[CMake] QT_DEFINITIONS

[Michael Hertling]

On 05/24/2010 05:20 PM, Clinton Stimpson wrote:
> 
> 
> 
> On Sunday, May 23, 2010 08:26:42 am Michael Hertling wrote:
>> Dear CMake community,
>>
>> after having taken a look into FindQt4.cmake, UseQt4.cmake and
>> Qt4ConfigDependentSettings.cmake, I wonder how the QT_DEFINITIONS
>> variable gets populated. As far as I can see, this variable remains
>> empty, perhaps except for -DQT_DLL on Windows; in particular, it does
>> not receive the -DQT_<library>_LIB flags which are solely enabled via
>> ADD_DEFINITIONS() by UseQt4.cmake, i.e. with INCLUDE(${QT_USE_FILE})
>> as suggested by FindQt4.cmake. Of course, from within a FindXXX.cmake
>> or XXXConfig.cmake for a package depending on Qt4, one does not want
>> to have the build environment modified by ADD_DEFINITIONS() et al.,
>> but convey the necessary settings by variables like QT_DEFINITIONS.
>> Therefore, my question is how to get to know about the flags for
>> compiling Qt4-dependent packages without including UseQt4.cmake.
>>
>> BTW, what is the recommendation for a package XXX which depends on a
>> package YYY regarding FindXXX.cmake and FindYYY.cmake, i.e. would one
>> expect FindXXX.cmake to call FIND_PACKAGE(YYY), or is the user expected
>> to call FIND_PACKAGE() for XXX and YYY in the correct order and, as the
>> case may be, with sufficient sets of components? In general, the latter
>> is easier for the finders, but the former would be more convenient for
>> the user.
>>
>> Any comments, suggestions etc. would be greatly appreciated.
> 
> I think the answer depends on whether the user of your software will be 
> writing Qt based code themselves, or whether Qt is hidden behind your apis.
> If they write Qt code themselves, they could choose to use a different set of 
> Qt modules, in which case compile flags could change.

Exactly, and this is one reason why I would be interested in learning
the compile flags right after a FIND_PACKAGE(Qt4 ...) to save them in
my own variables. There's no such problem w.r.t. libraries and include
directories since I can refer to QT_QTXXX_{FOUND,LIBRARY,INCLUDE_DIR},
and their values don't depend on the requested set of modules, but as
far as I can see, the flags aren't propagated by FindQt4.cmake to the
calling CMakeLists.txt in any manner.

As a suggestion, would it be possible to enhance FindQt4.cmake with two
sections similar to the last two of UseQt4.cmake, i.e. "list dependent
modules..." and "Qt modules...", but differing in that QT_DEFINITIONS
is populated with the flags according to the requested set of modules?
This shouldn't harm backward compatibility, and the user could save the
compile flags before FindQt4.cmake possibly runs anew. In UseQt4.cmake,
at first glance, ADD_DEFINITIONS(-DQT3_SUPPORT) would need to be moved
to FindQt4.cmake and transformed into an addition to QT_DEFINITIONS,
and ADD_DEFINITIONS(-DQT_${qt_module_def}_LIB) could be left out as
ADD_DEFINITIONS(${QT_DEFINITIONS}) is UseQt4.cmake's first command.

Regards,

Michael