Inherit from multiple partial implementations of an abstract base class?

Question

Is it possible to have a number of partial implementations of an abstract interface, and then collect these partial implementations into a single concrete class by using multiple inheritence?

I have the following example code:

#include <iostream>

struct Base
{
    virtual void F1() = 0;
    virtual void F2() = 0;
};

struct D1 : Base
{
    void F1() override { std::cout << __func__ << std::endl; }
};

struct D2 : Base
{
    void F2() override { std::cout << __func__ << std::endl; }
};

// collection of the two partial implementations to form the concrete implementation
struct Deriv : D1, D2
{
    using D1::F1; // I added these using clauses when it first didn't compile - they don't help
    using D2::F2;
};

int main()
{
    Deriv d;
    return 0;
}

This fails to compile with the following errors:

main.cpp: In function ‘int main()’:
main.cpp:27:11: error: cannot declare variable ‘d’ to be of abstract type ‘Deriv’
main.cpp:19:8: note:   because the following virtual functions are pure within ‘Deriv’:
main.cpp:5:18: note:    virtual void Base::F1()
main.cpp:6:18: note:    virtual void Base::F2()

Answer 1

Try inheriting virtually from Base:

struct D1 : virtual Base
{
    void F1() override { std::cout << __func__ << std::endl; }
};

struct D2 : virtual Base
{
    void F2() override { std::cout << __func__ << std::endl; }
};

Without the virtual inheritance, your multiple inheritance scenario looks like inheritance from two separate and incomplete base classes D1 and D2, neither of which can be instantiated.