[CMake] Forcibly run 'moc' on Qt files that are NOT part of the build

[Andreas Pakulat]

On 07.03.12 10:10:27, Michael Jackson wrote:
> In an effort to speed up the build of a project that uses Qt (and moc) I tried an alternate approach with the moc files. Normally I use the basic idea of gathering the headers that need to be "moc'ed" and feed those to moc with this type of CMake Code:
> 
> QT4_WRAP_CPP( FilterWidgets_Generated_MOC_SRCS ${QFilterWidget_HDRS} ${FilterWidget_GEN_HDRS}) 
> 
> The in the Add_Executable(...) call include the ${FilterWidgets_Generated_MOC_SRCS} variable to the list of sources. In my project I have at least 30 auto-generated files which all get moc'ed. That gives me an additional 60 compiled files. So I tried the idea of #include "moc_[some_file.cxx]" in each of the auto-generated .cpp files for each Widget. This would cut the number of files compiled in half. The issue is that since they are being #include'ed in the .cpp files then they do NOT need to be compiled themselves so I took the ${FilterWidgets_Generated_MOC_SRCS} out of the list of sources in the add_executable() call. What happened is that CMake did NOT run moc on those headers because there were now NOT included in the build.
> 
>  So for that version of the cmake code I have something like this:
> 
> QT4_WRAP_CPP( FilterWidgets_Generated_MOC_SRCS ${FilterWidget_GEN_HDRS}) 
> QT4_WRAP_CPP( FilterWidgets_MOC_SRCS ${QFilterWidget_HDRS} )
> 
> Is there a way to forcibly run the moc step even if the resulting source files are NOT directly included in the add_executable? Custom_Command? Add_Depends?

A few options I can think of:

- Use the automoc developed as part of KDE (its pure-Qt though)
    https://projects.kde.org/projects/kdesupport/automoc/repository
- Use the automoc function from FindQt4.cmake
- Use the new automoc function in CMake 2.8.7

All of these will help handle the qt4_wrap_cpp stuff for you as long as
you have the #include. Note that I think there's at least 2 or 3
differnet filenames for the include depending on which of the above you
use (foo.moc vs moc_foo.cpp vs. moc_foo.cxx or something like that).

If none of them is an option for you then I guess using
add_custom_target and add_dependencies is the way to go.

Andreas