C++ Multiple Inheritance - why you no work?

Question

I am trying to figure out an interesting multiple inheritance issue.

The grandparent is an interface class with multiple methods:

class A
{
public:
    virtual int foo() = 0;
    virtual int bar() = 0;
};

Then there are abstract classes that are partially completing this interface.

class B : public A
{
public:
    int foo() { return 0;}
};

class C : public A
{
public:
    int bar() { return 1;}
};

The class I want to use inherits from both of the parents and specifies what method should come from where via using directives:

class D : public B, public C
{
public:
    using B::foo;
    using C::bar;
};

When I try to instantiate a D I get errors for trying to instantiate an abstract class.

int main()
{
    D d; //<-- Error cannot instantiate abstract class.

    int test = d.foo();
    int test2 = d.bar();

    return 0;
}

Can someone help me understand the problem and how to best make use of partial implementations?

Answer 1

You don't have diamond inheritance. The B and C base classes of D each have their own A base class subobject because they do not inherit virtually from A.

So, in D, there are really four pure virtual member functions that need to be implemented: the A::foo and A::bar from B and the A::foo and A::bar from C.

You probably want to use virtual inheritance. The class declarations and base class lists would look like so:

class A
class B : public virtual A
class C : public virtual A
class D : public B, public C

If you don't want to use virtual inheritance then you need to override the other two pure virtual functions in D:

class D : public B, public C
{
public:
    using B::foo;
    using C::bar;

    int B::bar() { return 0; }
    int C::foo() { return 0; }
};