Guidelins for one codebase that builds with Qt5 and Qt6?

[Volker Krause]

On Montag, 5. August 2024 16:15:19 CEST Halla Rempt wrote:
> I seem to remember that there were KDE apps that could be built with
> both Qt6/Kf6 and Qt5/Kf5. Did I remember that right? And if so, is there
> some howto or documentation for that?

Right, that was the approach taken by many Frameworks, Plasma and Gear 
modules.

I'm not familiar enough with Krita's code to judge whether any of that is 
applicable, but here's roughly what we did there:

(1) Require Qt 5.15 and a recent KF5, and port away from deprecated API.

In many cases that already was the bulk of the code changes, and it's also 
what supported the direct Kate port Christoph mentioned for example.

This started in mid 2019.

(2) Using ECM will automatically give you a switch for the major Qt version 
(https://invent.kde.org/frameworks/extra-cmake-modules/-/blob/master/modules/
QtVersionOption.cmake, included by all modules that do anything Qt-related).

Replacing "Qt5" with "Qt${QT_MAJOR_VERSION}" and "KF5" with "KF$
{QT_MAJOR_VERSION}" tended to be the bulk of the necessary build system 
changes.

(3) A large chunk of the remaining changes for making things build with Qt 6 
can still be done without ifdefs (missing includes etc), but you wont be able 
to avoid them entirely. Similar for conditional code in the build system. 
Without (1) this can get significantly more painful though.

(4) As soon as the Qt 6 version is ready, drop Qt 5 support by removing 
conditional code and replacing ${QT_MAJOR_VERSION} with "6" in the build 
system. We got to this point in early 2024.

This worked ok for C++ code I think, the Qt 5 code remained releasable for the 
entire time. A few regressions got in during the process, but also a few 
version-independent issues got found by stricter compile-time checks on e.g. 
implicit conversions in Qt 6.

Supporting two major versions at the same time tends to get increasingly 
painful over time though, so I can only recommend to see this as "scaffolding" 
for a transient phase that shouldn't be longer than absolutely necessary. 
Apart from a few special exceptions (e.g. style plugins) we also didn't 
release anything officially supporting both versions.

What didn't work was anything involving larger amounts of QML code. There were 
a few attempts to support both versions as well with various levels of dirty 
hacks, but nothing that ended up being widely used.

Best regards,
Volker
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 195 bytes
Desc: This is a digitally signed message part.
URL: <http://mail.kde.org/pipermail/kde-devel/attachments/20240806/2eb372a6/attachment.sig>