Support for generator expressions ?

[BenLeadbetter]

It would be good to add support for generator expressions. Without it nothing from the generation stage can be tested with the framework.

For example the following test fails because the ct_assert_equal function is evaluated at configure time, rather than at generation time.

ct_add_test(NAME test_with_genex)
function(${test_with_genex})
    add_executable(test_executable test_executable.cpp)
    set(TEST_VAR $<TARGET_FILENAME:test_executable>)
    ct_assert_equal(TEST_VAR "test_executable")
endfunction()

[ryanmrichard]

@BenLeadbetter TL;DR I like this suggestion, but I don't know how to do it.

Full answer:

I really like the idea of being able to test generator expressions. FWIW, I tend to avoid them in my own CMake scripts simply because of how hard they are to debug. If we could unit test them that could be a game changer for me personally. That said, I'm not sure how to actually do it, or if it's even possible. More specifically CMakeTest is designed to be a unit testing framework for the CMake language, but since the CMake language is designed to run in the configure step, I'm not sure if it can be used to test things that happen in the generation step.

Just spit balling here, the canonical ways of getting at generator expression values are shown here. Conceivably, if under the hood CMakeTest could run one of those options in a subprocess/external project, then we could pull the value back into the unit testing framework. Off the top of my head, I'm not sure if we can forward the generator expressions to any of the CMake commands that run subprocesses at configure time though; my guess is no, since the generator expressions wouldn't have a value yet, but I don't know for sure.

IIRC CMakeTest actually does run the build step on the unit test (@AutonomicPerfectionist please correct me if I'm wrong). If it's not the case, we probably should make it the case. Anyways, assuming we do run the build step we could add a function ct_assert_genex($<...> VALUE_IT_SHOULD_HAVE) that somehow creates an assert which is run during the build step (maybe with the execute_process command?).

[AutonomicPerfectionist]

@ryanmrichard the build step is indeed ran during testing, it just doesn't do anything at the moment. I will have to do some digging on generator expressions later to see what I can come up with in regards to testing them