Signal without connection even in event loop?

[Guest]

Hey there,
I've got a short question about the Signal-Slot-Concept of Qt. If I have a class which emits a signal and no one is connected to this signal, will there be any data in the event loop of this "emit"-statement? Or just ignores the MOC this statement?

A short example:

main.cpp
@#include <QCoreApplication>
#include "MyClass.h"

int main(int argc, char argv[])
{
QCoreApplication app(argc, argv);
MyClass pObject = new MyClass();
pObject->start();

return app.exec();

}
@

MyClass.h
@
#include <QObject>

class MyClass : public QObject
{
Q_OBJECT
public:
explicit MyClass(QObject *parent = 0);

signals:
void emitSignal();

public slots:
void start();
};
@

MyClass.cpp
@
#include "MyClass.h"

MyClass::MyClass(QObject *parent) :
QObject(parent)
{
}

void MyClass::start() {
emit emitSignal();
}
@

Can anybody help? :-)

[raven-worx]

basically the emit enters the corresponding moc-class, which can't find any established connection to a signal/slot and just returns.

Btw. the event loop is not involved at all in most cases. Only if the connection is made with an queued type.

[JKSH]

raven-worx is right.

Every time you emit a signal, it calls QMetaObject::activate(), even if no slot is connected. (You never need to call this function directly though)

Read "How Qt Signals and Slots Work":http://woboq.com/blog/how-qt-signals-slots-work.html. The section called "Signal Emission" describes QMetaObject::activate():
[quote][If] there is nothing connected to this signal, ... we can return quickly, which means emitting a signal connected to no slot is extremely fast.[/quote]

[Guest]

Thanks to both of you!