macros declaration in headers

[kalyanreddy]

hi,

I have doubt about macros declaration in header files..

I saw one example in that header files are declared like this..

if temp.h is a header file
inside temp.h program is written like
#ifndef __temp_h__
#define __temp_h__
.
.
.
#endif

can any one explain what does it mean actually....

Thanks,
kalyan

[Disch]

It's called an include guard. It prevents errors if the header is included multiple times in the same source file.

#ifndef __temp_h__  // if '__temp_h__ isn't already defined...
#define __temp_h__  // ...then define it

// .. and compile the stuff in the header

#endif  // if __temp_h__ *was* already defined, the compiler skips to here
//  ie:  it skips over the entire header 

Example:

// main.cpp

#include "temp.h"  // first time it's #included, so __temp_h__ was not
 // defined yet.  The header will be compiled normally and now __temp_h__ has
 // been defined

#include "temp.h"  // __temp_h__ has already been defined, so this time
 // the header is skipped over 

[kalyanreddy]

Thanks Disch..I think I understood now...

so if I have a header file like xxxx.h

inside of xxxx.h i can call it __xxxx_h__

am I correct?

[Disch]

Yes.

You can name the guard whatever you want, just as long as it's unique so that no other header uses the same guard name.

[kalyanreddy]

cool Disch, once again thank you...

[kalyanreddy]

I also saw another post about this topic in detail
http://www.cplusplus.com/forum/beginner/30180/