[Qt-interest] Q_OBJECT in DLL - Release build warnings

[Catalin Roman]

Thank you. Problem solved.

Looking back, I think that the description for HEADERS (
http://doc.trolltech.com/4.6/qmake-variable-reference.html#headers) got me
confused, because it states: "qmake will also automatically detect if moc is
required by the classes in these headers, and *add the appropriate
dependencies and files to the project for generating and linking the moc
files*".

Thanks,
Catalin

On Wed, Feb 10, 2010 at 2:18 PM, <Oliver.Knoll at comit.ch> wrote:

> Catalin Roman wrote on Wednesday, February 10, 2010 11:22 AM:
>
> > ...
> > :( ... my intention was just to figure out if it's just a project
> > layout problem (since both samples performed basically the same
> > thing:calling a function from a DLL) or something deeper.
>
> Your problem is that you add the HEADER counterfoo.h to the CounterBar
> project! Why do you do that? Your intention is probably to specify a
> DEPENDENCY on that header, but that is NOT done with HEADERS += whatever.h!
>
> Check out
> http://doc.trolltech.com/4.6/qmake-variable-reference.html#dependpath for
> how the setup dependencies (so that when the headers in the CounterBar
> project change also dependend projects are re-build as needed).
>
> Anyway, once you remove the HEADERS += ../CounterFoo/counterfoo.h from your
> CounterBar.pro your example compiles fine without warnings (once I manually
> copy the generated DLL into the 'lib' subfolder, before compiling
> CounterBar).
>
> And it is very obvious why you received those warnings: by adding the
> header counterfoo.h AGAIN to your CounterBar project, it gets mock'ed ONCE
> AGAIN (because qmake realises that this header/class is derived from
> QObject, contains the Q_OBJECT macro etc.). And then the resulting
> moc_counterfoo.cpp gets compiled ONCE again into your *.exe, and the same
> code is now once in your DLL and once in your *.exe! And off course the
> second time the __declspec(dllimport) totally does not make sense for the
> compiler, hence the warnings ;)
>
> See the output WITH the HEADERS += specified:
>
> Z:\resources\work\Qt\CounterFooBar\CounterBar>mingw32-make
> ...
> g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE
> -DQT_LARGEFILE_SUPPORT -
> DQT_DLL -DQT_CORE_LIB -DQT_THREAD_SUPPORT
> -I"c:\Qt\2009.05\qt\include\QtCore" -I
> "c:\Qt\2009.05\qt\include" -I"c:\Qt\2009.05\qt\include\ActiveQt" -I"debug"
> -I"c:
> \Qt\2009.05\qt\mkspecs\win32-g++" -o debug\main.o main.cpp
>
> The main.cpp gets compiled, that's fine. Here the compilation should
> actually stop, and the link stage should begin, but instead...
>
> C:/Qt/2009.05/qt/bin\moc.exe -DUNICODE -... ..\CounterFoo\counterfoo.h -o
> debug\moc_counterfoo.cpp
>
> ... the counterfoo.h header is moc'ed ONCE AGAIN and ...
>
> g++ -... debug\moc_counterfoo.cpp
>
> ... compiled, resulting in those warnings:
>
> debug\moc_counterfoo.cpp:40: warning: 'CounterFoo::staticMetaObject'
> redeclared
> without dllimport attribute after being referenced with dll linkage
>
>
> It means exactly what it sais: you have re-declared the method
> CounterFoo::staticMetaObject (in the 2nd generated moc_counterfoo.cpp),
> after you had already #included (from your main.cpp) the same method from
> the 1st counterfoo.h header, where you (correctly) specified the dll linkage
> stuff (your CounterBar project sees __declspec(dllimport)). And
> __declspec(dllimport) is indeed different than "no DLL linkage
> specification", as in the 2nd case.
>
>
> Lesson learned: always read the output of your compiler/linker and take
> extra-care what gets mocked/compiled/linked ;) And take warnings seriously,
> it pais off (as you did ;)
>
> Cheers, Oliver
> --
> Oliver Knoll
> Dipl. Informatik-Ing. ETH
> COMIT AG - ++41 79 520 95 22
> _______________________________________________
> Qt-interest mailing list
> Qt-interest at trolltech.com
> http://lists.trolltech.com/mailman/listinfo/qt-interest
>
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