Qt, CMake, Visual Studio and Q_OBJECT in cpp files

Question

I'm developing a large project using Qt 4.6, CMake 2.8 and Visual Studio 2008 for the Windows platform.

As far the build system goes, it's all standard stuff: I'm using CMake's QT4_WRAP_CPP macro to generate moc files from header files, which are then linked into the final executable in the add_executable command. Everything is working as expected.

The only restriction with this setup is that I can't define widgets or helper using Q_OBJECT in .cpp files. This would be very convenient for small, context-specific helpers classes that should appear right next to where they're used.

I tried to pass the whole list of source files (both .h and .cpp) to QT4_WRAP_CPP, instead of just the header files, but that doesn't work (linking fails because some moc-related symbols are undefined).

I think the problem is that, for a given pair of files foo.h and foo.cpp, the QT4_WRAP_CPP macro will generate the same moc file (moc_foo.cxx) in the same directory, and obviously that means the first file will be overwritten by the second one, and as a result symbols will be missing at link-time.

Is there a way to fix or work around that problem? For instance, I tried to add a specific rule for foo.cpp of the form

QT4_GENERATE_MOC(directory/foo.cpp directory/foo.moc)

and then add

#include "foo.moc"

at the end of foo.cpp. I think this ought to work, but alas Visual Studio only allows one build rule per file, and .cpp files already have a build rule (compilation to object file), so this approach doesn't work, at least with Visual Studio.

Another idea that I had was to create a new macro, say QT4_WRAP_CPP_WITH_PREFIX, based on QT4_WRAP_CPP (which is defined in share/cmake-2.8/Modules/Qt4Macros.cmake), that would take an additional prefix argument and would add this prefix to the generated moc files. That way, I would call QT4_WRAP_CPP_WITH_PREFIX twice, once for .h files and once for .cpp files, with different prefixes. What I just dislike about this approach is that I'd be messing with the internals of CMake's Qt support, instead of using the public API.

Any better idea?

Cheerz, Franz

Answer 1

Recent versions of CMake have "automoc" which worked like a charm for me: http://blogs.kde.org/2011/11/01/cool-new-stuff-cmake-286-automoc

Simply add in the CMakeLists.txt:

set(CMAKE_AUTOMOC TRUE)

and then in the cpp (e.g. example.cpp) file:

#include "example.moc"

(the *.moc must match the cpp file's name).