Headers Including Each Other in C

C header files can include each other, but certain guidelines must be followed to avoid compilation errors.

Include Statement Placement

include statements should be placed inside preprocessor macros, known as include guards, to prevent multiple inclusions. This is especially important when headers include each other.

Forward Declarations

Consider the following code where two classes, A and B, include each other:

<code class="cpp">// A.h

#ifndef A_H_
#define A_H_

#include "B.h"

class A
{
public:
    A() : b(*this) {}

private:
    B b;
};

#endif</code>

<code class="cpp">// B.h

#ifndef B_H_
#define B_H_

#include "A.h"

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

private:
    A& a;
};

#endif</code>

In this scenario, the compiler encounters class B first, but A has not yet been declared. To resolve this, a forward declaration of A should be included before the definition of B:

<code class="cpp">// B.h

#ifndef B_H_
#define B_H_

class A;  // Forward declaration of class A

#include "A.h"

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

private:
    A& a;
};

#endif</code>

This forward declaration informs the compiler that A is a class, even though its complete definition is not yet available.

In Practice

In general, #include statements should be placed inside include guards, and forward declarations should be used when a header needs to refer to a class that is defined in a later included header. By following these guidelines, you can avoid compilation errors caused by circular includes.

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