CMake doesn't include header directory of submodule A within submodule B - c++

Question

I have a CMake project that looks like this:
project/
CMakeLists.txt
subprojectA/
CMakeLists.txt
include/
headerA.hpp
src/
libraryA.cpp
subprojectB/
CMakeLists.txt
src/
mainB.cpp
The "library" subproject, A, is compiled as a static library, becoming libsubprojectA.a. The "main" project, B, is compiled as a binary and depends on the library. mainB.cpp includes a reference to headerA.hpp.
Here is subprojectA/CMakeLists.txt:
project(SubProjectA)
include_directories(include)
add_library(subprojectA STATIC src/libraryA.cpp)
set(${PROJECT_NAME}_INCLUDE_DIRS
${PROJECT_SOURCE_DIR}/include
CACHE INTERNAL "${PROJECT_NAME}: Include Directories" FORCE)
And here is subprojectB/CMakeLists.txt:
project(SubProjectB)
include_directories(${SubProjectA_INCLUDE_DIRS})
add_executable(mainBinary src/mainB.cpp)
target_link_libraries(mainBinary subprojectA)
The main Project CMakeLists.txt looks like:
project(Project)
add_subdirectory(subprojectB)
add_subdirectory(subprojectA)
Note that subprojectB, the main project, is listed before subprojectA.
Here's the problem. When I first run "cmake" on this project, ${SubProjectA_INCLUDE_DIRS} is not set within SubProjectB.
What I think is happening is that the CMakeLists for SubProjectB loads first, when ${SubProjectA_INCLUDE_DIRS} has not yet been set. It sets its own include path to an empty string as a result. However, even though libsubprojectA.a gets built successfully before mainBinary, the include path was already set empty beforehand. As a result, I get this error when trying to make mainBinary:
subprojectB/src/mainB.cpp:1:23: fatal error: headerA.hpp: No such file or directory
#include "headerA.hpp"
^
It's a workaround to put subprojectA before subprojectB in the main Project CMakeLists in the declarative world of CMake. What I really want is to know the proper way to indicate to CMake that the include_directories(${SubProjectA_INCLUDE_DIRS}) line depends on the definitions that exist inside SubProjectA's CMakeLists. Is there a better way to do this?

Answer 1

If you want to express that include directory subprojectA/include is an interface of the library subprojectA, attach this property to the target with target_include_directories command:
subprojectA/CMakeLists.txt:
project(SubProjectA)
add_library(subprojectA STATIC src/libraryA.cpp)
# PUBLIC adds both:
# 1) include directories for compile library and
# 2) include directories for library's interface
target_include_directories(subprojectA PUBLIC include)
So any executable(or other library) which linked with subprojectA will have this include directory automatically:
subprojectB/CMakeLists.txt:
project(SubProjectB)
add_executable(mainBinary src/mainB.cpp)
target_link_libraries(mainBinary subprojectA)
Of course, for use last command properly you need to process directory with library before one with executable:
CMakeLists.txt:
project(Project)
add_subdirectory(subprojectA)
add_subdirectory(subprojectB)