Question

Is it legal to forward declare in another header’s file? For example:

#ifndef _MAIN_H_
#define _MAIN_H_
class ClassA;
class ClassB;
#include "classa.h"
#include "classb.h"
#endif

#ifndef _CLASSA_H_
#define _CLASSA_H_
#include "main.h"
class ClassA
{
public:
    ClassB b;
};
#endif

#ifndef _CLASSB_H_
#define _CLASSB_H_
#include "main.h"
class ClassB
{
public:
    ClassA a;
};
#endif

Both class A and class B depend on each other, and both have an object of the other type. What I did was forward declared both classes in another file. Is there a clean way to do this?

Answer 1

In general it is legal to forward-declare classes across headers.

However, in your example, you are instantiating both classes in each other, which is absolutely illegal!

To show the reasoning behind this, think about the following:

Let ClassA take 1 byte without b and ClassB also 1 byte without a.
Now, include b: ClassA now takes 2 byte. Now include a: ClassB now takes 3 bytes. Now we have to update the size of ClassA to 4 bytes due to the size increase of ClassB. Following that logic ClassB is now 5 bytes, ClassA 6 bytes, ClassB 7 bytes…….

To deal with this, you probably would like to change the type of (at least) one of a and b to a pointer or reference to the respective class. When doing this (in c/c++), make sure you understand your memory management!

The solution to your original problem could then look as follows:

ClassA.h

#pragma once
class ClassB;
class ClassA
{
public:
    ClassB* b;
}

ClassB.h

#pragma once
#include "ClassA.h"
class ClassB
{
public:
    ClassA a;
}

Note that ClassB.h requires ClassA.h since for demonstration purposes: ClassB contains a full object of type ClassA, so the definition of ClassA is required!