog.h
+++ b/src/qt/transactiondescdialog.h
@@ -20,7 +20,8 @@
     Q_OBJECT
 
 public:
-    explicit TransactionDescDialog(const QModelIndex &idx, QWidget *parent = 0);
+    explicit TransactionDescDialog(const QModelIndex &idx,
+                                   QWidget *parent = nullptr);
     ~TransactionDescDialog();
 
 private:
diff --git a/src/qt/transactionfilterproxy.h b/src/qt/transactionfilterproxy.h
--- a/src/qt/transactionfilterproxy.h
+++ b/src/qt/transactionfilterproxy.h
@@ -17,7 +17,7 @@
     Q_OBJECT
 
 public:
-    explicit TransactionFilterProxy(QObject *parent = 0);
+    explicit TransactionFilterProxy(QObject *parent = nullptr);
 
     /**
      * Earliest date that can be represented (far in the past).
diff --git a/src/qt/transactiontablemodel.h b/src/qt/transactiontablemodel.h
--- a/src/qt/transactiontablemodel.h
+++ b/src/qt/transactiontablemodel.h
@@ -29,7 +29,7 @@
 
 public:
     explicit TransactionTableModel(const PlatformStyle *platformStyle,
-                                   WalletModel *parent = 0);
+                                   WalletModel *parent = nullptr);
     ~TransactionTableModel();
 
     enum ColumnIndex {
diff --git a/src/qt/transactiontablemodel.cpp b/src/qt/transactiontablemodel.cpp
--- a/src/qt/transactiontablemodel.cpp
+++ b/src/qt/transactiontablemodel.cpp
@@ -193,7 +193,7 @@
             }
             return rec;
         }
-        return 0;
+        return nullptr;
     }
 
     QString describe(interfaces::Node &node, interfaces::Wallet &wallet,
diff --git a/src/qt/transactionview.h b/src/qt/transactionview.h
--- a/src/qt/transactionview.h
+++ b/src/qt/transactionview.h
@@ -36,7 +36,7 @@
 
 public:
     explicit TransactionView(const PlatformStyle *platformStyle,
-                             QWidget *parent = 0);
+                             QWidget *parent = nullptr);
 
     void setModel(WalletModel *model);
 
diff --git a/src/qt/transactionview.cpp b/src/qt/transactionview.cpp
--- a/src/qt/transactionview.cpp
+++ b/src/qt/transactionview.cpp
@@ -37,8 +37,9 @@
 
 TransactionView::TransactionView(const PlatformStyle *platformStyle,
                                  QWidget *parent)
-    : QWidget(parent), model(0), transactionProxyModel(0), transactionView(0),
-      abandonAction(0), columnResizingFixer(0) {
+    : QWidget(parent), model(nullptr), transactionProxyModel(nullptr),
+      transactionView(nullptr), abandonAction(nullptr),
+      columnResizingFixer(nullptr) {
     // Build filter row
     setContentsMargins(0, 0, 0, 0);
 
diff --git a/src/qt/walletmodel.cpp b/src/qt/walletmodel.cpp
--- a/src/qt/walletmodel.cpp
+++ b/src/qt/walletmodel.cpp
@@ -33,8 +33,8 @@
                          const PlatformStyle *platformStyle,
                          OptionsModel *_optionsModel, QObject *parent)
     : QObject(parent), m_wallet(std::move(wallet)), m_node(node),
-      optionsModel(_optionsModel), addressTableModel(0),
-      transactionTableModel(0), recentRequestsTableModel(0),
+      optionsModel(_optionsModel), addressTableModel(nullptr),
+      transactionTableModel(nullptr), recentRequestsTableModel(nullptr),
       cachedEncryptionStatus(Unencrypted), cachedNumBlocks(0) {
     fHaveWatchOnly = m_wallet->haveWatchOnly();
     addressTableModel = new AddressTableModel(this);
diff --git a/src/test/allocator_tests.cpp b/src/test/allocator_tests.cpp
--- a/src/test/allocator_tests.cpp
+++ b/src/test/allocator_tests.cpp
@@ -114,14 +114,14 @@
     // Go entirely wild: free and alloc interleaved, generate targets and sizes
     // using pseudo-randomness.
     for (int x = 0; x < 2048; ++x) {
-        addr.push_back(0);
+        addr.push_back(nullptr);
     }
     uint32_t s = 0x12345678;
     for (int x = 0; x < 5000; ++x) {
         int idx = s & (addr.size() - 1);
         if (s & 0x80000000) {
             b.free(addr[idx]);
-            addr[idx] = 0;
+            addr[idx] = nullptr;
         } else if (!addr[idx]) {
             addr[idx] = b.alloc((s >> 16) & 2047);
         }
@@ -159,7 +159,7 @@
             // Fake address, do not actually use this memory
             return reinterpret_cast<void *>(0x08000000 + (count << 24));
         }
-        return 0;
+        return nullptr;
     }
     void FreeLocked(void *addr, size_t len) override {}
     size_t GetLimit() override { return std::numeric_limits<size_t>::max(); }
diff --git a/src/wallet/db.cpp b/src/wallet/db.cpp
--- a/src/wallet/db.cpp
+++ b/src/wallet/db.cpp
@@ -398,7 +398,8 @@
         GetWalletEnv(file_path, walletFile);
     fs::path walletDir = env->Directory();
 
-    LogPrintf("Using BerkeleyDB version %s\n", DbEnv::version(0, 0, 0));
+    LogPrintf("Using BerkeleyDB version %s\n",
+              DbEnv::version(nullptr, nullptr, nullptr));
     LogPrintf("Using wallet %s\n", walletFile);
 
     // Wallet file must be a plain filename without a directory