[CMake] Where is QT_USE_IMPORTED_TARGETS not safe to use?

[Stephen Kelly]

Hi,

The FindQt4 module has this to say about QT_USE_IMPORTED_TARGETS:

> If QT_USE_IMPORTED_TARGETS is enabled, the QT_QTFOO_LIBRARY variables are
> set to point at these imported targets. This works better in general, and
> is also in almost all cases fully backward compatible. The only issue is
> when a project A which had this enabled then exports its libraries via
> export or EXPORT_LIBRARY_DEPENDENCIES(). In this case the libraries from
> project A will depend on the imported Qt targets, and the names of these
> imported targets will be stored in the dependency files on disk. 

The part that is not clear to me is this:

> This
> means when a project B then uses project A, these imported targets must be
> created again, otherwise e.g. "Qt4__QtCore" will be interpreted as name of 
> a library file on disk, and not as a target, and linking will fail.

What does it mean to say that the imported targets must be created again? Is 
it a source incompatibility? Does it mean that my downstreams have to use 
QT_USE_IMPORTED_TARGETS if I do?

The KDE techbase page has additional caveats such as checking for file 
endings of the libraries etc I guess. I don't do such things in Grantlee, 
but if the use of QT_USE_IMPORTED_TARGETS affects downstreams too (as above) 
it might be relevant.

http://techbase.kde.org/Development/CMake_KDE_4_4#FindQt4.cmake:_Imported_Targets_for_the_libraries

Would using the variable have any real benefit? I have the following in my 
CMakeLists which I suspect may be removable if I use the variable:


    # http://mail.kde.org/pipermail/kde-windows/2007-December/001692.html
    # http://lists.trolltech.com/pipermail/qt-interest/2009-July/009829.html
    # qt is always compiled with QT_NO_DEBUG under mingw,
    # so we need to compile stuff linked against it
    # the same or otherwise QPluginLoader rejects plugins
    add_definitions(-DQT_NO_DEBUG)

Thanks,

Steve.