diff --git a/examples/qmqtt/client/example.cpp b/examples/qmqtt/client/example.cpp
index 048f3b8..4620c96 100644
--- a/examples/qmqtt/client/example.cpp
+++ b/examples/qmqtt/client/example.cpp
@@ -43,7 +43,7 @@ class Publisher : public QMQTT::Client
 public:
     explicit Publisher(const QHostAddress& host = EXAMPLE_HOST,
                        const quint16 port = EXAMPLE_PORT,
-                       QObject* parent = NULL)
+                       QObject* parent = nullptr)
         : QMQTT::Client(host, port, parent)
         , _number(0)
     {
@@ -88,7 +88,7 @@ class Subscriber : public QMQTT::Client
 public:
     explicit Subscriber(const QHostAddress& host = EXAMPLE_HOST,
                         const quint16 port = EXAMPLE_PORT,
-                        QObject* parent = NULL)
+                        QObject* parent = nullptr)
         : QMQTT::Client(host, port, parent)
         , _qout(stdout)
     {
diff --git a/src/mqtt/qmqtt_client.cpp b/src/mqtt/qmqtt_client.cpp
index 78542e4..9a5ecd5 100644
--- a/src/mqtt/qmqtt_client.cpp
+++ b/src/mqtt/qmqtt_client.cpp
@@ -79,7 +79,7 @@ QMQTT::Client::Client(const QString& url,
 {
     Q_D(Client);
 #ifndef QT_NO_SSL
-    d->init(url, origin, version, NULL, ignoreSelfSigned);
+    d->init(url, origin, version, nullptr, ignoreSelfSigned);
 #else
     Q_UNUSED(ignoreSelfSigned)
     d->init(url, origin, version);
diff --git a/src/mqtt/qmqtt_client.h b/src/mqtt/qmqtt_client.h
index c502b25..7474a64 100644
--- a/src/mqtt/qmqtt_client.h
+++ b/src/mqtt/qmqtt_client.h
@@ -147,14 +147,14 @@ class Q_MQTT_EXPORT Client : public QObject
 public:
     Client(const QHostAddress& host = QHostAddress::LocalHost,
            const quint16 port = 1883,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 
 #ifndef QT_NO_SSL
     Client(const QString& hostName,
            const quint16 port,
            const QSslConfiguration& config,
            const bool ignoreSelfSigned=false,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 #endif // QT_NO_SSL
 
     // This function is provided for backward compatibility with older versions of QMQTT.
@@ -165,7 +165,7 @@ class Q_MQTT_EXPORT Client : public QObject
            const quint16 port,
            const bool ssl,
            const bool ignoreSelfSigned,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 
 #ifdef QT_WEBSOCKETS_LIB
     // Create a connection over websockets
@@ -173,7 +173,7 @@ class Q_MQTT_EXPORT Client : public QObject
            const QString& origin,
            QWebSocketProtocol::Version version,
            bool ignoreSelfSigned = false,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 
 #ifndef QT_NO_SSL
     Client(const QString& url,
@@ -181,7 +181,7 @@ class Q_MQTT_EXPORT Client : public QObject
            QWebSocketProtocol::Version version,
            const QSslConfiguration& config,
            const bool ignoreSelfSigned = false,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 #endif // QT_NO_SSL
 #endif // QT_WEBSOCKETS_LIB
 
@@ -189,7 +189,7 @@ class Q_MQTT_EXPORT Client : public QObject
     Client(NetworkInterface* network,
            const QHostAddress& host = QHostAddress::LocalHost,
            const quint16 port = 1883,
-           QObject* parent = NULL);
+           QObject* parent = nullptr);
 
     virtual ~Client();
 
diff --git a/src/mqtt/qmqtt_client_p.cpp b/src/mqtt/qmqtt_client_p.cpp
index 89329c3..0ac0568 100644
--- a/src/mqtt/qmqtt_client_p.cpp
+++ b/src/mqtt/qmqtt_client_p.cpp
@@ -73,7 +73,7 @@ void QMQTT::ClientPrivate::init(const QHostAddress& host, const quint16 port, Ne
     Q_Q(Client);
     _host = host;
     _port = port;
-    if(network == NULL)
+    if(network == nullptr)
     {
         init(new Network(q));
     }
diff --git a/src/mqtt/qmqtt_client_p.h b/src/mqtt/qmqtt_client_p.h
index 03d0768..72358db 100644
--- a/src/mqtt/qmqtt_client_p.h
+++ b/src/mqtt/qmqtt_client_p.h
@@ -60,7 +60,7 @@ class ClientPrivate
     ClientPrivate(Client* qq_ptr);
     ~ClientPrivate();
 
