Multiple Inheritance in C++, choose from which class take the member

Question

Consider the following code:

class A {
  public:
  virtual void f() {
    std::cout << "A-F" << std::endl;
  }

  virtual void g() {
    std::cout << "A-G" << std::endl;
  }
};

class B: public A {
  public:
  void f() {
    std::cout << "B-F" << std::endl;
  }
};

class C: public A {
  public:
  void g() {
    std::cout << "C-G" << std::endl;
  }
};

Now I want to define a class C whose member f is class B, and member g is from class A. Something like this:

class D: public B, public C {
  public:
  \\ Inheritate f from B

  \\ Inheritate g from C
};

How do I do it in c++?

Answer 1

You can achieve the same if your C only inherits from B, i.e. class C: public B {/*...*/}. If you really want your C to inherit from both, B and A, you have to use virtual inheritance (see the virtual keywords when inheriging from A):

#include <iostream>

class A {
  public:
  virtual void f() {
    std::cout << "A-F" << std::endl;
  }

  void g() {
    std::cout << "A-G" << std::endl;
  }
};

class B: virtual public A {
  public:
  void f() override {
    std::cout << "B-F" << std::endl;
  }
};

class C: virtual public A, public B {
  public:
};


int main() {

    C c;
    c.f();
    c.g();

    return 0;
}

This prints

B-F
A-G

Unreladed: I'd recommend to use the override (or final) keyword, especially when dealing with more complex class hierarchies.

EDIT

As OP changed the question:

#include <iostream>

class A {
  public:
  virtual void f() {
    std::cout << "A-F" << std::endl;
  }

  virtual void g() {
    std::cout << "A-G" << std::endl;
  }
};

class B: virtual public A {
  public:
  void f() override {
    std::cout << "B-F" << std::endl;
  }
};

class C: virtual public A {
  public:
  void g() override {
    std::cout << "C-G" << std::endl;
  }
};

class D: public B, public C {
  public:
  // Inherits f from B
  // Inherits g from C
};


int main() {

    D d;
    d.f();
    d.g();

    return 0;
}

This prints:

B-F
C-G

as required.

Note That I am not a big fan of virtual inheritance and maybe there is a better solution for your problem. However, this depends on your exact problem.