Declare abstract signal in interface class

Question

How to declare a Qt signal in an abstract class / interface when the implementing class is already derrived from QObject/QWidget?

class IEmitSomething {    public:      // this should be the signal known to others      virtual void someThingHappened() = 0; }  class ImplementEmitterOfSomething : public QWidget, public IEmitSomething {      // signal implementation should be generated here      signals: void someThingHappended(); } 

Answer 1

As I found out in the last days... the Qt way of doing this is like this:

class IEmitSomething {    public:      virtual ~IEmitSomething(){} // do not forget this     signals: // <- ignored by moc and only serves as documentation aid             // The code will work exactly the same if signals: is absent.      virtual void someThingHappened() = 0; }  Q_DECLARE_INTERFACE(IEmitSomething, "IEmitSomething") // define this out of namespace scope  class ImplementEmitterOfSomething : public QWidget, public IEmitSomething {    Q_OBJECT    Q_INTERFACES(IEmitSomething)     signals:       void someThingHappended(); } 

Now you can connect to those interface signals.

If you don't have access to the implementation when connecting to the signal your connect statement will require a dynamic cast to QObject:

IEmitSomething* es = ... // your implementation class  connect(dynamic_cast<QObject*>(es), SIGNAL(someThingHappended()), ...); 

... and this way you're not forced to expose the implementation class to subscribers and clients. Yeah!!!