Re: [boost] Guidence on #pragma once

On Sun, Nov 25, 2018 at 6:17 AM James E. King III via Boost < boost@lists.boost.org> wrote:

On Sun, Nov 25, 2018 at 8:02 AM charleyb123 via Boost wrote:

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On Sun, Nov 25, 2018 at 5:46 AM Olaf van der Spek via Boost < boost@lists.boost.org> wrote:

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On Sat, Nov 24, 2018 at 2:01 AM Andrey Semashev via Boost wrote:

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since `#pragma once` is non-standard and not reliable, there's little

Who has a sane reproducible test case showing the claimed

unreliability?

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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):

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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

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--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

Agree, except "current-dir" will be searched first for all `#include ""`, which would override the include search path order. --charley