How are header guards implemented in C and what code can appear between them?

Implementing Header Guards: Code Placement and Conventions

In software development, header guards play a crucial role in preventing multiple inclusions of header files. This article delves into the implementation of header guards and explores the content that can appear between them.

Header Guard Structure

Header guards typically follow a naming convention where the header filename is suffixed with _H, for example, ADD_H. The structure of a header guard is as follows:

#ifndef FILENAME_H
#define FILENAME_H

// Header file content

#endif

Content Between Header Guards

The code snippets included between the header guards constitute the header file. These snippets can include declarations, function prototypes, and macro definitions. For instance, in the provided example:

#ifndef ADD_H
#define ADD_H

#include "mymath.h"
int add(int x, int y);

#endif

Header Guard Conventions

The use of _H as a suffix for header guards is a widely adopted convention. However, it is not a requirement. You can define header guards using any unique name, such as:

#ifndef FLUFFY_KITTENS
#define FLUFFY_KITTENS

// Header file content

#endif

Placement of int main()

Note that the main() function should never be placed within a header file. Its location should always be in a .cpp file. Therefore, int main() does not come after the #endif directive in header guards.

Header Guard Functionality

Header guards serve as a safeguard against including a header file multiple times within the same .cpp file. If you attempt to include a header file that has already been included, the compiler will skip the code between #ifndef and #endif, preventing duplicate inclusions.

This mechanism ensures that all .cpp files can include a guarded header file exactly once, thereby avoiding potential conflicts and logical errors in your code.

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