[Development] Using '#pragma once' instead of include guards?

[Scott Bloom]

My two bits as a user of Qt.  Please make sure any exposed headers use the old-school guards and NOT the #pragma once.

I don’t care how the internal Qt headers work.  However, Ive been burned too many times by the "known issues" of the #pragma once to want to use them.

The problem isn’t if the two headers are the same, the problem is it not compiling because of multiple definitions of classes.

The main issue I have, is we build a tool with tons of sub libraries and modules, with a pretty poor build environment.  We have 800+ developers using it, so any change is done by a committee and takes years ☹

Our Qt gets places in a area everyone gets access to. Ie /usr/local/shared_data/Qt on linux and a mount Q:/Qt/ on windows, but Q: can all be accessed via //SHAREPOINT/shared_data/Qt

Under Qt might be Qt5.12.9 and Qt5.15.10, but for this example lets assume its not versioned.  Under each version, we then have the compiler sub directories.  Meaning win64VC15, linux_x86_64/debug and linux_x86_64/release.  The windows disty contains debug and release.  

What happens, is the compiler does not see Q:/Qt/win64VC15/include as the same path as //SHAREPOINT/shared_data/Qt/ win64VC15/include.  This is a known issue.

I 100% understand that we should fix out build system so they only access any data in the shared_data mount the same way.  However that is simply easier said than done relying on #pragma once just fails for us.  

Ive seen this issue at a number of companies that do multi-platform development but store their Qt versions on a server.

Scott

-----Original Message-----
From: Development <development-bounces at qt-project.org> On Behalf Of Volker Hilsheimer via Development
Sent: Monday, October 10, 2022 2:55 AM
To: development at qt-project.org
Subject: [Development] Using '#pragma once' instead of include guards?

Hi,


We are using `#pragma once` in a number of examples and tests in the Qt source tree, but I don’t think we have officially endorsed it in favour of explicit include guards.

#pragma once is “non-standard but widely supported” [1], with some caveats, e.g. when there are multiple header files with the same name (which each compiler then handles differently, as there is no standard).

From what I see, it should in practice be ok to use. But perhaps I’m missing something. Does anything speak against using ‘#pragma once’ in new files?


Volker

[1] https://en.wikipedia.org/wiki/Pragma_once

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