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Новый синтаксис сигналов и слотов в Qt 5

На этой странице можно узнать о новом синтаксисе сигналов и слотов, который используется в последних версиях Qt 5.

• Начальные сведения в блогах
• Как это работает (особенности реализации)

Подсказка: Старый синтаксис использования сигналов и слотов остается по прежнему и будет работать.

Состояние

• Уже слито в qtbase/master

Связи в Qt5

В Qt5 имеется несколько способов связать сигналы и слоты.

Старый синтаксис

В Qt 5 по прежнему осталась возможность использования старого синтаксиса для связи сигнала и слотов old string-based syntax определенные в QObject или любом наследнике класса QObject (включая QWidget).

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

Новый синтаксис

Способ связи сигнала и слота между двумя объектами класса QObjects и не строковыми параметрами:

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

pros

• Проверка существования сигнала и слота во время компиляции.
• Аргумент может иметь псевдоним typedefs или находится в другом пространстве имен.
• Автоматическое преобразование типов при неявном объявлении (например, из QString в QVariant)
• Возможность связать любую член-функцию QObject, необязательно слот.

cons

• Более сложный и непонятный синтаксис? (Необходимо указывать явно тип объекта)
• 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);

pro

• can be used with tr1::bind
• can be used with c+11 lambda expressions

connect(sender, &Sender::valueChanged,
 tr1::bind(receiver, &Receiver::updateValue, "senderValue", tr1::placeholder::_1) );

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 Qt5

As you might expect, there are some changes in how connections can be terminated in Qt5, 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 C11 it is possible to keep the code inline

void doYourStuff(const QByteArray &page)
{
 QTcpSocket *socket = new QTcpSocket;
 socket->connectToHost("qt.io", 80);
 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();
 });
}

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

forgot Q_OBJECT

#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(QString);
public:
 void someSlot1(int);
 void someSlot2(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. 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 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.

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);

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

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

Some macro could help (with c11 or typeof extensions)

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 QMetaObject::Connection 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 take a closure. One could add a list of object in the disconnection, or a new function like QMetaObject::Connection::require

auto c = connect(sender, &Sender::valueChanged, [=](const QString &newValue) {
 receiver->updateValue("senderValue", newValue);
 } , QList<QObject> { receiver } ); // solution 1
c.require(receiver); // solution 2

Callbacks

Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot. This do not work for the new method. If one wants to do callback c+ way, one should use std::function (or tr1) But we cannot use STL types in our ABI, so a QFunction should be done to copy std::function. This is anyway irrelevant for QObject connections.