C++ interfaces and inheritance

Question

I want to have an interface IA and another that extends it IB.

A then implements IA, and B inherits A and also implements IB.

However when compiling B gets errors saying the IA stuff is undefined, even though A defined it all :(

class IA
{
public:
    virtual ~IA(){}
    virtual void foo()=0;
};
class IB : public IA
{
public:
    virtual void bar()=0;
};


class A : public IA
{
public:
    A();
    void foo();
};
class B : public A, public IB
{
public:
    B();
    void bar();
};

error C2259: 'B' : cannot instantiate abstract class
due to following members:
'void IA::foo(void)' : is abstract

Answer 1

Take a look at the C++ FAQ, from the following point onwards: https://isocpp.org/wiki/faq/multiple-inheritance#mi-diamond

It explains the "dreaded diamond" and virtual inheritance in some detail.

Answer 2

(No, A does not define anything. You declared A() and void foo(), but you did not define them.)

The real problem is your multiple inheritance.

       B
     /   \
   A      IB
  /         \
IA          IA

As you can see, you have two "versions" of IA in your inheritance tree, and only A implements void IA::foo (though, as I noted above, this will give you a linker error as you didn't define the implementation).

void IB::IA::foo() remains unimplemented; thus, B itself "inherits the abstractness" and you can't instantiate it.

B will have to implement void foo() as well, or you might be able to use virtual inheritance to hack around it.