C++: using a base class as the implementation of an interface

If Base isn’t derived from Interface, then you’ll have to have forwarding calls in Derived. It’s only “overhead” in the sense that you have to write extra code. I suspect the optimizer will make it as efficient as if your original idea had worked.

class Interface {
    public:
        virtual void myfunction() = 0;
};

class Base {
    public:
        virtual void myfunction() {/*...*/}
};

class Derived : public Interface, public Base {
    public:
        void myfunction() { Base::myfunction(); }  // forwarding call
};

int main() {
   Derived d;
   d.myfunction();
   return 0;
}