Qt trigger signal on all instances of a given class

Question

Typically Qt signals are triggered on a specific instance of a QObject.

Is there a way to trigger it on all instances of a specific class via, perhaps, MetaObjects?

Or the only way is to maintain your own static list of all instances, perhaps by using constructors and destructors, and then just Q_FOREACH it?

Answer 1

How about creating a singleton behind the scenes, and connecting all your instances to a signal from that singleton (signal-to-signal connection)? When you want all your instances to emit the signal, just make the singleton emit it, and all instances will forward it.

Answer 2

Signals cannot be emitted outside of the class that defines them (or derives from the class that defines them) without invoking it through the QMetaObject system:

QMetaObject::invokeMethod( myObj, "mySignal",
                           Q_ARG( QString, "str" ),
                           Q_ARG( int, 42 ) );

However there doesn't appear to be an API method of getting all objects of all particular type to emit, the nearest I could find is:

for ( QWidget* widget : QApplication::allWidgets() ) {
    if ( dynamic_cast< myType* >( widget ) ) {
        QMetaObject::invokeMethod( widget, "mySignal",
                                   Q_ARG( QString, "str" ),
                                   Q_ARG( int, 42 ) );
    }
}

But obviously this only works for QWidget derived types, there doesn't appear to be a QObject equivalent.