I need to include two header files to each other not using forward declaration cause get "incomplete type" error

Question

I need to include two header files to each other but I have trouble doing this. Is there any way except using forward declaration and template to do this ? or I'm not allowed to do it in c++?

here is what I want to do:

// A.hpp file
#ifndef H_A_H
#define H_A_H
#include "B.hpp"
class A {
private:
  vector<B*> b;
public:
  void function() {
  // using methods of B
  }
};
#endif


// B.hpp file
#ifndef H_B_H
#define H_B_H
#include "A.hpp"
class B {
private:
  vector<A*> a;
public:
  void function() {
  // using methods of A
  }
};
#endif

1) What's stopping you from forward declaring `A` before declaration of `B`, and vice-versa? 2) You should define methods in .cpp files, where both of those headers are included. — Algirdas Preidžius, Jun 04 '18 at 10:07
You could also create a `C` class that operates on both `A` and `B` such that `A` and `B` need not "know" about each other .. or maybe use `std::map >` for your `A`->`B` mappings, then a `std::map >` for your `B`->`A` mappings and get rid of the `vector` in your classes ....? — txtechhelp, Jun 04 '18 at 10:13
@AlgirdasPreidžius the second point you mentioned is exactly one of my other problems. whenever I define methods in cpp file I get the "multiple definition of ..." error. — shirazy, Jun 04 '18 at 10:14
@txtechhelp thank you but I think that wouldn't be a good idea for my design. — shirazy, Jun 04 '18 at 10:15
@shirazy Then, you shouldn't: 1) include said .cpp file into other files, or 2) define it in both the header, AND a .cpp file. — Algirdas Preidžius, Jun 04 '18 at 10:16

score 4 · Accepted Answer · answered Jun 04 '18 at 10:43

You can't include two header files to each other. There should be forward declaration in one of the files and function definition has to be pushed to .cpp file where you can include the header file.

// HeaderA.h file
#ifndef H_A_H
#define H_A_H
#include "HeaderB.h"
class A {
private:
  int b;
public:
  void function() {
  // using methods of B
      B b;
      b.function();
  }
};
#endif

// HeaderB.h file
#ifndef H_B_H
#define H_B_H

class A;

class B {
private:
  int a;
public:
  void function();
};
#endif

// Main.cpp
#include "HeaderA.h"
#include "HeaderB.h"

 void B::function()
 {
  // using methods of A
      A a;
      a.function();
 }

int _tmain(int argc, _TCHAR* argv[])
{
    return 0;
}

score 1 · Answer 2 · answered Jun 04 '18 at 10:40

You have a cyclic dependency. This answer explains how to deal with them via forward declarations.

This article also deals with cyclic dependencies.

If you 100% don't want to use forward declarations and it is possible you can split logic in a different class and use composition.

// SomeLogic.h
class SomeLogic
{
};

// A.h
#include "SomeLogic.h"
class A
{
    SomeLogic someLogic;
};

// B.h
#include "SomeLogic.h"
class B
{
    SomeLogic someLogic;
};

I need to include two header files to each other not using forward declaration cause get "incomplete type" error

2 Answers2