qt4 vs qt5 build for dependencies

[Ben Cooksley]

On Fri, Apr 5, 2013 at 10:27 PM, David Faure <faure+bluesystems at kde.org> wrote:
> When I ported attica, soprano, libdbusmenu-qt and other such "kdesupport-
> level" dependencies to Qt5, I used
>
> find_package(Qt5Core QUIET)
> if (Qt5Core_FOUND)
>   message(STATUS "Building with Qt5 support")
>   ...
> else()
>   find_package(Qt4 REQUIRED)
> endif()
>
> i.e. use Qt5 if it's in the user's environment (CMAKE_PREFIX_PATH) otherwise
> look for Qt4.
> I thought this was perfect, it made things "work out of the box" without any
> manual intervention, for both types of users.
> And it didn't require a separate branch for the qt5 support (we only have this
> in a few modules which started to use qt5 stuff, but for for the majority of
> modules, there's no active development, so branching is not wanted).
>
> Now here's the problem: nowadays distros are starting to have Qt5 in /usr,
> i.e. it's being found automatically. Which makes it impossible to build this
> code for Qt4.
>
> So I'm afraid I have to drop the idea of automatic detection, and let people
> choose explicitely. My current idea would be to reintroduce -DQT5_BUILD=true
> (like we had in kdelibs at some point; but this discussion isn't about kdelibs
> obviously), i.e. default to the Qt4 build, since that's what KDE SC releases
> are still about.
>
> Later on, when we're about to release KF5, we want to turn this around, i.e.
> make it easy to build things for Qt5, and if we still need qt4 support at that
> point, adding a -DQT4_BUILD=true (and dropping the QT5_BUILD variable,
> obviously).
>
> Opinions? Better ideas?

Assuming the user had given no guidance, would it be possible to
attempt a fallback to Qt 5 if it was not possible to find Qt 4?
Obviously if the user gives guidance that should override.

I'm otherwise neutral - just let me know beforehand please so I can
adjust the CI system to pass the new argument(s).

Regards,
Ben

>
> --
> David Faure, faure at kde.org, http://www.davidfaure.fr
> Sponsored by BlueSystems and KDAB to work on KDE Frameworks
>
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