[CMake] Where is QT_USE_IMPORTED_TARGETS not safe to use?

[Stephen Kelly]

Hi Alex, thanks for the more detailed explanation. It is indeed more clear.

However, I still don't think there is a linker error if I omit 
QT_USE_IMPORTED_TARGETS from the config file. I tried to re-create the error 
condition with a trivial project and could not re-create it.

Alexander Neundorf wrote:

>> 
>> IMPORTED_LINK_INTERFACE_LIBRARIES_DEBUG "Qt4::QtCore"
> 
> The statement above has the effect than when a user of Grantlee in a his
> project named Foo links against the Grantlee library, it will additionally
> link against "Qt4::QtCore".
> 
> If there is no target (imported or "normal", this doesn't matter) with
> that name in the project Foo, then "Qt4::QtCore" is considered a direct
> parameter for the linker and cmake will append (under Linux) a
> "-lQt4::QtCore to the linker command line.
> Which would be wrong.
> 
> So for that to work correctly, an imported target with the name
> "Qt4::QtCore" must be created somewhere in the project Foo.
> Typically by FindQt4, which should then create these imported targets.

"Qt4::QtCore" seems to be defined as an import unconditionally in FindQt4. 
Well, almost uncondiditionally. qmake has to be found, and it has to be 
either debug or release mode, but it doesn't look like FindQt4 is very 
useful if either case is false anyway.

So this works just fine:

$ cat CMakeLists.txt main.cpp

cmake_minimum_required(VERSION 2.8)

project(cmdline)

find_package(Qt4 REQUIRED QtCore )

INCLUDE_DIRECTORIES(${QT_INCLUDE_DIR})


add_executable(cmdline
  main.cpp
)

target_link_libraries(cmdline
  Qt4::QtCore
)

#include <QtCore/QCoreApplication>

int main(int argc, char **argv)
{
  QCoreApplication app(argc, argv);

  return app.exec();
}


That example seems to indicate to me that even if using 
set(QT_USE_IMPORTED_TARGETS 1) in the Config file is the right thing to do 
and I should do it anyway, it won't cause a linker error if I leave it out.

Is there some way I can modify that example to re-create the linker error 
described in the comment?

> You can still test in e.g. GrantleeConfig.cmake whether the target
> Qt4::QtCore exists and fail with FATAL_ERROR if it doesn't.
> 
> I strongly recommend using these imported targets, one reason is because
> otherwise when installing a binary package of Grantlee under Windows, it
> will not be possible to freely choose the install location, or better the
> location where Qt must be is then already hardcoded in
> GrantLeeConfig.cmake (because then there won't be "Qt4::QtCore" in
> GrantleeConfig.cmake, but something like
> "c:/where/was/Qt/at/buildtime/QtCore.dll"). When then linking against
> Grantleee, and Qt is installed somewhere else, it will fail.

Yes, this makes sense. I think that's something we almost covered in the 
review in Randa :). I am adding the setter to use the imported target 
feature so it will be in the next release, but I also want to understand it 
fully.

Thanks,

Steve.