Why Do Include Guards Fail to Prevent Mutual Recursion and Multiple Definitions?

Why aren't include guards preventing mutual, recursive inclusion?

Include guards do protect header files from mutual, recursive inclusion.

The problem arises when there are dependencies between the definitions of data structures in mutually-including headers. For example:

// a.h
#include "b.h"

struct A
{
    ...
};

// b.h
#include "a.h"

struct B
{
    A* pA; // error: class A is forward-declared but not defined
};

To resolve this, forward declarations should be used instead of include guards:

// b.h
#include "a.h"

// Forward declaration of A
struct A;

struct B
{
    A* pA;
};

Why aren't include guards preventing multiple definitions?

Include guards do protect a header from redundant inclusions in the same translation unit. However, multiple definitions can still occur due to their presence in different translation units.

To resolve this, the inline keyword can be used to allow multiple definitions in different translation units:

// header.h
inline int f()
{
    ...
}

Alternatively, the function definition can be moved to a separate source file to prevent multiple definitions:

// source.cpp
int f()
{
    ...
}

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