[Development] #pragma once

[Mathias Hasselmann]

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.

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
>