Interfaces must inherit QObject?!

kshegunov

@JonB said in Interfaces must inherit QObject?!:

Is that a requirement of C++?

It is not a requirement, but there's a significant difference.
Here's links, I know you love them sources:
https://isocpp.org/wiki/faq/multiple-inheritance (What is the “dreaded diamond”? being key)
https://en.cppreference.com/w/cpp/language/derived_class (Virtual base classes is relevant)

JonB

@kshegunov
Yeah, thanks, this looks good. public virtual Base, that's a new one for me, I like it. The more complexity in C++ the better... ;-)

JKSH

@JonB said in Interfaces must inherit QObject?!:

@kshegunov
Is that a requirement of C++? The OP claimed
class VictoriaSpongeCake : public ICake, public QObject
So if I change ICake to inherit like this:
class ICake: public QObject
It all compiles without a problem.

That is not quoting me, it is quoting him.

OP forgot to re-run qmake (or delete the build directory) after making ICake inherit QObject and adding the Q_OBJECT macro.

@JonB said in Interfaces must inherit QObject?!:

How then can the OP here arrange to write an "interface" which inherits from QObject and apply it to classes which inherit from QObject too?

They can't.

QObjects are designed to be "identity objects", as opposed to "value objects": https://doc.qt.io/qt-5/object.html#qt-objects-identity-vs-value (for example, an image is a value but a window is an identity). I'd say that an "interface object" is neither an identity nor a value.

One more thought: If a class inherits from 2 differnent QObjects, what should obj->staticMetaObject.superClass() return?

P.S. I think you'll enjoy the thoroughness of this article: https://www.ics.com/blog/multiple-inheritance-qt

JonB

@JKSH said in Interfaces must inherit QObject?!:

OP forgot to re-run qmake (or delete the build directory) after making ICake inherit QObject and adding the Q_OBJECT macro.

They can't.

Thank you, this makes a lot more sense then!