How do I access the friend function objects with my derived classes in seperate files?

Question

I have 3 separate header files. Class A, Class B is derived A and Class.

class A {
public:
    virtual void foo(C ...object...);
};

class B : public A {
public:
    void foo(C ...object...);
};

class C {
public:
    friend class A;
    friend class B;
private:
    A *arr[num][num];
};

Arr is pointer of 2D array.The inside of the 2D array is filled with B objects.How can I access the C class object from the header file of class B? Is it possible?If possible, how should the "include .h" seperates header files and foo function prototype be?

Answer 1

A common way to split your code into separate files is following:

• A.h

  #pragma once
  class C;
  
  class A {
  public:
      virtual void foo(C c);
  };
  

  Class A doesn't need any type information of C. The declaration of member function foo doesn't need complete type information but a declaration because the function expects a parameter of type C.

• A.cpp

  #include "A.h"
  #include "C.h"
  
  void A::foo(C c) {}
  

  The definition of function A::foo(C c) needs complete type information of C. Therefore the class definition of C is included.

• B.h

  #pragma once
  #include "A.h"
  
  class C;
  
  class B : public A {
  public:
       void foo(C c) override;
  };
  

  Class B needs complete type information of A for inheritance. Therefore the class definition of A is included. Complete type information of class C is not necessary (see A.h).

• B.cpp

  #include "B.h"
  #include "C.h"
  
  void B::foo(C c) {}
  

  Same as in A.cpp

• C.h

  #pragma once
  class A;
  
  class C {
  public:
       friend class A;
       friend class B;
  private:
       A *arr[5][5];
  };
  

  Class C just contains friend declarations of A and B and a pointer to type A. A friend declaration don't need a declaration or full type information. Since all pointers to objects have same size complete type information for class A is not necessary. It will become necessary when the pointer is dereferenced.

All source files also contain an include of the corresponding header file.