New Signal Slot Syntax: Difference between revisions

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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.
This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.
* {{DocLink|signalsandslots-syntaxes||Differences between String-Based and Functor-Based Connections}} (Official documentation)
 
* [http://woboq.com/blog/new-signals-slots-syntax-in-qt5.html Introduction] (Woboq blog)
*{{DocLink|signalsandslots-syntaxes||Differences between String-Based and Functor-Based Connections}} (Official documentation)
* [http://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html Implementation Details] (Woboq blog)
*[http://woboq.com/blog/new-signals-slots-syntax-in-qt5.html Introduction] (Woboq blog)
*[http://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html Implementation Details] (Woboq blog)


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


== Connecting in Qt 5 ==
==Connecting in Qt 5==


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


=== Old syntax ===
===Old syntax===


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


<pre>
<syntaxhighlight lang="c++">
connect(
connect(
     sender, SIGNAL( valueChanged( QString, QString ) ),
     sender, SIGNAL( valueChanged( QString, QString ) ),
     receiver, SLOT( updateValue( QString ) )
     receiver, SLOT( updateValue( QString ) )
);
);
</pre>
</syntaxhighlight>


=== New: connecting to QObject member ===
===New: connecting to QObject member===


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


<pre>
<syntaxhighlight lang="c++">
connect(
connect(
     sender, &Sender::valueChanged,
     sender, &Sender::valueChanged,
     receiver, &Receiver::updateValue
     receiver, &Receiver::updateValue
);
);
</pre>
</syntaxhighlight>


==== Pros ====
====Pros====


* Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
*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.
*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)
*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.
*It is possible to connect to any member function of QObject, not only slots.


==== Cons ====
====Cons====


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


=== New: connecting to simple function ===
===New: connecting to simple function===


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


<pre>
<syntaxhighlight lang="c++">
connect(
connect(
     sender, &Sender::valueChanged,
     sender, &Sender::valueChanged,
     someFunction
     someFunction
);
);
</pre>
</syntaxhighlight>


==== Pros ====
====Pros====


* Can be used with <tt>std::bind</tt>:
*Can be used with <tt>std::bind</tt>:


<pre>
<syntaxhighlight lang="c++">
connect(
connect(
     sender, &Sender::valueChanged,
     sender, &Sender::valueChanged,
     std::bind( &Receiver::updateValue, receiver, "senderValue", std::placeholders::_1 )
     std::bind( &Receiver::updateValue, receiver, "senderValue", std::placeholders::_1 )
);
);
</pre>
</syntaxhighlight>


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


<pre>
<syntaxhighlight lang="c++">
connect(
connect(
     sender, &Sender::valueChanged,
     sender, &Sender::valueChanged,
     [=]( const QString &newValue ) { receiver->updateValue( "senderValue", newValue ); }
     [=]( const QString &newValue ) { receiver->updateValue( "senderValue", newValue ); }
);
);
</pre>
</syntaxhighlight>


==== Cons ====
====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).
*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 ==
==Disconnecting in Qt 5==


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


=== Old way ===
===Old way===


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


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


=== Symetric to the function pointer one ===
===Symetric to the function pointer one===


<pre>
<syntaxhighlight lang="c++">
disconnect(
disconnect(
     sender, &Sender::valueChanged,
     sender, &Sender::valueChanged,
     receiver, &Receiver::updateValue
     receiver, &Receiver::updateValue
);
);
</pre>
</syntaxhighlight>


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


=== New way using QMetaObject::Connection ===
===New way using QMetaObject::Connection===


<pre>
<syntaxhighlight lang="c++">
QMetaObject::Connection m_connection;
QMetaObject::Connection m_connection;
// …
// …
Line 116: Line 117:
// …
// …
QObject::disconnect( m_connection );
QObject::disconnect( m_connection );
</pre>
</syntaxhighlight>


Works in all cases, including lambda functions or functors.
Works in all cases, including lambda functions or functors.


== Asynchronous made easier ==
==Asynchronous made easier==


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


<pre>
<syntaxhighlight lang="c++">
void doYourStuff( const QByteArray &page )
void doYourStuff( const QByteArray &page )
{
{
     QTcpSocket *socket = new QTcpSocket;
     QTcpSocket *socket = new QTcpSocket;
    socket->connectToHost( "qt.io", 80 );
     QObject::connect(
     QObject::connect(
         socket, &QTcpSocket::connected,
         socket, &QTcpSocket::connected,
Line 151: Line 151:
         }
         }
     );
     );
    socket->connectToHost( "qt.io", 80 );
}
}
 
</syntaxhighlight>
</pre>


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


<pre>
<syntaxhighlight lang="c++">
void Doc::saveDocument()
void Doc::saveDocument()
{
{
Line 173: Line 173:
     );
     );
}
}
</pre>
</syntaxhighlight>


Another example using [http://blog.nikhilmarathe.me/2011/02/qhttpserver-web-apps-in-qt.html QHttpServer] : http://pastebin.com/pfbTMqUm
Another example using [http://blog.nikhilmarathe.me/2011/02/qhttpserver-web-apps-in-qt.html QHttpServer] : http://pastebin.com/pfbTMqUm


== Error reporting ==
==Error reporting==


Tested with GCC.
Tested with GCC.
Line 183: Line 183:
Fortunately, IDEs like Qt Creator simplifies the function naming
Fortunately, IDEs like Qt Creator simplifies the function naming


