[CMake] QT_DEFINITIONS

[Clinton Stimpson]

On Monday, May 24, 2010 03:07:43 pm Michael Hertling wrote:
> 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.

I don't quite follow why you want to save those values.  Are you assuming the 
installation location of Qt is the same for you and all of the software users?

I was thinking that if the user of your software was also using Qt, 
FindQt4.cmake should be used on their machine instead of saved information 
extracted from FindQt4.cmake on someone else's machine.

One possibility is to make your own UseXXX.cmake file that includes 
UseQt4.cmake.  The UseXXX.cmake could set QT_USE_QT* flags that XXX depends on.  
That's a bit awkward I guess.

Another possibility is for your XXX_variables to include the values of the 
QT_* variables, and if the user decides to use additional Qt modules, the 
burden would be on them.  They could have their own find_package(Qt4), as its 
ok to include it multiple times.
This latter method is what FindQt4.cmake does as some of the Qt modules have 
extra dependencies such as zlib, OpenGL, Xlib, etc, but its probably a bit 
simpler in the case of FindQt4.cmake because most of those 3rd party 
dependencies are hidden behind the Qt api.
The current FindQt4.cmake will do a find_package(OpenGL), find_package(X11), 
etc.. for those dependencies.

Clint