Include guard

The include guard or include Guardian is a programming technique in C and C ++ to solve the problem of multiple integration.

This problem occurs when the same header file is included several times within a translation unit . This is usually done unintentionally, e.g. B. if several modules use the same library .

Example of a multiple integration

The following two header files A.h and are given B.h. a. define one class each :

// A.h
const int M = 123;

class A
{ /* ... */ };

// B.h
#include "A.h"

class B : public A
{ /* ... */ };

In the main program, both class A and B should now be used:

1 // program.cpp
2 #include "A.h"
3 #include "B.h"
4 
5 int main() { /* ... */ }

The program cannot be compiled without errors because class A is defined twice: The first definition is made when the header file is included A.hin line 2. The second definition is made because the header file B.h, which is included in line 3, itself A.hinvolves. The class (and possibly other variables) from this file have already been defined beforehand, which leads to a compilation error.

Solution with preprocessor macros (#ifndef)

The #ifndef wrapper is the traditional and C-compliant approach. The problem of multiple integration is solved by defining a unique preprocessor macro when the header file is first included. If the macro was already defined, the following definitions of the header are skipped:

// A.h
#ifndef A_H
#define A_H

class A
{ /* ... */ };

#endif /* A_H */

The above preprocessor commands cause the first time incorporation of Ah (from program.cpp ) the macro A_H is not yet defined and the preprocessor passes through the definitions. With the second integration (from Bh ) the macro is already defined and the preprocessor skips the block #ifndef… #endif .

Since macros exist in the global namespace, problems can arise here if an attempt is made to use the same name elsewhere (so-called namespace pollution ). This can be prevented by specifying naming conventions, but it does not solve the problem. Identifiers with a leading underscore (_) are reserved for compiler-specific definitions and definitions in the standard libraries. A leading underscore may therefore not be used in the application code according to the current C and C ++ standards.

The CPP , the preprocessor of the GCC , recognizes such constructs automatically, notes the corresponding files and skips them when they are integrated again.

Solution via language extensions (#pragma once)

Most of the C ++ compilers in use today support the language extension #pragma once. This also ensures that a (header) file is only integrated once, but starts at a higher level, directly at the preprocessor, and does not introduce macros into the global namespace.

To use it, it is sufficient to #pragma onceinsert the preprocessor directive within the header file :

// A.h
#pragma once

class A
{ /* ... */ };

This language extension is supported by the following C ++ implementations:

• GCC ,
• Clang ,
• Microsoft Visual C ++ ,
• Intel C ++ compiler ,
• Comeau C / C ++ ,
• C ++ Builder XE3,
• Digital Mars C ++ .

Individual evidence

1. ↑ Eric Fleegal's WebLog  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. on MSDN Blogs; Retrieved August 19, 2011.  
2. ↑ C ++ standard (ISO / IEC 14882); C ++ working draft N3242 (PDF), section 17.6.4.3.2
3. ↑ C standard (ISO / IEC 9899); Committee Draft N1124 (PDF), Sections 6.10.8 and 7.1.3 / 1.
4. ↑ Once-Only Headers in the GNU CPP Online Manual. Retrieved August 19, 2011.
5. ↑ GCC 3.4 Release Series - Changes, New Features, and Fixes . Retrieved May 16, 2014.
6. ↑ clang: Pragma.cpp Source File ( Memento of the original from April 4, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . Retrieved May 16, 2014.  
7. ↑ MS Developer Network - once (C / C ++) . Retrieved May 16, 2014.
8. ↑ Comeau C ++ 4.0 ( Memento of the original from December 11, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. - Pre-Release - User Documentation: Pragmas ( Memento of the original dated December 11, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . Retrieved May 16, 2014.  
9. ↑ #pragma once - RAD Studio XE3 . Retrieved May 16, 2014.
10. ↑ Pragmas #pragma once . Retrieved May 16, 2014.