(maybe) stupid C++ question Topic is solved

[theigor]

What's the difference between using

Code: Select all

#pragma once

in the beginning of a header file and a

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#ifndef FILE_H
#define FILE_H
...
#endif

[protocol]

Thats not a stupid question to me.

They are both the same but:

#pragma once (and #import)
- is compiler specific

&&

the #define conditional works everywhere

Also: http://en.wikipedia.org/wiki/Pragma_once

[eco]

The wikipedia article is incorrect. #pragma once also works on GCC 3.4+ (it was un-deprecated) and inclusion guards and #pragma once aren't quite the same thing. The result is the same but #pragma once improves compilation speed (see this Games from Within article for details). Use both to get the speed of #pragma once but still fallback to inclusion guards for compilers which don't support it.

[cpp]

mhh, Isnt #pragma once part of ISO compliant C++??

[lowjoel]

Yes, it's meant for "allowing various instructions to be given to a compiler". It is standard, but there may be some features present in compiler 'a' that isn't in compiler 'b', since it is implementation defined. IIRC only VC has #pragma once.

Most compilers wont stop and give an error if they encounter an unknown pragma, most will just issue a warning.

[eco]

To clarify what lowjoel was saying: "#pragma" is part of the standard. "#pragma once" is not. "#pragma" is for issuing compiler specific directives not covered by the standard. VC, for instance, supports warning disabling like this: "#pragma warning ( disable : 4101)". Also, as I said before, VC isn't the only compiler that supports "#pragma once". I know both GCC 3.4+ and Metrowerks support it. Others probably do as well.

[cpp]

is it safe to say that #pragrma once (not just pragma), is supported by all modern & good compilers?

[lowjoel]

no... IIRC I'd only dare use it under a VC-only environment

[jbacigal]

I generally use the #ifndef #defne header guards and then #pragma once beneath it. I don't think there is any adverse affect to using both - get the compile time boost if #pragma once is supported, if not, you still have inclusion guards.