CMake append objects from different CMakeLists.txt into one library

Question

I would like to create a single library from objects from multiple sub-directories, each one containing their own CMakeLists.txt with OBJECT library trick to have multiple t

Answer 1

Several OBJECT libraries can be incorporated into one INTERFACE library:

# Create lib1 OBJECT library with some sources, compile flags and so.
add_library(lib1 OBJECT ...)

# Create lib2 OBJECT library with some sources, compile flags and so.
add_library(lib2 OBJECT ...)

# Create INTERFACE library..
add_library(libs INTERFACE)
# .. which combines OBJECT libraries
target_sources(libs INTERFACE $<TARGET_OBJECTS:lib1> $<TARGET_OBJECTS:lib2>)

Resulted library can be used with standard target_link_libraries:

add_library(mainLib <some-source>)
target_link_libraries(mainLib libs)

It is possible to combine INTERFACE libraries futher:

# Create libs_another INTERFACE library like 'libs' one
add_library(libs_another INTERFACE)
..

# Combine 'libs' and 'libs_another' together
add_library(upper_level_libs INTERFACE)
target_link_libraries(upper_level_libs INTERFACE libs libs_another)

Resulted libraries' "chain" could be of any length.

---

While this way for combination of OBJECT libraries looks hacky, it is actually allowed by CMake documentation:

Although object libraries may not be named directly in calls to the target_link_libraries() command, they can be "linked" indirectly by using an Interface Library whose INTERFACE_SOURCES target property is set to name $<TARGET_OBJECTS:objlib>.

Answer 2

I just collect objects from all places using set with PARENT_SCOPE.

root CMakeLists.txt:

set(OBJECTS)

add_subdirectory(lib1)
add_subdirectory(lib2)

add_library(lib STATIC ${OBJECTS})

CMakeLists.txt in subdirectories:

add_subdirectory(lib11)

add_library(${PROJECT_NAME} OBJECT src1.c)
list(APPEND OBJECTS $<TARGET_OBJECTS:${PROJECT_NAME}>)
set(OBJECTS ${OBJECTS} PARENT_SCOPE)