[CMake] Advice on how to remove duplicated code for object and interface library setup

[Deniz Bahadir]

Am 16.03.2018 um 03:17 schrieb Stephen McDowell:
> I am trying to obey modern CMake target-based practices, with a twist 
> that everything is available as an OBJECT library if desired.  I have 
> this working and can explain why if desired, but I feel it is extraneous 
> for this question.  What I create:
> 
> - mylib <- the main library
>      - mylib_obj <- contains all of the actual objects for the library 
> (OBJECT library)
>      - mylib_interface <- linked with mylib
> 
> So it looks like this:
> 
>      add_library(*mylib_obj OBJECT* "")
> **add_library(*mylib_interface INTERFACE*)
> 
> Based on various options / settings, *target_sources(mylib_obj)* will be 
> set and all of the various compiler operations etc.  At the end, I have
> 
>      # MYLIB_LIBRARY_TYPE is either SHARED or STATIC
>      add_library(*mylib* ${MYLIB_LIBRARY_TYPE} 
> *$<TARGET_OBJECTS:mylib_obj>*)
>      target_link_libraries(*mylib mylib_interface*)
> 
> This works nicely, but internally I have to do the same operations for 
> both *mylib_obj* and *mylib_interface*.
> 
>      target_include_directories(*mylib_obj PUBLIC* ${SomeLib_INCLUDE_DIRS})
>      target_include_directories(*mylib_interface INTERFACE* 
> ${SomeLib_INCLUDE_DIRS})
>      target_link_libraries(*mylib_interface INTERFACE* ${SomeLib_LIBRARIES})
> *
> *
> Without *mylib_interface*, I cannot include the future linking 
> information as well as include / compiler directive information will not 
> be propagated with *target_link_libraries(something mylib)*.  But 
> *mylib_obj* also needs include information, etc. *But* I can’t do
> 
>      target_link_libraries(*mylib_obj mylib_interface*)
> *
> *
> because *mylib_obj* is an object library.

Best idea is to use the latest development-version of CMake from master 
branch (which will become CMake 3.12). That one supports linking of 
object-libraries natively like other targets, too. (See: 
https://gitlab.kitware.com/cmake/cmake/merge_requests/1524)

> 
> Is there a way to avoid having to set the same thing for both 
> *mylib_obj* and *mylib_interface* given the requirement that 
> *mylib_obj* is intended to house all of the actual objects?
> 
> Thank you for any guidance, I’m sorry if this question doesn’t make much 
> sense.  I’m trying to unlearn all of my outdated self-taught CMake!
> 
> -Stephen
> 

Hope that helps,
Deniz