In the C and C++ programming languages, an #include guard, sometimes called a macro guard, is a particular construct used to avoid the problem of double inclusion when dealing with the include directive. The addition of #include guards to a header file is one way to make that file idempotent.
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The following C code demonstrates a problem that can arise if #include guards are missing:
struct foo { int member; };
#include "grandfather.h"
#include "grandfather.h" #include "father.h"
Here, the file "child.c" has indirectly included two copies of the text in the header file "grandfather.h". This causes a compilation error, since the structure type foo is apparently defined twice. In C++, this would be a violation of the One Definition Rule.
#ifndef GRANDFATHER_H #define GRANDFATHER_H struct foo { int member; }; #endif
#include "grandfather.h"
#include "grandfather.h" #include "father.h"
Here, the first inclusion of "grandfather.h" causes the macro GRANDFATHER_H to be defined. Then, when "child.c" includes "grandfather.h" the second time, the #ifndef test returns false, and the preprocessor skips down to the #endif, thus avoiding the second definition of struct foo. The program compiles correctly.
Different naming conventions for the guard macro may be used by different programmers. Other common forms of the above example include GRANDFATHER_INCLUDED, CREATORSNAME_YYYYMMDD_HHMMSS (with the appropriate time information substituted), and names generated from a UUID. (However, names starting with one or two underscores, such as _GRANDFATHER_H and __GRANDFATHER_H, are reserved to the implementation and must not be used by the user.[1][2]) It is important to avoid the common pitfall of duplicating the name in a different file (even one in a different project), which defeats the purpose of include guards.
In order 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, many C and C++ implementations provide the non-standard directive #pragma once. This directive, inserted at the top of a header file, will ensure that the file is only included once. This approach, however, can be thwarted by the potential difficulty of telling whether two #include directives in different places actually refer to the same header (for example, via a symbolic link on Unix-like systems). Also, since #pragma once is not a standard directive, its semantics may be subtly different on different implementations. The Objective-C language (which is a superset of C) introduced an #import directive, which works exactly like #include, except that it only includes each file once, thus obviating the need for #include guards.[3]