New Signal Slot Syntax

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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.

• Differences between String-Based and Functor-Based Connections (Official documentation)
• Introduction (Woboq blog)
• Implementation Details (Woboq blog)

Note: This is in addition to the old string-based syntax which remains valid.

Connecting in Qt 5

There are several ways to connect a signal in Qt 5.

Old syntax

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget)

connect(
    sender, SIGNAL( valueChanged( QString, QString ) ),
    receiver, SLOT( updateValue( QString ) )
);

New: connecting to QObject member

Here's Qt 5's new way to connect two QObjects and pass non-string objects:

connect(
    sender, &Sender::valueChanged,
    receiver, &Receiver::updateValue
);

Pros

• Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
• Argument can be by typedefs or with different namespace specifier, and it works.
• Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
• It is possible to connect to any member function of QObject, not only slots.

Cons

• More complicated syntax? (you need to specify the type of your object)
• Very complicated syntax in cases of overloads? (see below)
• Default arguments in slot is not supported anymore.

New: connecting to simple function

The new syntax can even connect to functions, not just QObjects:

connect(
    sender, &Sender::valueChanged,
    someFunction
);

Pros

• Can be used with std::bind:

connect(
    sender, &Sender::valueChanged,
    std::bind( &Receiver::updateValue, receiver, "senderValue", std::placeholders::_1 )
);

• Can be used with C++11 lambda expressions:

connect(
    sender, &Sender::valueChanged,
    [=]( const QString &newValue ) { receiver->updateValue( "senderValue", newValue ); }
);

Cons

• There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a "context object". When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).

Disconnecting in Qt 5

As you might expect, there are some changes in how connections can be terminated in Qt 5, too.

Old way

You can disconnect in the old way (using SIGNAL, SLOT) but only if

• You connected using the old way, or
• If you want to disconnect all the slots from a given signal using wild card character

Symetric to the function pointer one

disconnect(
    sender, &Sender::valueChanged,
    receiver, &Receiver::updateValue
);

Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card) In particular, does not work with static function, functors or lambda functions.

New way using QMetaObject::Connection

QMetaObject::Connection m_connection;
// …
m_connection = QObject::connect( /* … */ );
// …
QObject::disconnect( m_connection );

Works in all cases, including lambda functions or functors.

Asynchronous made easier

With C++11 it is possible to keep the code inline

void doYourStuff( const QByteArray &page )
{
    QTcpSocket *socket = new QTcpSocket;
    QObject::connect(
        socket, &QTcpSocket::connected,
        [socket, page]() { socket->write( QByteArray( "GET " + page + "" ) ); }
    );
    QObject::connect(
        socket, &QTcpSocket::readyRead,
        [socket]() { qDebug() << "GOT DATA " << socket->readAll(); }
    );
    QObject::connect(
        socket, &QTcpSocket::disconnected,
        [socket]() {
            qDebug() << "DISCONNECTED ";
            socket->deleteLater();
        }
    );
    QObject::connect(
        socket, static_cast<void ( QTcpSocket::* )( QAbstractSocket::SocketError )>( &QAbstractSocket::error ),
        [socket]( QAbstractSocket::SocketError ) {
            qDebug() << "ERROR " << socket->errorString();
            socket->deleteLater();
        }
    );
    socket->connectToHost( "qt.io", 80 );
}

Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:

void Doc::saveDocument()
{
    QFileDialog *dlg = new QFileDialog();
    dlg->open();
    QObject::connect(
        dlg, &QDialog::finished,
        [dlg, this]( int result ) {
            if ( result ) {
                QFile file( dlg->selectedFiles().first() );
                // …
            }
            dlg->deleteLater();
        }
    );
}

Another example using QHttpServer : http://pastebin.com/pfbTMqUm

Error reporting

Tested with GCC.

Fortunately, IDEs like Qt Creator simplifies the function naming

Missing Q_OBJECT in class definition

#include <QtCore>

class Goo : public QObject
{
    Goo() {
        connect( this, &Goo::someSignal, this, &QObject::deleteLater );
    }
    
    signals:
        void someSignal();
};

qobject.h: In member function 'void QObject::qt_check_for_QOBJECT_macro(const T&&) const [with T = Goo]':
qobject.h:535:9: instantiated from 'static typename QtPrivate::QEnableIf<((int)
(QtPrivate::FunctionPointer<Func>::ArgumentCount) >= (int)
(QtPrivate::FunctionPointer<Func2>::ArgumentCount)), void*>::Type QObject::connect(const typename
QtPrivate::FunctionPointer<Func>::Object*, Func1, const typename QtPrivate::FunctionPointer<Func2>::Object*,
Func2, Qt::ConnectionType) [with Func1 = void (Goo::*)(), Func2 = void (QObject::*)(), typename
QtPrivate::QEnableIf<((int)(QtPrivate::FunctionPointer<Func>::ArgumentCount) >= (int)
(QtPrivate::FunctionPointer<Func2>::ArgumentCount)), void*>::Type = void*, typename 
QtPrivate::FunctionPointer<Func>::Object = Goo, typename QtPrivate::FunctionPointer<Func2>::Object = QObject]'
main.cc:4:68: instantiated from here
qobject.h:353:5: error: void value not ignored as it ought to be
make: '''* [main.o] Error 1

