pseyfert/cmake_implicit_depends

trying implicit depends with cmake

small study on dependency treatment in CMake

There are three files: base.h derived.h main.cpp

base.h is a "fake" header. It's include is disabled through the preprocessor options. The executable main should thus be rebuilt whenever main.cpp or derived.h get touched.

The CMake option IMPLICIT_DEPENDS is tested for a custom command. It is set up to have the same depends as main.

The script runall.sh runs CMake a few times (generation and compilation) for both generators and touches files to se what rebuild steps get done/omitted. The results are in the result file.

observation with make

CMake is set up to figure dependencies for C/C++ files out by itself, and doesn't use the gcc preprocessor. It therefore misses that base.h is only for confusion and rebuilds everything when base.h is touched.

It "correctly" applies the same dependencies for the custom command

observation with ninja

With ninja, the compiler's dependency resolution is used instead of the CMake dependency scan. When base.h is touched, no recompilation is done.

Yet, IMPLICIT_DEPENDS is not implemented for the ninja generator, and doesn't recognize the need to rerun grep when derived.h gets touched.

small print

In the runall.sh script, I added a few sleep commands, because ninja runs too fast and the time checking doesn't realise the touch should've happened after the build.