-    void init(const QHostAddress& host, const quint16 port, NetworkInterface* network = NULL);
+    void init(const QHostAddress& host, const quint16 port, NetworkInterface* network = nullptr);
 #ifndef QT_NO_SSL
     void init(const QString& hostName, const quint16 port, const QSslConfiguration& config,
               const bool ignoreSelfSigned=false);
diff --git a/src/mqtt/qmqtt_network_p.h b/src/mqtt/qmqtt_network_p.h
index 0bb5413..6600197 100644
--- a/src/mqtt/qmqtt_network_p.h
+++ b/src/mqtt/qmqtt_network_p.h
@@ -59,23 +59,23 @@ class Network : public NetworkInterface
     Q_OBJECT
 
 public:
-    Network(QObject* parent = NULL);
+    Network(QObject* parent = nullptr);
 #ifndef QT_NO_SSL
-    Network(const QSslConfiguration& config, QObject* parent = NULL);
+    Network(const QSslConfiguration& config, QObject* parent = nullptr);
 #endif // QT_NO_SSL
 #ifdef QT_WEBSOCKETS_LIB
 #ifndef QT_NO_SSL
     Network(const QString& origin,
             QWebSocketProtocol::Version version,
             const QSslConfiguration* sslConfig,
-            QObject* parent = NULL);
+            QObject* parent = nullptr);
 #endif // QT_NO_SSL
     Network(const QString& origin,
             QWebSocketProtocol::Version version,
-            QObject* parent = NULL);
+            QObject* parent = nullptr);
 #endif // QT_WEBSOCKETS_LIB
     Network(SocketInterface* socketInterface, TimerInterface* timerInterface,
-            QObject* parent = NULL);
+            QObject* parent = nullptr);
     ~Network();
 
     void sendFrame(const Frame &frame);
diff --git a/src/mqtt/qmqtt_networkinterface.h b/src/mqtt/qmqtt_networkinterface.h
index f37be35..a431d6b 100644
--- a/src/mqtt/qmqtt_networkinterface.h
+++ b/src/mqtt/qmqtt_networkinterface.h
@@ -53,7 +53,7 @@ class Q_MQTT_EXPORT NetworkInterface : public QObject
 {
     Q_OBJECT
 public:
-    explicit NetworkInterface(QObject* parent = NULL) : QObject(parent) {}
+    explicit NetworkInterface(QObject* parent = nullptr) : QObject(parent) {}
     virtual ~NetworkInterface() {}
 
     virtual void sendFrame(const Frame& frame) = 0;
diff --git a/src/mqtt/qmqtt_router.h b/src/mqtt/qmqtt_router.h
index 019aa5e..35b04d7 100644
--- a/src/mqtt/qmqtt_router.h
+++ b/src/mqtt/qmqtt_router.h
@@ -46,7 +46,7 @@ class Q_MQTT_EXPORT Router : public QObject
 {
     Q_OBJECT
 public:
-    explicit Router(Client *parent = 0);
+    explicit Router(Client *parent = nullptr);
 
     RouteSubscription *subscribe(const QString &route);
     Client *client() const;
diff --git a/src/mqtt/qmqtt_routesubscription.h b/src/mqtt/qmqtt_routesubscription.h
index 17bc39d..f3925ae 100644
--- a/src/mqtt/qmqtt_routesubscription.h
+++ b/src/mqtt/qmqtt_routesubscription.h
@@ -65,7 +65,7 @@ private Q_SLOTS:
 private:
     friend class Router;
 
-    explicit RouteSubscription(Router *parent = 0);
+    explicit RouteSubscription(Router *parent = nullptr);
     void setRoute(const QString &route);
 
     QPointer<Client> _client;
diff --git a/src/mqtt/qmqtt_socket_p.h b/src/mqtt/qmqtt_socket_p.h
index 838233a..cd971af 100644
--- a/src/mqtt/qmqtt_socket_p.h
+++ b/src/mqtt/qmqtt_socket_p.h
@@ -50,7 +50,7 @@ class Socket : public SocketInterface
 {
     Q_OBJECT
 public:
-    explicit Socket(QObject* parent = NULL);
+    explicit Socket(QObject* parent = nullptr);
     virtual	~Socket();
 
     virtual QIODevice *ioDevice();
diff --git a/src/mqtt/qmqtt_socketinterface.h b/src/mqtt/qmqtt_socketinterface.h
index 6c80415..88257ef 100644
--- a/src/mqtt/qmqtt_socketinterface.h
+++ b/src/mqtt/qmqtt_socketinterface.h
@@ -54,7 +54,7 @@ class Q_MQTT_EXPORT SocketInterface : public QObject
 {
     Q_OBJECT
 public:
-    explicit SocketInterface(QObject* parent = NULL) : QObject(parent) {}
+    explicit SocketInterface(QObject* parent = nullptr) : QObject(parent) {}
     virtual ~SocketInterface() {}
 
