Re: [boost] Guidence on #pragma once

Subject: Re: [boost] Guidence on #pragma once
From: James E. King III (jking_at_[hidden])
Date: 2018-11-25 13:16:37

On Sun, Nov 25, 2018 at 8:02 AM charleyb123 via Boost
<boost_at_[hidden]> wrote:
>
> On Sun, Nov 25, 2018 at 5:46 AM Olaf van der Spek via Boost <
> boost_at_[hidden]> wrote:
>
> > On Sat, Nov 24, 2018 at 2:01 AM Andrey Semashev via Boost
> > <boost_at_[hidden]> wrote:
> > > since `#pragma once` is non-standard and not reliable, there's little
> >
> > Who has a sane reproducible test case showing the claimed unreliability?
> >
>
> The '#pragma once' discussion occasionally comes up in the Qt community,
> and they repeatedly decide to not use it.
>
> It came up again last month, and here is a use case of claimed
> unreliability (from Thiago):
>
> > For example, I have ~/src as a bind-mount to ~/dev/src. That means
> >       ~/src/qt/qt5/qtbase/src/corelib/global/qglobal.h
> >       ~/dev/src/qt/qt5/qtbase/src/corelib/global/qglobal.h
> > are actually the same file, but they aren't for the effects of #pragma once
> > because the paths differ.
> >
> > Another problem is qcompilerdetection.h, qprocessordetection.h,
> > qsystemdetection.h, qtypeinfo.h, etc. which depend on the header guards to
> > break the include cycle. Ditto for qlist.h + qstringlist.h and
> > qbytearraylist.h
>
> Excerpt from:
>   *-
> https://lists.qt-project.org/pipermail/development/2018-October/033733.html
>
> --charley
>

The include search path ordering for the compilation unit does not change
during compilation.  Either ~/src or ~/dev/src will be first in the
search path,
therefore #pragma once would work properly for that case.

- Jim