[CMake] [cmake-developers] Obtaining header file dependencies of a source file manually

[Dan Liew]

On 30 November 2015 at 18:35, Brad King <brad.king at kitware.com> wrote:
> On 11/30/2015 01:32 PM, Dan Liew wrote:
>> It works but only for makefile generators... That's an annoying
>> limitation. I'll file a feature request to get this implemented for
>> other generators.
>
> It happens to work for Makefile generators because it was easy to
> implement since those generators already do their own scanning.
> For other generators it is not possible to implement.

Well I found the open issue [1]. It's been open for a while.

It doesn't look completely impossible. For Ninja, it looks like it has
some support for compiler
generated dependency files [2]. Incorporating that into
``add_custom_command()`` would require
an extra call to the host compiler to generate the dependencies when
the custom command is invoked.

For other generators that are Makefile or Ninja based I guess that
approach wouldn't work.

There is an alternative which I suggested in the post. Have CMake
determine the dependencies of the files passed to ``IMPLICIT_DEPENDS``
at configure
time and spit that into the build files of the generator (that would
work for any generator). Then have any changes made to the files
passed to ``IMPLICIT_DEPENDS``
trigger a reconfigure so that the dependencies can be recomputed. For
Makefiles you can don't need to do this and can take the "fast path"
instead.

[1] https://cmake.org/Bug/view.php?id=13234
[2] https://ninja-build.org/manual.html#_depfile

Dan.