     virtual QIODevice* ioDevice() = 0;
diff --git a/src/mqtt/qmqtt_ssl_socket_p.h b/src/mqtt/qmqtt_ssl_socket_p.h
index 28ccf7f..45fbc7c 100644
--- a/src/mqtt/qmqtt_ssl_socket_p.h
+++ b/src/mqtt/qmqtt_ssl_socket_p.h
@@ -54,7 +54,7 @@ class SslSocket : public SocketInterface
 {
     Q_OBJECT
 public:
-    explicit SslSocket(const QSslConfiguration& config, QObject* parent = NULL);
+    explicit SslSocket(const QSslConfiguration& config, QObject* parent = nullptr);
     virtual ~SslSocket();
 
     virtual QIODevice *ioDevice();
diff --git a/src/mqtt/qmqtt_timer_p.h b/src/mqtt/qmqtt_timer_p.h
index fc48dfc..699008e 100644
--- a/src/mqtt/qmqtt_timer_p.h
+++ b/src/mqtt/qmqtt_timer_p.h
@@ -43,7 +43,7 @@ class Timer : public TimerInterface
 {
     Q_OBJECT
 public:
-    explicit Timer(QObject *parent = 0);
+    explicit Timer(QObject *parent = nullptr);
     virtual ~Timer();
 
     bool isSingleShot() const;
diff --git a/src/mqtt/qmqtt_timerinterface.h b/src/mqtt/qmqtt_timerinterface.h
index de176d5..2d87415 100644
--- a/src/mqtt/qmqtt_timerinterface.h
+++ b/src/mqtt/qmqtt_timerinterface.h
@@ -42,7 +42,7 @@ class Q_MQTT_EXPORT TimerInterface : public QObject
 {
     Q_OBJECT
 public:
-    explicit TimerInterface(QObject* parent = NULL) : QObject(parent) {}
+    explicit TimerInterface(QObject* parent = nullptr) : QObject(parent) {}
     virtual ~TimerInterface() {}
 
     virtual bool isSingleShot() const = 0;
diff --git a/src/mqtt/qmqtt_websocket.cpp b/src/mqtt/qmqtt_websocket.cpp
index c824786..bb1fa5b 100644
--- a/src/mqtt/qmqtt_websocket.cpp
+++ b/src/mqtt/qmqtt_websocket.cpp
@@ -16,7 +16,7 @@ QMQTT::WebSocket::WebSocket(const QString& origin,
     , _ioDevice(new WebSocketIODevice(_socket, this))
 {
     initialize();
-    if (sslConfig != NULL)
+    if (sslConfig != nullptr)
         _socket->setSslConfiguration(*sslConfig);
     connect(_socket, &QWebSocket::sslErrors, this, &WebSocket::sslErrors);
 }
diff --git a/src/mqtt/qmqtt_websocket_p.h b/src/mqtt/qmqtt_websocket_p.h
index 6d159fa..ca77363 100644
--- a/src/mqtt/qmqtt_websocket_p.h
+++ b/src/mqtt/qmqtt_websocket_p.h
@@ -62,12 +62,12 @@ class WebSocket : public SocketInterface
     WebSocket(const QString& origin,
               QWebSocketProtocol::Version version,
               const QSslConfiguration* sslConfig,
-              QObject* parent = NULL);
+              QObject* parent = nullptr);
 #endif // QT_NO_SSL
 
     WebSocket(const QString& origin,
               QWebSocketProtocol::Version version,
-              QObject* parent = NULL);
+              QObject* parent = nullptr);
 
     virtual ~WebSocket();
 
diff --git a/src/mqtt/qmqtt_websocketiodevice_p.h b/src/mqtt/qmqtt_websocketiodevice_p.h
index 9fb169a..6f79727 100644
--- a/src/mqtt/qmqtt_websocketiodevice_p.h
+++ b/src/mqtt/qmqtt_websocketiodevice_p.h
@@ -51,7 +51,7 @@ class WebSocketIODevice : public QIODevice
 {
     Q_OBJECT
 public:
-    explicit WebSocketIODevice(QWebSocket *socket, QObject *parent = NULL);
+    explicit WebSocketIODevice(QWebSocket *socket, QObject *parent = nullptr);
 
     bool connectToHost(const QNetworkRequest &request);
 
