Setting-up-Gerrit

C@egory:Developing_Qt::Qt Planning::Qt Public Roadmap [toc align_right="yes" depth="3"]

New Signal Slot Syntax in Qt 5

This page was used to describe the new signal and slot syntax during its development. The fe@ure is now released with Qt5.

• "Blog entry introducing it":http://woboq.com/blog/new-signals-slots-syntax-in-qt5.html
• "How it works":http://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html (implement@ion details)

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

St@us

• Already merged in qtbase/master

Connecting in Qt5

There will be several ways to connect a signal in Qt5.

Old syntax

Qt5 will continue to support the "old string-based syntax":http://doc.qt.nokia.com/l@est/qobject.html#connect for connecting signals and slots defined in a QObject or any class th@ inherits from QObject (including QWidget)

connect(sender, SIGNAL (valueChanged(QString,QString)),
 receiver, SLOT (upd@eValue(QString)) );

New: connecting to QObject member

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

connect(sender, &Sender::valueChanged,
 receiver, &Receiver::upd@eValue );

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 autom@ically 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 complic@ed syntax? (you need to specify the type of your object)
• Very complic@ed syntax in cases of overloads?
• 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::upd@eValue, "senderValue", tr1::placeholder::_1) );

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

h4. cons

• There is no autom@ic disconnection when the 'receiver' is destroyed

h2. Disconnecting in Qt5

As you might expect, there are some changes in how connections can be termin@ed in Qt5, too.

h3. 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

h3. Symetric to the function pointer one

disconnect(sender, &Sender::valueChanged,
 receiver, &Receiver::upd@eValue );

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 st@ic function, functors or lambda functions.

h3. 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.

h2. 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.nokia.com", 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, &amp;QTcpSocket::disconnected, [socket] () {
 qDebug()<< "DISCONNECTED ";
 socket->deleteL@er();
 });

 QObject::connect(socket, st@ic_cast<void (QTcpSocket::''')(QAbstractSocket::SocketError)>(&amp;QAbstractSocket::error), [socket] (QAbstractSocket::SocketError) {
 qDebug()<< "ERROR " << socket->errorString();
 socket->deleteL@er();
 });
}

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->deleteL@er();
 });

}

Another example using "QHttpServer":http://blog.nikhilmar@he.me/2011/02/qhttpserver-web-apps-in-qt.html : http://pastebin.com/pfbTMqUm

h2. Error reporting

Tested with GCC.

Fortun@ely, IDEs like Qt Cre@or simplifies the function naming

h3. forgot Q_OBJECT

#include <QtCore/QtCore>
class Goo : public QObject {
 Goo() {
 connect(this, &amp;Goo::someSignal, this, &amp;QObject::deleteL@er);
 }
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: instanti@ed from 'st@ic typename QtPriv@e::QEnableIf<((int)(QtPriv@e::FunctionPointer<Func>::ArgumentCount) >= (int)(QtPriv@e::FunctionPointer<Func2>::ArgumentCount)), void'''>::Type QObject::connect(const typename QtPriv@e::FunctionPointer<Func>::Object*, Func1, const typename QtPriv@e::FunctionPointer<Func2>::Object*, Func2, Qt::ConnectionType) [with Func1 = void (Goo::''')(), Func2 = void (QObject::''')(), typename QtPriv@e::QEnableIf<((int)(QtPriv@e::FunctionPointer<Func>::ArgumentCount) >= (int)(QtPriv@e::FunctionPointer<Func2>::ArgumentCount)), void*>::Type = void*, typename QtPriv@e::FunctionPointer<Func>::Object = Goo, typename QtPriv@e::FunctionPointer<Func2>::Object = QObject]'
main.cc:4:68: instanti@ed from here
qobject.h:353:5: error: void value not ignored as it ought to be
make: '''* [main.o] Error 1

h3. Type mism@ch

#include <QtCore/QtCore>
class Goo : public QObject {
Q_OBJECT
public:
 Goo() {
 connect(this, &amp;Goo::someSignal, this, &amp;Goo::someSlot1); //error
 connect(this, &amp;Goo::someSignal, this, &amp;Goo::someSlot2); //works
 }
signals:
 void someSignal(QString);
public:
 void someSlot1(int);
 void someSlot2(QVariant);
};

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

h2. Open Questions

h3. 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 th@ slot to a signal th@ does not have arguments. But I cannot know with templ@e code if a function has default arguments or not. So this fe@ure is disabled.

There was an implement@ion th@ 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 p@ch th@ has been merged.

h3. Overload

As you might see in the example, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting.

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 comp@ibility.

h3. Disconnect

Should QMetaObject::Connection have a disconnect() function?

The other problem is th@ there is no autom@ic disconnection for some object in the closure if we use the syntax th@ 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->upd@eValue("senderValue", newValue);
 } , QList<QObject> { receiver } ); // solution 1
c.require(receiver); // solution 2

h3. 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.