=== Missing Q_OBJECT in class definition ===
===Missing Q_OBJECT in class definition===


<pre>
<syntaxhighlight lang="c++">
#include <QtCore>
#include <QtCore>


Line 197: Line 197:
         void someSignal();
         void someSignal();
};
};
</pre>
</syntaxhighlight>


<pre>
<pre>
Line 214: Line 214:
</pre>
</pre>


=== Type mismatch ===
===Type mismatch===


<pre>
<syntaxhighlight lang="c++">
#include <QtCore>
#include <QtCore>


Line 230: Line 230:
          
          
     signals:
     signals:
         void someSignal( QString );
         void someSignal( const QString & );
          
          
     public:
     public:
         void someSlot1( int );
         void someSlot1( int );
         void someSlot2( QVariant );
         void someSlot2( const QVariant & );
};
};
</pre>
</syntaxhighlight>


<pre>
<pre>
Line 260: Line 260:
</pre>
</pre>


== Open questions ==
==Open questions==


=== Default arguments in slot ===
===Default arguments in slot===


If you have code like this:
If you have code like this:


class A : public QObject {
<syntaxhighlight lang="c++">
    Q_OBJECT
class A : public QObject {
  Q_OBJECT
     public slots:
     public slots:
         void someSlot(int foo = 0);
         void someSlot(int foo = 0);
};
};
</syntaxhighlight>


The old method allows you to connect that slot to a signal that does not have arguments.
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.
But I cannot know with template code if a function has default arguments or not.
So this feature is disabled.


There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.
This problem can be overcome with the help of a small lambda function:
This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.
<syntaxhighlight lang="c++">
A *m_a;          // member of the MyClass
QTimer *m_timer; // member of the MyClass


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


As you might see in the [[#Asynchronous made easier|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:
As you might see in the [[#Asynchronous made easier|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:


<code>connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));</code>
<syntaxhighlight lang="c++">
connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));
</syntaxhighlight>


cannot be simply converted to:
cannot be simply converted to:


connect(
<syntaxhighlight lang="c++">
connect(
     mySpinBox, &QSpinBox::valueChanged,
     mySpinBox, &QSpinBox::valueChanged,
     mySlider, &QSlider::setValue
     mySlider, &QSlider::setValue
);
); // does not compile
</syntaxhighlight>


...because {{DocLink|QSpinBox}} has {{DocLink|QSpinBox|signals|two signals named <tt>valueChanged()</tt>}} with different arguments. Instead, the new code needs to be:
...because {{DocLink|QSpinBox}} has {{DocLink|QSpinBox|signals|two signals named <tt>valueChanged()</tt>}} with different arguments. Instead, the new code has to be:


connect(
<syntaxhighlight lang="c++">
     mySpinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged),
// for c++14 and above
connect(
     mySpinBox, qOverload<int>(&QSpinBox::valueChanged),
     mySlider, &QSlider::setValue
     mySlider, &QSlider::setValue
);
);
 
// for c++11
Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:
connect(
 
    mySpinBox, QOverload<int>::of(&QSpinBox::valueChanged),
void (QSpinBox::* mySignal)(int) = &QSpinBox::valueChanged;
connect(
    mySpinBox, mySignal,
     mySlider, &QSlider::setValue
     mySlider, &QSlider::setValue
);
);
 
</syntaxhighlight>
Some macro could help (with C++11 or ''typeof'' extensions). A template based solution was introduced in Qt 5.7: {{DocLink|QtGlobal|qOverload}}
{{DocLink|QtGlobal|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:
 
The best thing is probably to recommend not to overload signals or slots …
 
… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.
 
=== Disconnect ===
 
Should <tt>QMetaObject::Connection</tt> have a disconnect() function?
 
The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.
One could add a list of objects in the disconnection, or a new function like <tt>QMetaObject::Connection::require</tt>
 
// Solution 1
auto c = connect(
    sender, &Sender::valueChanged,
    [=]( const QString &newValue ) { receiver->updateValue( "senderValue", newValue ); },
    QList<QObject> { receiver }
);
// Solution 2 (needs a new definition of QMetaObject::Connection::require)
c.require( receiver );


<syntaxhighlight lang="c++">
connect(
    mySpinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged),
    mySlider, &QSlider::setValue
)
</syntaxhighlight>


=== Callbacks ===
===Callbacks===


Function such as <tt>QHostInfo::lookupHost</tt> or <tt>QTimer::singleShot</tt> or <tt>QFileDialog::open</tt> take a <tt>QObject</tt> receiver and <tt>char*</tt> slot.
Function such as <tt>QHostInfo::lookupHost</tt> (until Qt 5.9) or <tt>QTimer::singleShot</tt> (until Qt 5.4) or <tt>QFileDialog::open</tt> take a <tt>QObject</tt> receiver and <tt>char*</tt> slot.
This does not work for the new method.
This does not work for the new method.
If one wants to do callback C++ way, one should use <tt>std::function</tt>
If one wants to do callback C++ way, one should use <tt>std::function</tt>
But we cannot use STL types in our ABI, so a <tt>QFunction</tt> should be done to copy <tt>std::function</tt>.
In any case, this is irrelevant for <tt>QObject</tt> connections.

Latest revision as of 14:16, 30 December 2022

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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.

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

Retrieved from "https://wiki.qt.io/index.php?title=New_Signal_Slot_Syntax&oldid=40014"