condition dependant compilation error

[Deamon]

Hi forum,

i made a small test app along my c++ study and found an odd circumstance. I made a console project that contains three files with the following test code.
-------------------------------------------------
//main.cpp

#include <iostream>
#include "includestd.h"

#ifndef _process_h
#include "process.h"
#endif

int main()
{
   std::cout << "Hello world!" << std::endl;
   return 0;
}
--------------------------------------------------
// process.h

class vehicle{int i;};
--------------------------------------------------
// includestd.h

#ifndef _process_h
#include "process.h"
#endif
--------------------------------------------------

When i try to compile this i get this errors:

process.h:4: redefinition of `class vehicle'
process.h:4: previous definition of `class vehicle'

Obviously this is becose of an double inclusion of the process.h, but why ?

I have included this code in to the main.cpp to prevent a double inclusion but it still includes it double:

#ifndef _process_h
#include "process.h"
#endif

Do i have a misconception here ?

BTW: When i change the process.h code to:
-------------------------
#ifndef _process_h
#define _process_h

class vehicle{int i;};

#endif
-------------------------

it works fine but shouldn't it work even without it ?

regards,
Deamon

[anarxia]

The "standard" way to avoid multiple includes is the 2nd way (that you also confirmed that it works).
Why the other way doesn't work? You never define _process_h!

[Deamon]

Why the other way doesn't work? You never define _process_h!

That means the first method is pointless ? That means that if i have defined the .h then i can include it where ever i want it without fear of an double inclusion in the for of #include "process.h" ?

I don't need to check it via:

#ifndef _process_h
#include "process.h"
#endif

???

But is there a way to check if a .h was already included and include it only if it's not ?

some sort of:

#ifninc _process_h
#include "process.h"
#endif

???

regards,
Deamon

[TDragon]

Remember, #ifndef / #define / #endif is just a fancy (ab)use of the preprocessor for preventing multiple inclusion; it's not magical in any way.

#ifndef _process_h -- IF "_process_h" has not already been defined
#define _process_h -- then define it now
... (code) ... -- and process the following code
#endif -- (otherwise the intervening code is skipped over, because _process_h was already defined)

If you don't include the line "#define _process_h", then obviously it will always process the code between the #ifndef / #endif, because _process_h is never #defined.

Use "#ifdef" to check if a macro HAS been defined (note the lack of an 'n'). #ifdef and #ifndef are shortened forms of "#if defined" and "#if !defined".

[Deamon]

Ok i think i got it now. Thank you