> On 13 Oct 2022, at 16:56, Kyle Edwards via Development > <development@qt-project.org> wrote: > > On 10/13/22 10:42, Jean-Michaël Celerier wrote: >> >The only way you’d have a strong case with this is if it has some other >> >significant benefit, like compilation speedup. >> >> The main benefit to me is that it entirely removes possibilities for >> conflict due to headers having the same name. At least Qt takes great care >> of avoiding this but still, notice that e.g. the authors of Qt3D's >> Qt3DCore::QTransform had to be careful to not just do #ifndef QTRANSFORM_H >> >> Now what happens when someone develops a different library but with a header >> guard similar to Qt's? >> >> If I grep into the various cloned projects on my hard drive, for instance I >> see >> >> #ifndef QRENDERER_H >> #ifndef QGLRENDERER_H >> #ifndef QSEARCHFIELD_H >> #ifndef QLOG_H >> #ifndef QJACK_H >> #ifndef QENC_H >> #ifndef QRANGESLIDER_H >> #ifndef QDOUBLERANGESLIDER_H >> >> etc... is the Qt project 100% confident that it will *never ever* use these >> names? With pragma once this is a 100% non-problem. > > I agree with previous points that while #pragma once can work well for a > standalone program, it has the potential to cause problems when used in > libraries that other developers use. Even CMake omitted #pragma once from the > one (admittedly deprecated) header that may be consumed externally > (cmCPluginAPI.h). >
> However, there are ways to enforce the use of unique header guards. > clang-tidy has an extensible header guard check that can be customized > per-project, and plugin loading functionality. Qt could create a clang-tidy > plugin that sets up this header guard check and enforces a unique-enough > header guard in CI. That works to avoid clashes inside a project, but doesn't help if user applications mix Qt + their code + other libraries not under their control, which is similar to the issues of #pragma once. Header guards do have downsides too, but I suppose software developers got used to them and have the necessary workarounds in place. Which is a valid argument for not introducing #pragma once in Qt, which would require different workarounds, and create friction that would need a very compelling argument to inflict. -- Eike Ziller Principal Software Engineer The Qt Company GmbH Erich-Thilo-Straße 10 D-12489 Berlin eike.zil...@qt.io http://qt.io Geschäftsführer: Mika Pälsi, Juha Varelius, Jouni Lintunen Sitz der Gesellschaft: Berlin, Registergericht: Amtsgericht Charlottenburg, HRB 144331 B _______________________________________________ Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
