private/public qt signals

Question

Can Qt signals be public or private? Can I create internal signals, which are seen only inside the class?

Update: I have a class with some internal signals. How can I make those signals invisible for other classes (encapsulation & information hiding)?

Answer 1

No. Signals cannot be public or private. Qt signals are protected class methods.

"signals" keyword is defined in qobjectdefs.h (line 69 as for Qt 4.6.1):

#   define signals protected

UPDATE: signals are only protected upto and including all minor versions of Qt 4. From Qt 5.0 onwards they are public. See https://stackoverflow.com/a/19130831.

Answer 2

A common way, e.g. seen in kdelibs, is this:

Q_SIGNALS:
#ifndef Q_MOC_RUN
    private: // don't tell moc, doxygen or kdevelop, but those signals are in fact private
#endif

   void somePrivateSignal();

This makes the signal private, i.e. it can only be emitted by the class itself but not by its subclasses. To not make the "private:" overrule Q_SIGNALS (moc wouldn't see somePrivateSignal as signal then), it's inside Q_MOC_RUN, which is only defined when moc runs.

Edit: This approach doesn't work for the new-style connects introduced with Qt 5 (connect(a, &A::someSignal, b, &B::someSlot)), as they require the signal to be accessible.

Answer 3

Slots are simple methods which can be public, protected, or private.

As Andrei pointed it out, signal are only a redefinition of protected, meaning they can only be emitted by the class in which they are defined.

If you want to make a class emit a signal from anoter one, you have to add it a public method (or slot) like this one:

void emitTheSignal(...) {
  emit theSignal(...);
}

Answer 4

Signals was protected in Qt4, in Qt5 they are public. Int Qt5 you can make them private by adding QPrivateSignal as the last argument. More on this: http://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html

Answer 5

Qt signals are public in the sense that any object can connect to any signal.

Answer 6

You can use the PIMPL pattern for that. Your private signals exists in the private implementation only.