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Subject: Re: [boost] [#pragma once]
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-11-17 06:29:35

On Sat, Nov 17, 2012 at 4:33 AM, Matt Calabrese <rivorus_at_[hidden]> wrote:
> I'm basically with everyone who's been saying to just not bother using
> "#pragma once" since the idiomatic ifdef guards work fine and unless there
> are some real benchmarks, I don't think it's worthwhile trying to bend over
> backwards to take advantage of something that's non-standard. I'm just not
> convinced there are any actual gains here.
> On Fri, Apr 10, 2009 at 11:33 AM, Remko Tronçon <remko_at_[hidden]> wrote:
>> Anyway, I understand that a library like Boost doesn't want to rely on
>> this, but personally, but it sounds sane to me that other projects
>> (that are less depended on and don't need to support a gazillion of
>> compilers) use it.
> Alright, I'll agree with this. While I'm not convinced it's a great idea
> for boost to use #pragma once, a library macro for users that uses it when
> available might make sense. Assuming _Pragma support, it's trivial to
> provide a macro BOOST_INCLUDE_ONCE that is defined as _Pragma( "once" ) in
> compilers that support it and nothing in compilers that don't (avoiding
> possible unrecognized pragma warnings). An alternative is a macro that
> produces a compile-time error if it's not supported.

I would rather prefer BOOST_HAS_PRAGMA_ONCE macro in Boost.Config. Not
all compilers support _Pragma (MSVC for one, and it's the primary
target for the optimization), so I would still use #pragma guarded
with the config macro.

I know, in order to get BOOST_HAS_PRAGMA_ONCE defined I will have to
include boost/config.hpp, but I do it anyway (so as most of Boost
libraries, I presume).

Disclaimer: I did not run performance tests to confirm the compile
speed gains. But that #pragma once thing is almost a habit of mine, so
it's not a big deal for me to add it to my headers.

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