Type mismatch

#include <QtCore>

class Goo : public QObject
{
    Q_OBJECT
    
    public:
        Goo() {
            connect( this, &Goo::someSignal, this, &Goo::someSlot1 ); // Error
            connect( this, &Goo::someSignal, this, &Goo::someSlot2 ); // Works
        }
        
    signals:
        void someSignal( const QString & );
        
    public:
        void someSlot1( int );
        void someSlot2( const QVariant & );
};

qobject.h: In static member function 'static typename QtPrivate::QEnableIf<((int)
(QtPrivate::FunctionPointer<Func>::ArgumentCount) >= (int)
(QtPrivate::FunctionPointer<Func2>::ArgumentCount)), void*>::Type QObject::connect(const typename
QtPrivate::FunctionPointer<Func>::Object*, Func1, const typename QtPrivate::FunctionPointer<Func2>::Object*,
Func2, Qt::ConnectionType) [with Func1 = void (Goo::*)(QString), Func2 = void (Goo::*)(int), typename
QtPrivate::QEnableIf<((int)(QtPrivate::FunctionPointer<Func>::ArgumentCount) >= (int)
(QtPrivate::FunctionPointer<Func2>::ArgumentCount)), void*>::Type = void*, typename 
QtPrivate::FunctionPointer<Func>::Object = Goo, typename QtPrivate::FunctionPointer<Func2>::Object = Goo]':
main.cc:6:62: instantiated from here
qobject.h:538:163: error: no type named 'IncompatibleSignalSlotArguments' in 'struct 
QtPrivate::CheckCompatibleArguments<QtPrivate::List<QString, void>, QtPrivate::List<int, void>, true>'
qobject.h: In static member function 'static void QtPrivate::FunctionPointer<Ret (Obj::*)(Arg1)>::call
(QtPrivate::FunctionPointer<Ret (Obj::*)(Arg1)>::Function, Obj*, void*) [with Args = QtPrivate::List<QString, void>,
Obj = Goo, Ret = void, Arg1 = int, QtPrivate::FunctionPointer<Ret (Obj::*)(Arg1)>::Function = void (Goo::*)(int)]':
qobject.h:501:13: instantiated from 'void QObject::QSlotObject<Func, Args>::call(QObject*, void**) [with Func =
void (Goo::*)(int), Args = QtPrivate::List<QString, void>, QObject = QObject]'
main.cc:14:2: instantiated from here
qobject.h:109:13: error: cannot convert 'QtPrivate::RemoveRef<QString>::Type' to 'int' in argument passing
make: *** [main.o] Error 1

Open questions

Default arguments in slot

If you have code like this:

class A : public QObject {
   Q_OBJECT
     public slots:
         void someSlot(int foo = 0);
};

The old method allows you to connect that slot to a signal that does not have arguments. This does not work with the new syntax out-of-the-box.

This problem can be overcome with the help of a small lambda function:

A *m_a;          // member of the MyClass
QTimer *m_timer; // member of the MyClass

MyClass::foo()
{
  connect(m_timer, &QTimer::timeout, this, [this]() { m_a->someSlot(); });
}

Overload

As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:

connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));

cannot be simply converted to:

connect(
     mySpinBox, &QSpinBox::valueChanged,
     mySlider, &QSlider::setValue
); // does not compile

...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code has to be:

// for c++14 and above
connect(
     mySpinBox, qOverload<int>(&QSpinBox::valueChanged),
     mySlider, &QSlider::setValue
);
// for c++11
connect(
     mySpinBox, QOverload<int>::of(&QSpinBox::valueChanged),
     mySlider, &QSlider::setValue
);

qOverload and its helper class QOverload (for compilers who don't understand c++14) were added in Qt5.7. For older versions, the following construct has to be used:

connect(
    mySpinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged),
    mySlider, &QSlider::setValue
)

Callbacks

Function such as QHostInfo::lookupHost (until Qt 5.9) or QTimer::singleShot (until Qt 5.4) or QFileDialog::open take a QObject receiver and char* slot. This does not work for the new method. If one wants to do callback C++ way, one should use std::function