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Subject: Re: [boost] [#pragma once]
From: Sid Sacek (ssacek_at_[hidden])
Date: 2009-04-13 12:15:35

[ Graham wrote: ]
> Incredibuild (xoreax) and other parallel compilation systems have
problems with pragma once

I'm not sure what the "problems with pragma once" are by Graham's
statement above.

So far, I have heard that #pragma once doesn't work right on some
platforms. That tells me the programmer cannot rely on it, but that
doesn't indicate that it cannot be used in the program.

What is clear is that #pragma once works on some platforms. Also, for
the platforms that it doesn't work on, the use of #ifndef is absolutely
required. Boost is a cross-platform library, so that means
cross-platform programs needs to look something like this:

    #pragma once

If the above code gives a pragma warning, the warning can be turned off
or simply be ignored. I have not yet heard any objections for the above
code. If that's still not good enough for every compiler that's out
there, the variation below - which was already proposed to boost -
should be good enough. Each platform can decide what to do with their
version of the BOOST_ONCE macro.

    #pragma BOOST_ONCE

I'm starting to feel like the people in charge of the boost libraries
aren't even listening in on this conversation. I would like to hear some
opinions and some votes about the above code, especially opinions from
the boost heavy-weights. I need to know I'm not just wasting everybody's
time here. If something like the above code was acceptable to the boost
custodians, I would be happy to make the appropriate changes myself.

I realize that this topic has already been discussed for the C++
language in general and, for the most part, has been ignored. However,
I'm going to add a twist to the pragma-once statement that was never
brought up in discussion. What should solve all of the issues concerning
the header guards is the following variation to #pragma once:

    #pragma once( HEADER_FILE_GUARD )

The above line of code includes both the pragma-once statement, but is
also protected by the unique HEADER_FILE_GUARD token. Its meaning is
clear and concise. This implies that the #ifndef, #define, and #endif
are no longer required, and satisfies everybody's compiling needs. It
would be great if the current preprocessor could be used to implement
that, but I don't think it can. The current existing alternative within
the boost code is just plain hideous. Just to remind you all, here it is
one more time:

    #ifndef __BOOST_HEADER_GUARD__
    #define __BOOST_HEADER_GUARD__

    #if defined( _MSC_VER ) && ( _MSC_VER >= 1020 )
    # pragma once

-Sid Sacek

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