Question

Is there a way to use any C++ function as a Qt slot, without having its class inheriting from QWidget?

Answer 1

You cannot in Qt versions < Qt 5.

In order to use signals/slots the meta object compiler has to be invoked. To make this happen your class should meet the following requirements:

• Inherit from QObject or any other subclass (eg QWidget, QPushButton etc)
• The Q_OBJECT macro should be defined in the private section of the class in order to enable meta-object features such as slots
• Use the Qt keywords slots and signals in order to declare which functions should be handles by the meta compiler as slots or signals

For more details check the corresponding documentation pages about the meta-object system and the signals & slots

Also check the QObject documentation:

Notice that the Q_OBJECT macro is mandatory for any object that implements signals, slots or properties. You also need to run the
Meta Object Compiler on the source file. We strongly recommend the use
of this macro in all subclasses of QObject regardless of whether or
not they actually use signals, slots and properties, since failure to
do so may lead certain functions to exhibit strange behavior.

Edit: Since Qt 5, functors and lambda expressions can be used as slot. See New Signal Slot Syntax in Qt 5