Is pragma once a safe include guard

Question

I ve read that there is some compiler optimization when using  pragma once which can result in faster compilation   I recognize that is non-standard  and thus could pose a cross-platform compatibility issue   Is this something that is supported by most modern compilers on non-windows platforms  gcc    I want to avoid platform compilation issues  but also want to avoid the extra work of fallback guards    pragma once  ifndef HEADER H  define HEADER H        endif    HEADER H   Should I be concerned  Should I expend any further mental energy on this

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GCC supports  pragma once since 3 4  see http   en wikipedia org wiki Pragma once for further compiler support   The big upside I see on using  pragma once as opposed to include guards is to avoid copy paste errors   Let s face it  most of us hardly start a new header file from scratch  but rather just copy an existing one and modify it to our needs  It is much easier to create a working template using  pragma once instead of include guards  The less I have to modify the template  the less I am likely to run into errors  Having the same include guard in different files leads to strange compiler errors and it takes some time to figure out what went wrong   TL DR   pragma once is easier to use

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Using  pragma once should work on any modern compiler  but I don t see any reason not to use a standard  ifndef include guard  It works just fine  The one caveat is that GCC didn t support  pragma once before version 3 4   I also found that  at least on GCC  it recognizes the standard  ifndef include guard and optimizes it  so it shouldn t be much slower than  pragma once

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Not always   http   gcc gnu org bugzilla show bug cgi id 52566 has a nice example of two files meant to both be included  but mistakenly thought to be identical because of identical timestamps and content  not identical file name

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Using gcc 3 4 and 4 1 on very large trees  sometimes making use of distcc   I have yet to see any speed up when using  pragma once in lieu of  or in combination with standard include guards   I really don t see how its worth potentially confusing older versions of gcc  or even other compilers since there s no real savings  I have not tried all of the various de-linters  but I m willing to bet it will confuse many of them   I too wish it had been adopted early on  but I can see the argument  Why do we need that when ifndef works perfectly fine    Given C s many dark corners and complexities  include guards are one of the easiest  self explaining things  If you have even a small knowledge of how the preprocessor works  they should be self explanatory    If you do observe a significant speed up  however  please update your question

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Additional note to the people thinking that an automatic one-time-only inclusion of header files is always desired  I build code generators using double or multiple inclusion of header files since decades  Especially for generation of protocol library stubs I find it very comfortable to have a extremely portable and powerful code generator with no additional tools and languages  I m not the only developer using this scheme as this blogs X-Macros show  This wouldn t be possible to do without the missing automatic guarding

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The main difference is that the compiler had to open the header file to read the include guard  In comparison  pragma once causes the compiler to keep track of the file and not do any file IO when it comes across another include for the same file  While that may sound negligible  it can easily scale up with huge projects  especially ones without good header include disciplines   That said  these days compilers  including GCC  are smart enough to treat include guards like pragma once  i e  they dont open the file and avoid the file IO penalty   In compilers that dont support pragma I ve seen manual implementations that are a little cumbersome     ifdef FOO H  include  foo h   endif   I personally like  pragma once approach as it avoids the hassle of naming collisions and potential typo errors  It s also more elegant code by comparison  That said  for portable code  it shouldn t hurt to have both unless the compiler complains about it

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If we use msvc or Qt  up to Qt 4 5    since GCC up to 3 4    msvc both  support  pragma once  I can see no reason for not using  pragma once   Source file name usually same equal class name  and we know  sometime we need refactor  to rename class name  then we had to change the  include XXXX also   so I think manual maintain the  include xxxxx is not a smart work   even with Visual Assist X extension  maintain the  xxxx  is not a necessary work

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I don t know about any performance benefits but it certainly works   I use it in all my C   projects  granted I am using the MS compiler   I find it to be more effective than using   ifndef HEADERNAME H  define HEADERNAME H      endif   It does the same job and doesn t populate the preprocessor with additional macros     GCC supports  pragma once officially as of version 3 4

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The performance benefit is from not having to reopen the file once the  pragma once have been read  With guards  the compiler have to open the file  that can be costly in time  to get the information that it shouldn t include it s content again   That is theory only because some compilers will automatically not open files that didn t have any read code in  for each compilation unit   Anyway  it s not the case for all compilers  so ideally  pragma once have to be avoided for cross-platform code has it s not standard at all   have no standardized definition and effect  However  practically  it s really better than guards   In the end  the better suggestion you can get to be sure to have the best speed from your compiler without having to check the behavior of each compiler in this case  is to use both pragma once and guards    ifndef NR TEST H  define NR TEST H  pragma once   include  Thing h   namespace MyApp               endif   That way you get the best of both  cross-platform and help compilation speed     As it s longer to type  I personally use a tool to help generate all that in a very wick way  Visual Assist X

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I use it and I m happy with it  as I have to type much less to make a new header  It worked fine for me in three platforms  Windows  Mac and Linux    I don t have any performance information but I believe that the difference between  pragma and the include guard will be nothing comparing to the slowness of parsing the C   grammar  That s the real problem  Try to compile the same number of files and lines with a C  compiler for example  to see the difference   In the end  using the guard or the pragma  won t matter at all

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I wish  pragma once  or something like it  had been in the standard   Include guards aren t a real big deal  but they do seem to be a little difficult to explain to people learning the language   but it seems like a minor annoyance that could have been avoided   In fact  since 99 98  of the time  the  pragma once behavior is the desired behavior  it would have been nice if preventing multiple inclusion of a header was automatically handled by the compiler  with a  pragma or something to allow double including   But we have what we have  except that you might not have  pragma once

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pragma once does have one drawback  other than being non-standard  and that is if you have the same file in different locations  we have this because our build system copies files around  then the compiler will think these are different files

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Using   pragma once  might not have any effect  it is not supported everywhere - though it is increasingly widely supported   so you need to use the conditional compilation code anyway  in which case  why bother with   pragma once    The compiler probably optimizes it anyway   It does depend on your target platforms  though   If all your targets support it  then go ahead and use it - but it should be a conscious decision because all hell will break loose if you only use the pragma and then port to a compiler that does not support it

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Today old-school include guards are as fast as a  pragma once  Even if the compiler doesn t treat them specially  it will still stop when it sees  ifndef WHATEVER and WHATEVER is defined  Opening a file is dirt cheap today  Even if there were to be an improvement  it would be in the order of miliseconds   I simply just don t use  pragma once  as it has no benefit  To avoid clashing with other include guards I use something like  CI APP MODULE FILE H --  CI   Company Initials  APP   Application name  the rest is self-explanatory

[c++] Is #pragma once a safe include guard?

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