# Include guard

In the C and C++ programming languages, an #include guard, sometimes called a macro guard, header guard or file guard, is a way to avoid the problem of double inclusion when dealing with the include directive.
The C preprocessor processes inclusion directives like #include "foo.h" to include "foo.h" and transcludes the code of that file into a copy of the main file often called the translation unit.
However, if an #include directive for a given file appears multiple times during compilation, the code will effectively be duplicated in that file. If the included file includes a definition, this can cause a compilation error due to the One Definition Rule, which says that definitions (such as the definition of a class) cannot be duplicated in a translation unit. #include guards prevent this by defining a preprocessor macro when a header is first included. In the event that header file is included a second time, the #include guard will prevent the actual code within that header from being compiled.
An alternative to #include guards is #pragma once. This non-standard but commonly supported directive among C and C++ compilers has the same purpose as an #include guard, but has less code and does not require the definition of a variable.
Modules, introduced in C++20, eliminate the necessity of #include guards, due to not being handled by the preprocessor.

## Double inclusion
### Example

The following C code demonstrates a real problem that can arise if #include guards are missing:

#### Result

Here, the file "child.c" has indirectly included two copies of the text in the header file "grandparent.h". This causes a compilation error, since the structure type foo will thus be defined twice. In C++, this would be called a violation of the one definition rule.

## Use of #include guards
### Example

The same code as the previous section is used with the addition of #include guards. The C preprocessor preprocesses the header files, including and further preprocessing them recursively. This will result in a working source file.

#### Result

Here, the first inclusion of "grandparent.h" has the macro GRANDPARENT_H defined. When "child.c" includes "grandparent.h" at the second time (while including "parent.h"), as the #ifndef test returns false, the preprocessor skips down to the #endif, thus avoiding the second definition of struct foo. The program compiles correctly.

### Discussion

Different naming conventions for the guard macro may be used by different programmers. Other common forms of the above example include GRANDPARENT_INCLUDED, CREATORSNAME_YYYYMMDD_HHMMSS (with the appropriate time information substituted), and names generated from a UUID. (However, names starting with one underscore and a capital letter (C and C++) or any name containing double underscore (C++ only), such as _GRANDPARENT_H and GRANDPARENT__H, are reserved to the language implementation and should not be used by the user.)
Of course, it is important to avoid duplicating the same include-guard macro name in different header files, as including the 1st will prevent the 2nd from being included, leading to the loss of any declarations, inline definitions, or other #includes in the 2nd header.

## Difficulties

For #include guards to work properly, each guard must test and conditionally set a different preprocessor macro. Therefore, a project using #include guards must work out a coherent naming scheme for its include guards, and make sure its scheme doesn't conflict with that of any third-party headers it uses, or with the names of any globally visible macros.
For this reason, most C and C++ implementations provide a non-standard #pragma once directive. This directive, inserted at the top of a header file, will ensure that the file is included only once. The Objective-C language (which is a superset of C) has an #import directive, which works exactly like #include, except that it includes each file only once, thus obviating the need for #include guards.

## Other languages

Some languages support specifying that the code should be included only once, in the including file, rather than in the included one (as with C/C++ include guards and #pragma once):
* PL/I uses the %INCLUDE statement as the equivalent to C's #include directive. IBM Enterprise PL/I also supports the %XINCLUDE statement which will "incorporate external text into the source program if it has not previously been included." (It also offers an XPROCEDURE statement, similar to a PROCEDURE statement, which will ignore the second and subsequent occurrences of an XPROCEDURE with the same name.)
* Objective-C's #import directive (see above)
* PHP's include_once