Question

I must admit I’m having trouble formulating this question, but I’ll try my best to be precise. I have tried to search for an answer to my question, but I suspect I have been unable to find what I’m looking for, as I’m not exactly sure what to call this.

I have a base class A, and several child classes inheriting from this base class. I then make another class X that inherits from some of the mentioned child classes. The problem I’m now facing is that each of the classes X inherits, have their own instance of class A. The code below should give a better understanding of what I mean.

class A;
class B : public A;
class C : public A;

class X : public B, public C;

Is there a way to make class B and C share the same instance of class A, when they are both acting as indirect base classes for the same class?

To give an example of why I want this, lets look at this code.

    class A
    {
        int _x;
    };

    class B : public A
    {
        void outputX(){std::cout << A::_x << std::endl;
    };

    class C : public A
    {
       void setX(int x){A::_x=x;}
    };

    class X : public B, public C
    {
        C::setX(5);

        // this will output an un-initialized _x, 
        // as B and C have their own version of A
        B::outputX() 
    };

Now I realize this seems rather unnecessary in this example here, but in my real situation I like to think it would be a good solution if B and C shared instance of A in class X.

Is this at all possible?

Answer 1

You can solve this by using virtual inheritance:

class B : virtual public A;
class C : virtual public A;

class X : virtual public B, virtual public C;

See more on the diamond problem.