How to Handle Circular Header Inclusions in C ?

Headers Including Each Other in C

Question:
When working with multiple header files in C , should the #include statements be placed inside or outside of macros? Specifically, what happens when two classes include each other?

Answer:

Macro Placement:
#include statements should always be placed inside the macros (#ifndef include guards) to prevent infinite recursion during compilation.

Circular Inclusions:
Circular inclusions occur when two classes include each other's headers. To resolve this, a forward declaration should be used before defining a class that includes a reference to another class.

Example:

Consider the following header files A.h and B.h:

<code class="cpp">// A.h
#ifndef A_H_
#define A_H_

#include "B.h" // Circular inclusion

class A {
  B b;
};
#endif

// B.h
#ifndef B_H_
#define B_H_

class A; // Forward declaration

class B {
  A& a;
};
#endif</code>

Main Function:

<code class="cpp">// main.cpp
#include "A.h"

int main() {
  A a;
}</code>

Explanation:

Circular Inclusion Issue: If the #include statements were placed outside the macros, the compiler would encounter an infinite recursion while attempting to include both headers.

Forward Declaration: In B.h, a forward declaration of class A; is used. This informs the compiler that A is a class, without including its definition. This allows B to declare a reference to A.

Order of Inclusion: The order of header inclusions is also important. A.h must be included before B.h to allow for the forward declaration.

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