My attempt at a small hierarchy. Why doesn't my code compile?

See example code below. The instantiation of a Concrete object (see
main function) makes the compiler (VS 2003) complain that Concrete is
an abstract class because MethodsA::callA() and MethodsB::callB() are
not defined.

Hopefully it's obvious what I'm trying to acheive. Can someone tell me
where I'm going wrong?

Thanks,

Pete

// Low level interface
struct MethodsA
{
    virtual ~MethodsA() {}
    virtual void callA() = 0;
};

// Another low level interface
struct MethodsB
{
    virtual ~MethodsB() {}
    virtual void callB() = 0;
};

// Higher level interface - the main base class of concrete objects
struct MainBase
    : public MethodsA,
         public MethodsB
{
};

// A suggested implementation of a first low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfAMixin : public MethodsA
{
    void callA() {}
};

// A suggested implementation of a second low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfBMixin : public MethodsB
{
    void callB() {}
};

// Concrete class
class Concrete
    : public MainBase, // "isa" main base
         public ImplOfAMixin, // pull in this mixin
         public ImplOfBMixin // and this mixin
{
};

int main()
{
    Concrete obj;

    return 0;
}