Question

Let’s say I have this signal:

signals:
    void progressNotification(int progress);

I only recently learned about the emit keyword in Qt. Until now, I used to execute signals by just calling them like a regular function. So instead of:

emit progressNotification(1000 * seconds);

I would write:

progressNotification(1000 * seconds);

Calling them like that seemed to work, and all the connected slots would execute, so does using the emit keyword cause a different behaviour, or is it just syntactic sugar?

Answer 1

emit is just syntactic sugar. If you look at the pre-processed output of function that emits a signal, you’ll see emit is just gone.

The “magic” happens in the generated code for the signal emitting function, which you can look at by inspecting the C++ code generated by moc.

For example a foo signal with no parameters generates this member function:

void W::foo()
{
    QMetaObject::activate(this, &staticMetaObject, 0, 0);
}

And the code emit foo(); is pre-processed to simply foo();

emit is defined in Qt/qobjectdefs.h (in the open-source flavor of the source anyway), like this:

#ifndef QT_NO_EMIT
# define emit
#endif

(The define guard is to allow you to use Qt with other frameworks that have colliding names via the no_keywords QMake config option.)