Headers Including Each Other in C

In C , it is occasionally necessary for headers to include each other. However, this can lead to issues, especially when it comes to where to place the #include statements.

Inside or Outside Macros

In general, #include statements should be placed inside macros, such as #ifndef include guards. This prevents infinite recursion during compilation, as demonstrated in the following example:

// A.h
#ifndef A_H_
#define A_H_

#include "B.h"

class A {
    private:
        B b;
    public:
        A() : b(*this) {}
};

#endif // A_H_

// B.h
#ifndef B_H_
#define B_H_

#include "A.h"

class B {
    private:
        A& a;
    public:
        B(A& a) : a(a) {}
};

#endif // B_H_

Placing the #include statements outside the macros results in the compiler recursing indefinitely due to the mutual inclusion between A.h and B.h.

Undeclared Types

However, placing the #include statements inside the macros can lead to issues with undeclared types. For example, consider the following code:

// A.h
#ifndef A_H_
#define A_H_

class A;  // Forward declaration

#include "B.h"

class A {
    private:
        B b;
    public:
        A() : b(*this) {}
};

#endif // A_H_

// B.h
#ifndef B_H_
#define B_H_

#include "A.h"

class B {
    private:
        A a;  // Directly include A
    public:
        B(A& a) : a(a) {}
};

#endif // B_H_

In this case, the compiler will complain that A is an undeclared type within B.h. This is because the forward declaration in A.h is not visible when B.h is included.

Solution: Forward Declarations

To resolve these issues, it is best to use forward declarations and include the header containing the full definition where necessary. In this example, a forward declaration of A should be added to B.h before the definition of B:

// B.h
#ifndef B_H_
#define B_H_

class A;  // Forward declaration

#include "A.h"

class B {
    private:
        A a;  // Directly include A
    public:
        B(A& a) : a(a) {}
};

#endif // B_H_