KF 5 & C++14?

[ahiemstra at heimr.nl]

On Monday, 27 April 2020 14:53:32 CEST Friedrich W. H. Kossebau wrote:
> Am Montag, 27. April 2020, 00:45:41 CEST schrieb David Faure:
> > On Sunday, April 26, 2020 5:30:37 PM CEST Friedrich W. H. Kossebau wrote:
> > > Hi,
> > > 
> > > I just saw that at least kimageformats, knewstuff & kquickcharts all set
> > > this: set(CMAKE_CXX_STANDARD 14)
> > > 
> > >     set(CMAKE_CXX_STANDARD_REQUIRED ON)
> > > 
> > > Which ignores a bit that so far C++11 has been the minimum standard
> > > officially supported in/by KDE Frameworks (by mainly following what Qt 5
> > > supports).
> > > 
> > > Compare also the current text of the policy
> > > "Frameworks compiler requirements and C++11":
> > > --- 8< ---
> > > The following minimal compiler versions are supported by KDE Frameworks:
> > > * GCC 4.8
> > > * Clang 3.3
> > > * VS2013 (MSVC12)
> > > 
> > > This means all of the C++11 standards can be used.
> > > --- 8< ---
> > > * https://community.kde.org/Frameworks/
> > > Policies#Frameworks_compiler_requirements_and_C.2B.2B11
> > > 
> > > What to make of this? Might it be the time to raise the bars a bit, and
> > > how
> > > much? Surely the lower limit is what the oldest Qt version currently
> > > supported by KDE Frameworks claims to support:
> > > https://doc.qt.io/qt-5.12/supported-platforms.html
> > > 
> > > Should we go above this?
> > > Or should kimageformats, knewstuff & kquickcharts be fixed to use C++11
> > > only again?
> > > 
> > > No own opinion, so far only stumbled over what seems a policy breakage.
> > 
> > The goal was to align with the requirements of the Qt version we require.
> > 
> > But it's hard to know if anyone is actually using gcc 4.8 to build the
> > current version of KF5. One way to find out is to... do nothing. Leave the
> > above as is and see if anyone actually complains that it doesn't match our
> > promise.
> 
> For reference, the CMAKE_CXX_STANDARD 14 is present in those modules...
> KQuickCharts:  since KF 5.65 (== addition of module)
> KNewStuff:     since KF 5.69
> KImageFormats: since KF 5.70
> So relatively new, meaning the field observation phase only just began :)
> 

The reason Quick Charts is using C++14 is that I seem to remember that being 
the minimum required version for Frameworks. But apparently I was mistaken, 
might have been me mixing up Plasma and Frameworks requirements.

> > This isn't however a green light for doing this everywhere, not until we
> > wait multiple months for feedback. Otherwise the porting effort (down to
> > C++11) will be huge.
> 
> One personal motivation to ask was the hope that we could go C++14 across
> all KF modules now. Not needing to support C++11 anymore, but relying on
> C++14 deprecation tagging features would simplify ECMGenerateExportHeader
> code and, what triggered me here, make it simple to extend version-based
> deprecation tagging also for enumerators.
> Guess I am out of luck then for now, or have to post-pone the feature :)

Personally, I'd be in favour of ramping up required C++ version support in 
light of the requirement of C++17 for Qt 6. So require 14 now, require 17 once 
Qt 6 is out. That way you're taking care of one bit of the transition before 
Qt 6 actually hits.

Note also that Qt 5.14 already stopped supporting GCC <5 [1] (which is the 
version that supports C++14 fully) and even for 5.12 GCC 5 versions are better 
supported [2].

- Arjen

> 
> Cheers
> Friedrich

[1]: https://codereview.qt-project.org/c/qt/qtdoc/+/288825
[2]: https://doc.qt.io/qt-5.12/linux.html