building KF5 projects against a different Qt5 version (than the one the KF5 frameworks were built against)

[David Faure]

On Sunday, April 26, 2020 3:21:34 PM CEST René J.V. Bertin wrote:
> On Sunday April 26 2020 14:12:01 David Faure wrote:
> >> (and possibly LD_PRELOAD).
> >
> >I don't see why you would set LD_PRELOAD in this configuration.
> 
> This may be necessary if even one of the multiple libraries that get loaded
> has an old-style link to a Qt library which makes it ignore
> LD_LIBRARY_PATH. I've seen that with libQt5QtCore (and also with libdbus).

This should only ever happen if RPATH is used instead of RUNPATH.
I haven't seen this in 15 years or so.

> >I think this is because the KF5 target brings its own include paths as part
> >of the imported target.
> 
> Probably, yes. And that's enough to stop the build I was attempting, so I
> cannot affirm nor infirm if something similar won't happen with linker
> paths.

Right. Those are most likely full paths too.

> >Build against the old Qt, run against the new Qt?
> >Then it's back to "only" LD_LIBRARY_PATH switcheroo which works well, while
> >at the buildsystem level it's all based upon the old Qt so no mixup there.
>
> Yeah, except that then you can't test those #if QT_VERSION hacks...

Well, yeah, you can't have it all.
Or, you can, but you need two full builds.
 
> Talking about version test hacks (or not-so-hacks): why is it that
> KFOO_VERSION isn't defined systematically when you include any header of
> the FOO KF5 framework? With Qt you never have to worry about QT_VERSION
> being defined. 

Well, everything in Qt ends up including global.h
There's no such central header in KF5 frameworks (which are more modular, by 
definition), so you need to include the framework_version.h header.

> This would be less of an issue if -Wundef was part of -Wall
> (or if IDE parsers always flagged undefined macros used in a preprocessor
> expression that isn't a simple #ifdef (or #if defined)...

We have -Wundef by default.

extra-cmake-modules/kde-modules/KDECompilerSettings.cmake:
367:set(_KDE_GCC_COMMON_WARNING_FLAGS "-Wall -Wextra -Wcast-align -Wchar-
subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef")

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5