[CMake] QT_DEFINITIONS

[Michael Hertling]

On 05/24/2010 11:39 PM, Clinton Stimpson wrote:
> 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?

No, I don't. "Saving the values" means to transfer the results delivered
by FindQt4.cmake to variables for further use, e.g. to return the values
from the finder of a Qt4-dependent package as being recommended by
${CMAKE_ROOT}/Modules/readme.txt.

> 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.

Of course, FindQt4.cmake is called on the target machine in this way,
but that is not my concern. Let me explain by means of the following
example: Package XXX uses QtCore, QtGui and Qt...; consequently, its
finder calls FIND_PACKAGE(Qt4 COMPONENTS QtCore QtGui Qt...), but not
INCLUDE(${QT_USE_FILE}) because of the latter's impact on the build
environment. The user of XXX is informed about libraries and include
directories via XXX_LIBRARIES containing QT_QT{CORE,GUI,...}_LIBRARY
and XXX_INCLUDE_DIRS containing QT_QT{CORE,GUI,...}_INCLUDE_DIR, but
the compile flags can not be propagated via XXX_DEFINITIONS because
FindQt4.cmake doesn't reveal them. So, XXX's user can't find out the
potentially necessary flags for the compilation that encompasses the
Qt4 headers before eventually enabling them in the build environment
later with INCLUDE(${QT_USE_FILE}). Now, my concern is how to get to
know about the compile flags after a call to FIND_PACKAGE(Qt4 ...),
and AFAIK, this is what variables like QT_DEFINITIONS are meant for.

> 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 is what I'm doing when writing finders for Qt4-dependent packages,
but there's no QT_* variable which contains the compile flags; thus, I
can't convey them via the XXX_DEFINITIONS variable as it is officially
recommended. The suggested modifications aim at FindQt4.cmake setting
up QT_DEFINITIONS with the compile flags, so there is a possibility to
get them without influencing the build environment, but currently, the
only way to enable the flags is to issue INCLUDE(${QT_USE_FILE}) after
all direct and indirect calls to FindQt4.cmake has been made, and even
then, the flags can't be accessed in a convenient manner.

For these reasons, I have asked the question if it is possible to learn
the compile flags in some way and, as the case may be, if FindQt4.cmake
et al. could be slightly modified in order to populate QT_DEFINITIONS
accordingly.

Best regards,

Michael