Re: [boost] Proposal for #pragma once support

Subject: Re: [boost] Proposal for #pragma once support
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-06-09 14:30:11

On Tue, Jun 9, 2009 at 1:26 PM, Vladimir Prus <vladimir_at_[hidden]>wrote:

> Zachary Turner wrote:
>
> > I would like to propose conditional support for #pragma once be added to
> all
> > header files in boost.  This would involve, at the beginning of every
> header
> > file, a preprocessor check to determine whether or not the particular
> > compiler is on a whitelist of known compilers to optimized #pragma once.
>  If
> > so, #pragma once is used.  so, something like this:
> >
> > #if defined(_MSC_VER) /* || defined(OTHER_PRAGMA_ONCE_ENABLED_COMPILER)
> ...
> > */
> > #pragma once
> > #endif
>
> If you are proposing to add this to every single file to boost, I am afraid
> you get to either do it, or provide a script to do this automatically. You
> might also have to patch the inspect program to automatically catch headers
> that don't have such preamble. Finally, you have to some form of the check
> that does not requires that every single header be updated if new compiler
> is added to white-list.
>
> Because Boost has >90 libraries now, there's zero chance to have such
> global
> change to happen unless you do necessary automation. And, it would be
> surely
> interesting if Boost itself can benefit from this approach, before we
> commit
> to it.
>

I agree that it would need to be automated.  But the script itself should be
very trivial, I'd be willing to try it myself.  And perhaps the best
approach is what I mentioned in a previous post, to force the programmer to
explicitly define BOOST_USE_PRAGMA_ONCE before including a boost header
file.  This would solve the problem of having to modify every header when
new compilers were released.  And since by default it would be undefined, an
out-of-the-box boost installation would work exactly as it does now.  I'll
work on this a bit later today perhaps, and try to post some benchmarks for
for compiling boost itself, and for compiling a commercial product which
uses boost.