Multiple inheritance tree of pure abstract classes

I have to clean a piece of code... it looks something like this:

struct InterfaceA { virtual void foo () = 0; };
struct InterfaceB : public  InterfaceA { virtual void var () = 0; };

struct ImplA : public InterfaceA
{
    void foo () { std::cout << "foo" << std::endl; }
};

struct ComplexImpl : public InterfaceB, public ImplA
{
    void var () { std::cout << "var" << std::endl; }
    //void foo () { ImplA::foo (); } //  <<=== I want to erase this
};

void herenciaCompleja ()
{
    ImplA x;
    x.foo ();

    ComplexImpl y;
    y.foo ();
}

The problem comes in the commented line, when commented, fails with the error "cannot instantiate abstract class".

I'll probabily use a reference in the InterfaceA replacing the inheritance... but, Is there any way to do it while maintaining the inheritance?