#ifndef FOO
#define FOO
...
#endif
#pragma once
Author
I was under the impression that this was specific to Visual C++. But according to MSDN it isn't. Is this basically the same thing as doing the following?
AFAIK it is not consistent across compilers. .net uses just #pragma once but I would not like to trust to portability on that one.
quote:Original post by Trip99
AFAIK it is not consistent across compilers.
Correct. #pragma once will not work on all compilers.
With #ifndef, #define, and #endif, there might be a very small chance for name collision, where as with #pragma once, there ins''t.
Visual C++ uses both methods because it''s just one of those things that''s in there, along with all the other weird stuff.
I beleive #pragma once will prevent the file from being opened a second time, the regular header guards won''t work without opening the file and possibly including some 40M lines of headers above the guards.
#pragma once works in GCC too
I just wish that they could release the version that does...
#pragma once works in GCC too
I just wish that they could release the version that does...
I read somewhere that VC''s implementation of #pragma once is broken - it can be fooled by including via relative paths. It''s typically only used as an optimization - regular include guards are necessary to guarantee single inclusion. Other compilers are able to detect include guards and keep the information in a table (what #pragma once does in VC, apparently).
quote:Original post by Jan WassenbergDo you have any source of information? I''d be interested in details, since I''ve been using VC''s #pragma once for years in both small and large projects and never seen any problems with it.
I read somewhere that VC''s implementation of #pragma once is broken - it can be fooled by including via relative paths. It''s typically only used as an optimization - regular include guards are necessary to guarantee single inclusion. Other compilers are able to detect include guards and keep the information in a table (what #pragma once does in VC, apparently).
If I recall correctly, 
- CD
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[edited by - Code_Dark on April 13, 2004 7:46:56 PM]
#pragma once - CD
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[edited by - Code_Dark on April 13, 2004 7:46:56 PM]
I have only my memory, and a post I just found describing the problem.
This probably won''t happen in most projects, but I sometimes find myself including via (different) relative paths, so it''d blow up with #pragma once only.
This probably won''t happen in most projects, but I sometimes find myself including via (different) relative paths, so it''d blow up with #pragma once only.
quote:Original post by Leffe
#pragma once works in GCC too
It''s "deprecated" in current versions, and you''ll get warnings if you use it, but they''re bringing it back in GCC 3.4.
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