Qt multiple inheritance and signals

Question

I\'m having a problem with QT regarding multiple enheritance because of QObject. I know that a lot of others have the same problems but I don\'t know how I should fix it.

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Answer 1

Use virtual inheritence, e.g.

class X : public virtual Y 
{
};

class Z : public virtual Y
{
};

class A : public virtual X, public virtual Z
{
};

will only have one copy of the base class Y

Answer 2

You should use virtual inheritance.

see http://en.allexperts.com/q/C-1040/virtual-inheritance.htm

Seems like you are experiencing the diamond issue, see also:

http://www.cprogramming.com/tutorial/virtual_inheritance.html

Do it like this:

class NavigatableItem : public virtual QObject
class Button : public virtual NavigatableItem, public virtual QToolButton
class MainMenuOption : public Button

Answer 3

It requires a bit more code, but what I have done in the past is make one of them (your NavigatableItem in this case) a pure virtual class, i.e. interface. Instead of using the "signals" macro, make them pure virtual protected functions. Then multiply-inherit from your QObject-derived class as well as the interface, and implement the methods.

I know it is somewhat controversial, but avoiding multiple implementation inheritance at all costs does solve a host of problems and confusion. The Google C++ Style Guidelines recommend this, and I think it is good advice.

class NavigatableItemInterface
{
    // Don't forget the virtual destructor!
    protected:
        virtual void deselected() = 0;
        virtual void selected() = 0;
        virtual void activated() = 0;
};

class Button : public NavigatableItemInterface, public QToolButton
{
    Q_OBJECT
    ...
    signals:
        virtual void deselected();
        ...
}