Headers

Header is a .h file used mainly for declarations.

Example add.h:

#ifndef ADD_H
#define ADD_H

int add(int x, int y);

#endif

Put definitions in .cpp files, and declarations in headers.

The paired .cpp file usually has the same base name as the header and includes it directly:

#include "add.h"

int add(int x, int y)
{
    return x + y;
}

Include my own headers with quotes:

#include "add.h"

Include standard headers with angle brackets:

#include <iostream>

Do not include .cpp files. Add them to compilation instead.

Header guard#

Header guard protects a header from being included multiple times into the same translation unit.

#ifndef SOME_UNIQUE_NAME_H
#define SOME_UNIQUE_NAME_H

// declarations here

#endif

How it works:

1. #ifndef checks whether the macro has not been defined yet.
2. #define defines the macro.
3. On the next include of the same header, the code between #ifndef and #endif is skipped.

Header guard uses preprocessor directives.

Header guard does not block including the same header in different .cpp files.

Header guard also does not replace the rule: declarations in .h, function definitions in .cpp.

#pragma once works similarly and is convenient, but it is not part of the C++ standard.