C++: How can I avoid "invalid covariant return type" in inherited classes without casting?

Question

I have a quite complex class hierarchy in which the classes are cross-like depending on each other: There are two abstract classes A and C containing a method that returns an instance of C and A, respectively. In their inherited classes I want to use a co-variant type, which is in this case a problem since I don't know a way to forward-declare the inheritance relation ship.

I obtain a "test.cpp:22: error: invalid covariant return type for ‘virtual D* B::outC()’"-error since the compiler does not know that D is a subclass of C.

class C;

class A {
public:
        virtual C* outC() = 0;
};

class C {
public:
        virtual A* outA() = 0;
};


class D;

class B : public A {
public:
        D* outC();
};

class D : public C {
public:
        B* outA();
};

D* B::outC() {
        return new D();
}

B* D::outA() {
        return new B();
}

If I change the return type of B::outC() to C* the example compiles. Is there any way to keep B* and D* as return types in the inherited classes (it would be intuitive to me that there is a way)?

Answer 1

I know of no way of having directly coupled covariant members in C++. You'll have either to add a layer, or implement covariant return yourself.

For the first option

class C;

class A {
public:
        virtual C* outC() = 0;
};

class C {
public:
        virtual A* outA() = 0;
};


class BI : public A {
public:
};

class D : public C {
public:
        BI* outA();
};

class B: public BI {
public:
        D* outC();
};

D* B::outC() {
        return new D();
}

BI* D::outA() {
        return new B();
}

and for the second

class C;

class A {
public:
        C* outC() { return do_outC(); }
        virtual C* do_outC() = 0;
};

class C {
public:
        virtual A* outA() = 0;
};


class D;

class B : public A {
public:
        D* outC();
        virtual C* do_outC();
};

class D : public C {
public:
        B* outA();
};

D* B::outC() {
        return static_cast<D*>(do_outC());
}

C* B::do_outC() {
        return new D();
}

B* D::outA() {
        return new B();
}

Note that this second option is what is done implicitly by the compiler (with some static checks that the static_cast is valid).

Answer 2

As far as I know, there's no way to do this without explicit casting. The problem is that the definition of class B can't know that D is a subclass of C until it sees a full definition of class D, but the definition of class D can't know that B is a subclass of A until it sees a full definition of class B, and so you have a circular dependency. This can't be resolved with forward-declarations because a forward declaration unfortunately cannot specify an inheritance relationship.

There's a similar problem with trying to implement a covariant clone() method using templates, which I found can be solved, but the analogous solution still fails here because the circular reference remains impossible to resolve.