Retrieving compile definitions with genex evaluated

Code
1

I have a build which uses the unifdef tool and hence is dependent on having the compile definitions for built targets available as input to this. Now that I was adding the compile definitions using genexes I’d like to learn a method to retrieve them and have the genexes evaluated. get_target_property gives me them unevaluated.
Is there a canonical way (without reverting to just store all definitions in a list instead)?

2

By definition, generator expressions (“genexes”) are evaluated at generate time (hence their name), so you cannot access their evaluated value at configure time — it may not even be fully defined yet. You can of course send unevaluated genexes into a context which will eventually evaluate them, such as using them in the arguments of a command run via add_custom_command(). Perhaps something like this:

add_custom_command(
  OUTPUT result.ext
  COMMAND some.exe --defines $<TARGET_PROPERTY:MyTarget,COMPILE_DEFINITIONS>
)

For more complex situations, you may also want to look at the genexes $<GENEX_EVAL> and $<TARGET_GENEX_EVAL>.

3

Thanks, I kind of managed to get it to work for unifdef (which can accept a newline separated file with defines) with the following beauty:
file(GENERATE OUTPUT defs.txt CONTENT $<LIST:JOIN,$<LIST:TRANSFORM,$<TARGET_PROPERTY:my_tgt,INTERFACE_COMPILE_DEFINITIONS>,PREPEND,-D>,\n>)
But then I realized I need to pass on the definitions to Doxygen as well, which needs a space separated string. So I wonder if there is a way to have this stored in a variable instead of written to disk (and then read up again, and that is when it’s starting to get hard to justify)?

4

Variables don’t really exist at generate time, so you can’t get the value in a variable. Instead, a $<SPACE> genex (which doesn’t exist yet, but an issue would help) may be useful here as the join string.