Transitive target_include_directories on OBJECT libraries

Question

Here is snippet from make CMakeLists.txt:

add_library(foo-object OBJECT src/foo.cpp)
target_include_directories(foo-object PUBLIC include)
add_library(foo SHARED $<TARGET_OBJECTS:${PROJECT_NAME}-object>)
add_library(foo_static STATIC $<TARGET_OBJECTS:${PROJECT_NAME}-object>)

Now, this all works fine, both libraries are generated. However I have a problem when I try to use it:

add_executable(bar src/main.cpp)
target_link_libraries(bar foo)

Target bar doesn't compile, because include directories from foo-object are not propagated. If I add target_include_directories directly on foo as well, everything will compile fine.

How can I make both foo and foo_static automatically use (and forward to stuff depending on them) include directories from foo-object?

Answer 1

Hm, at the moment I came up with following:

add_library(foo-object OBJECT src/foo.cpp)
target_include_directories(foo-object PUBLIC include)

get_property(object_include_dirs TARGET foo-object PROPERTY INCLUDE_DIRECTORIES)
get_property(object_link_libs TARGET foo-object PROPERTY LINK_LIBRARIES)

add_library(foo SHARED $<TARGET_OBJECTS:${PROJECT_NAME}-object>)
target_include_directories(foo PUBLIC ${object_include_dirs})
target_link_libraries(foo PUBLIC ${object_link_libs})

add_library(foo_static STATIC $<TARGET_OBJECTS:${PROJECT_NAME}-object>)
target_include_directories(foo_static PUBLIC ${object_include_dirs})
target_link_libraries(foo_static PUBLIC ${object_link_libs})

but come on, there must be better way :/

Answer 2

It seems that transitive properties only work when targets are linked through a chain of target_link_library calls. In your case, you do not have such a link between foo-object and foo.

If you add a source file to foo, that one should also not be able to see the include directory from foo-object.

This might be an oversight in the design of OBJECT libraries, as it essentially breaks the transitive properties for those.