pragma once

In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation. Thus, #pragma once serves the same purpose as include guards, but with several advantages, including: less code, avoidance of name clashes, and sometimes improvement in compilation speed.^[1]

Example

File "grandparent.h"

#pragma once

struct foo 
{
    int member;
};

File "parent.h"

#include "grandparent.h"

File "child.c"

#include "grandparent.h"
#include "parent.h"

In this example, the inclusion of grandparent.h in both parent.h and child.c would ordinarily cause a compilation error, because a struct with a given name can only be defined a single time in a given compilation. The #pragma once directive serves to avoid this by ignoring subsequent inclusions of grandparent.h.

Advantages

The most common alternative to #pragma once is to use #define to set an #include guard macro, the name of which is picked by the programmer to be unique to that file. For example,

#ifndef GRANDPARENT_H
#define GRANDPARENT_H
... contents of grandparent.h
#endif /* !GRANDPARENT_H */

This is more complicated, possibly less efficient, and prone to error as there are no mechanisms to prevent a programmer accidentally using the same macro name in more than one file, which would result in only one of the files being included. Since the compiler itself is responsible for handling #pragma once, the programmer cannot make errors which cause name clashes.

Using #pragma once instead of include guards will, for some compilers, improve compilation speed since it is a higher-level mechanism; the compiler itself can compare filenames or inodes without having to invoke the C preprocessor to scan the header for #ifndef and #endif. Yet, since include guards appear very often and the overhead of opening files is significant, it's common for compilers to optimize the handling of include guards, making them as fast as #pragma once.^[2][3][4]

Caveats

Identifying the same file on a file system is not a trivial task.^[5] Symbolic links and especially hard links may cause the same file to be found under different names in different directories. Compilers may use a heuristic that compares file size, modification time and content.^[6] Additionally, #pragma once can do the wrong thing if the same file is intentionally copied into several parts of a project, e.g. when preparing the build. Whereas include guards would still protect from double definitions, #pragma once may or may not treat them as the same file in a compiler-dependent way. These difficulties, together with difficulties related to defining what constitutes the same file in the presence of hard links, networked file systems etc. so far prevented the standardization of #pragma once.

Portability

References

External links

• P0538R0 A Qualified Replacement for #pragma once