[Development] #pragma once

[Konstantin Tokarev]

25.01.2018, 13:27, "Mathias Hasselmann" <mathias at taschenorakel.de>:
> Let's see what people who know much more about compiler features than
> any of us think about "#pragma once". Let's check what GCC and Clang do
> for their C++ library headers:
>
>    $ grep -r pragma.*once /usr/include/clang/5.0.0/include
> /usr/include/c++/7.2.0/
>    $
>
> ...and this is about headers that target exactly one compiler and are
> known to be locally installed.

The latter statement is not exactly true, both libstdc++ and libc++ can be used
by many compilers, and they also aren't guaranteed to be locally installed.

>
> Hope this helps,
> Mathias
>
> Am 24.01.2018 um 13:19 schrieb Mitch Curtis:
>>>  -----Original Message-----
>>>  From: Ville Voutilainen [mailto:ville.voutilainen at gmail.com]
>>>  Sent: Wednesday, 24 January 2018 1:11 PM
>>>  To: Mitch Curtis <mitch.curtis at qt.io>
>>>  Cc: Alexander Nassian <nassian at bitshift-dynamics.com>; development at qt-
>>>  project.org
>>>  Subject: Re: [Development] #pragma once
>>>
>>>  On 24 January 2018 at 12:34, Mitch Curtis <mitch.curtis at qt.io> wrote:
>>>>>  -----Original Message-----
>>>>>  From: Ville Voutilainen [mailto:ville.voutilainen at gmail.com]
>>>>>  Sent: Wednesday, 24 January 2018 11:25 AM
>>>>>  To: Alexander Nassian <nassian at bitshift-dynamics.com>
>>>>>  Cc: Mitch Curtis <mitch.curtis at qt.io>; development at qt-project.org
>>>>>  Subject: Re: [Development] #pragma once
>>>>>
>>>>>  On 24 January 2018 at 12:22, Alexander Nassian <nassian at bitshift-
>>>>>  dynamics.com> wrote:
>>>>>>  Maybe because it’s not part of the C++ standard?
>>>>>
>>>>>  #pragma once is not a replacement for include guards.
>>>>
>>>>  Why not?
>>>>
>>>>>  It's not part of the C++ standard because it doesn't always work
>>>>
>>>>  In which ways? My quick search gave me these:
>>>>
>>>>  https://stackoverflow.com/a/1946730/904422
>>>>  https://en.wikipedia.org/wiki/Pragma_once#Caveats
>>>
>>>  That wikipedia link seems to describe the problems fairly accurately.
>>
>>  Do we have that issue in Qt?
>>
>>>>>  and modules are a superior solution anyway.
>>>>  How so?
>>>
>>>  Because you can import the same module multiple times without concerns
>>>  about re-definitions, and that import is much faster than parsing a header
>>>  file.
>>  _______________________________________________
>>  Development mailing list
>>  Development at qt-project.org
>>  http://lists.qt-project.org/mailman/listinfo/development
>
> _______________________________________________
> Development mailing list
> Development at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/development

-- 
Regards,
Konstantin