diff --git a/tests/gtest/tests/customprinter.cpp b/tests/gtest/tests/customprinter.cpp
index 2cac54a..64b1d80 100644
--- a/tests/gtest/tests/customprinter.cpp
+++ b/tests/gtest/tests/customprinter.cpp
@@ -40,7 +40,7 @@ void CustomPrinter::OnTestCaseStart(const TestCase& test_case)
     qout << "[----------] "
          << (test_case.test_to_run_count() == 1 ? "test" : "tests")
          << " from " << test_case.name();
-    if(NULL == test_case.type_param())
+    if(nullptr == test_case.type_param())
     {
         qout << endl;
     }
@@ -88,18 +88,18 @@ void CustomPrinter::OnTestEnd(const TestInfo& test_info)
     PrintTestName(test_info.test_case_name(), test_info.name());
     if (test_info.result()->Failed())
     {
-        if (NULL != test_info.type_param() || NULL != test_info.value_param())
+        if (nullptr != test_info.type_param() || nullptr != test_info.value_param())
         {
             qout << ", where ";
-            if (NULL != test_info.type_param())
+            if (nullptr != test_info.type_param())
             {
                 qout << "TypeParam = " << test_info.type_param();
-                if (NULL != test_info.value_param())
+                if (nullptr != test_info.value_param())
                 {
                     qout << " and ";
                 }
             }
-            if (NULL != test_info.value_param())
+            if (nullptr != test_info.value_param())
             {
                 qout << "GetParam() = " << test_info.value_param();
             }
diff --git a/tests/gtest/tests/routedmessagetests.cpp b/tests/gtest/tests/routedmessagetests.cpp
index 16767de..d27ea89 100644
--- a/tests/gtest/tests/routedmessagetests.cpp
+++ b/tests/gtest/tests/routedmessagetests.cpp
@@ -19,7 +19,7 @@ private slots:
 };
 
 RoutedMessageTests::RoutedMessageTests()
-    : _uut(NULL)
+    : _uut(nullptr)
 {
 }
 
diff --git a/tests/gtest/tests/routertests.cpp b/tests/gtest/tests/routertests.cpp
index ac5b4f3..42da51c 100644
--- a/tests/gtest/tests/routertests.cpp
+++ b/tests/gtest/tests/routertests.cpp
@@ -23,7 +23,7 @@ private slots:
 };
 
 RouterTests::RouterTests()
-    : _uut(NULL)
+    : _uut(nullptr)
 {
 }
 
@@ -46,7 +46,7 @@ void RouterTests::cleanup()
 void RouterTests::subscribe_Test()
 {
 //    QMQTT::RouteSubscription* subscription = _uut->subscribe("route");
-//    QVERIFY(NULL != subscription);
+//    QVERIFY(nullptr != subscription);
 //    QCOMPARE(subscription->route(), QString("route"));
 }
 
diff --git a/tests/gtest/tests/routesubscriptiontests.cpp b/tests/gtest/tests/routesubscriptiontests.cpp
index da8cfda..7e01952 100644
--- a/tests/gtest/tests/routesubscriptiontests.cpp
+++ b/tests/gtest/tests/routesubscriptiontests.cpp
@@ -19,7 +19,7 @@ private slots:
 };
 
 RouteSubscriptionTests::RouteSubscriptionTests()
-    : _uut(NULL)
+    : _uut(nullptr)
 {
 }
 
diff --git a/tests/gtest/tests/socketmock.h b/tests/gtest/tests/socketmock.h
index db85e83..16e8724 100644
--- a/tests/gtest/tests/socketmock.h
+++ b/tests/gtest/tests/socketmock.h
@@ -9,7 +9,7 @@
 class SocketMock : public QMQTT::SocketInterface
 {
 public:
-    SocketMock(QObject* parent = NULL)
+    SocketMock(QObject* parent = nullptr)
         : QMQTT::SocketInterface (parent)
         , mockIoDevice(new IODeviceMock)
     {
diff --git a/tests/gtest/tests/tcpserver.cpp b/tests/gtest/tests/tcpserver.cpp
index 8eda19f..a188ba8 100644
--- a/tests/gtest/tests/tcpserver.cpp
+++ b/tests/gtest/tests/tcpserver.cpp
@@ -3,7 +3,7 @@
 #include <QTcpSocket>
 
 TcpServer::TcpServer(const QHostAddress host, const quint16 port)
-    : _socket(NULL)
+    : _socket(nullptr)
 {
     connect(this, &QTcpServer::newConnection, this, &TcpServer::on_newConnection);
     listen(host, port);
@@ -15,16 +15,16 @@ TcpServer::~TcpServer()
 
 void TcpServer::on_newConnection()
 {
-    if(NULL != _socket)
+    if(nullptr != _socket)
     {
         disconnect(_socket, &QTcpSocket::readyRead, this, &TcpServer::on_readyRead);
         _socket->disconnectFromHost();
         _socket->deleteLater();
-        _socket = NULL;
+        _socket = nullptr;
         _data.clear();
     }
     _socket = nextPendingConnection();
-    if(NULL != _socket)
+    if(nullptr != _socket)
     {
         connect(_socket, &QTcpSocket::readyRead, this, &TcpServer::on_readyRead);
     }