qt4 vs qt5 build for dependencies

[Alexander Neundorf]

On Friday 05 April 2013, Ben Cooksley wrote:
> 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.

Yes, the user should be able to decide which one he wants.

Not sure whether a bool (QT4_BUILD or QT5_BUILD) or e.g. version flag (-
DDESIRED_QT_VERTSION=5) is better.

CMake comes with a FindQt.cmake (which has a DESIRED_QT_VERSION variable), 
which can find Qt3 and Qt4. I haven't used it since years.
Maybe this could be modernized to work also with Qt5.
Is there a plan what to do with Qt5Transitional.cmake from e-c-m ?

Alex