Cross referencing included headers in c++ program

Question

I am curious about a scenario set up like the following example:

Here is code that would be placed in a file called Header1.h:

#ifndef HEADER1_H #define HEADER1_H  #include "Header2.h"  class Class1 {  Class2 class2Instance; };  #endif 

Here is code that would be placed in a file called Header2.h:

#ifndef HEADER2_H #define HEADER2_H  #include "Header1.h"  class Class2 {  Class1 class1Instance; };  #endif 

I get error messages when I do this (because of the includes I assume), but it feels like I would need to do this in order to include each of the objects in the separate classes. Can anyone help me accomplish this, what am I doing wrong?

Answer 1

The problem is that the size of Class1 depends on Class2, and vice-versa. Therefore, there's no way to calculate the size for either one. Forward-declare one of the classes, and change one of the attributes to be a pointer or reference:

#ifndef HEADER2_H #define HEADER2_H  class Class1; class Class2 {  Class1 *class1Instance;  // or  Class1 &class1Instance; };  #endif 

Answer 2

file:header1.h

#ifndef HEADER1_H #define HEADER1_H  class Class2; // no need to include Header2  class Class1 {     Class2* class2Instance; }  #endif 

file:header1.cpp

#include "header2.h" // do include Header2 here, otherwise you will get errors 

adapted from here