Basic question on use of #ifndefine

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I need to ask a very basic question. Can anyone explain the use of these statements in the same file? What does these statements signify when written together?

C++

#ifndef ABC_H
#define ABC_H
//code
#endif

Posted 4-Apr-12 1:16am

Member 8576081

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3 solutions

Solution 3

The code between #ifndef ABC_H and #endif is compiled only when ABC_H isn't defined.

The #define ABC_H line, defines the ABC_H identifier.

This combination is used often, in order to ensure that a header file is included only one time. At the first time that the compiler enters to the file, the ABC_H identifier isn't defined, so the content of the file is included (and the ABC_H identifier is defined). At the next time that the compiler enters to the file, the ABC_H identifier is already defined...

Posted 4-Apr-12 1:42am

Shmuel Zang

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Pranit Kothari · Accepted Answer · 2012-04-04T01:23:00

Solution 1

This is known as inclusion guard. In C/C++ we include header files. It is possible that one header file may be included in server other header files. Suppose you have two such header files, which includes say "File.h", your preprocessor expands File.h twice. To avoid this we use inclusion guards.

For more details see this[^]

Posted 4-Apr-12 1:23am

Pranit Kothari

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Member 8576081 4-Apr-12 7:28am

Thanx a lot for such a quick response...

[no name] 4-Apr-12 7:48am

Thx

Pablo Aliskevicius · Accepted Answer · 2012-04-04T01:25:00

Solution 2

These are 'inclusion guards'.
See http://en.wikipedia.org/wiki/Include_guard[^]
Basically, they let you include the same header more than once in a module, without generating an error.

Hope this helps,

Pablo.

Posted 4-Apr-12 1:25am

Pablo Aliskevicius

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Member 8576081 4-Apr-12 7:29am

Thanx Pablo!