[cmake-developers] Current deficiencies of automoc

[Alan W. Irwin]

On 2016-10-22 19:49+0200 Sebastian Holtermann wrote:

[...]
> Actually I made an implementation in 3.6.0 that generated the moc files
> in the a build tree subdirectory correspoding to the header path.
> But that blew up on some projects because the generated paths got too long 
> for some compiĺers. That implementation was theefore reverted.
> In 3.7  there is a new approatch that generates the moc files in
> ${CMAKE_BINARY_DIR}/
>    ${TARGETNAME}_automoc.dir/
>        ${HEADERNAME}_${HEADERPATHCHECKSUM}.cpp
> This ensures that the paths don't get too long and that there won't be any 
> name collisions.

Hi Sebastian:

After a substantial break to finish off a different project, I have
now had a chance to return to the present topic using my attached
test_automoc project.  (Also available in the cmake/test_automoc
subdirectory of the PLplot git master branch that can be accessed at
<https://sourceforge.net/p/plplot/plplot/ci/master/tree/>.) The
project builds 6 separate simple Qt5 applications that #include a
header that needs moc processing.  The only differences between the
source code files for the various executables is the name and
directory location of that #include'd file and the various instructions
given to automoc (and in one case qt5_wrap_cpp) to process that file.

With that test project, I now confirm (sorry about the noise to the
contrary before) that automoc does use the approach you mentioned
above for CMake-3.7.0 to reduce the name collisions that occur for
that same project for earlier versions of CMake.

Concerning the collisions still expected for 3.7.0, the automoc
documentation (see
<https://cmake.org/cmake/help/v3.7/prop_tgt/AUTOMOC.html>) states the
following:

"However, if multiple source files in different directories do this
[i.e., contain an "#include "moc_<headerbasename>.cpp" when the header
name is <headerbasename>.h] then their generated moc files would
collide. In this case a diagnostic will be issued."

For the life of me, I cannot find a way to generate that diagnostic.
Instead, three different targets (two targets concerning the same
implementation code and header in the same directory and a third
target for a copy of that same implementation code and header in a
different directory) generate at build time a file named
moc_test_q_object.cpp in the same build directory at different times.
This is a clear example of a three-way name collision, but CMake does
not issue a diagnostic for any of these targets concerning that
collision contrary to the above documentation.

Could you please take a look at the three last executables configured
for this test case (helloworld_automoc_same_include,
helloworld_automoc_same_include1, and
helloworld_automoc_same_include2) to see why that collision diagnostic
is not being issued with cmake-3.7.0?  Better yet, of course, would be
to solve these remaining name collisions completely by using a
modification of your idea above, i.e., for this special case where
users implementation code contained

#include "moc_<headerbasename>.cpp"

then moc should generate the file

${CMAKE_BINARY_DIR}/${TARGETNAME}_automoc.dir/${HEADERPATHCHECKSUM}/moc_${HEADERNAME}.cpp

Note, the generated moc file has the name expected by the #include so
I believe this idea would work so long as automoc automatically
appended
${CMAKE_BINARY_DIR}/${TARGETNAME}_automoc.dir/${HEADERPATHCHECKSUM} to
the target INCLUDE_DIRECTORIES property.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
__________________________

Linux-powered Science
__________________________
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