Transitive target_include_directories on OBJECT libraries - cmake

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.

Answer 3

On CMake <3.12, use the following:
add_library(foo SHARED $<TARGET_OBJECTS:${PROJECT_NAME}-object>)
target_include_directories(foo
PRIVATE
$<TARGET_PROPERTY:${PROJECT_NAME}-object,INTERFACE_INCLUDE_DIRECTORIES>)
On CMake >=3.12, take a look at this answer (thanks #ian5v for the suggestion)
How it works:
target_include_directories(...)
...
PUBLIC and INTERFACE items will populate the INTERFACE_INCLUDE_DIRECTORIES property of <target>.
Therefore ${PROJECT_NAME}-object has INTERFACE_INCLUDE_DIRECTORIES set on it. We need to fetch this property and insert it into our own include path.
This looks like a job for "generator expressions"!. In particular, $<TARGET_PROPERTY:tgt,prop> looks like it will come in handy here.
Our tgt will be ${PROJECT_NAME}-object, and we're trying to extract all of the values of INTERFACE_INCLUDE_DIRECTORIES, so INTERFACE_INCLUDE_DIRECTORIES will be prop.
This comes out to $<TARGET_PROPERTY:${PROJECT_NAME}-object,INTERFACE_INCLUDE_DIRECTORIES>, which is exactly what we've used in the code above.

Answer 4

For me something like the following seems to be working:
add_library(foo_objects OBJECT src/foo.cpp src/foo.hpp)
set_property(TARGET foo_objects PROPERTY POSITION_INDEPENDENT_CODE ON)
target_include_directories(foo_objects PUBLIC
"$<BUILD_INTERFACE:src>"
"$<INSTALL_INTERFACE:include>")
add_library(foo_shared SHARED)
add_library(foo_static STATIC)
target_link_libraries(foo_shared PUBLIC foo_objects)
target_link_libraries(foo_static PUBLIC foo_objects)