CMake: How to build external projects and include their targets

Question

I have a Project A that exports a static library as a target:

install(TARGETS alib DESTINATION lib EXPORT project_a-targets)
install(EXPORT project_a-targets DESTINATION lib/alib)

Now I want to use Project A as an external project from Project B and include its built targets:

ExternalProject_Add(project_a
  URL ...project_a.tar.gz
  PREFIX ${CMAKE_CURRENT_BINARY_DIR}/project_a
  CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
)

include(${CMAKE_CURRENT_BINARY_DIR}/lib/project_a/project_a-targets.cmake)

The problem is that the include file does not exist yet when CMakeLists of Project B is run.

Is there a way to make the include dependent on the external project being built?

Update: I wrote a short CMake by Example tutorial based on this and other common problems I encountered.

Answer 1

I think you're mixing up two different paradigms here.

As you noted, the highly flexible ExternalProject module runs its commands at build time, so you can't make direct use of Project A's import file since it's only created once Project A has been installed.

If you want to include Project A's import file, you'll have to install Project A manually before invoking Project B's CMakeLists.txt - just like any other third-party dependency added this way or via find_file / find_library / find_package.

If you want to make use of ExternalProject_Add, you'll need to add something like the following to your CMakeLists.txt:

ExternalProject_Add(project_a
  URL ...project_a.tar.gz
  PREFIX ${CMAKE_CURRENT_BINARY_DIR}/project_a
  CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
)

include(${CMAKE_CURRENT_BINARY_DIR}/lib/project_a/project_a-targets.cmake)

ExternalProject_Get_Property(project_a install_dir)
include_directories(${install_dir}/include)

add_dependencies(project_b_exe project_a)
target_link_libraries(project_b_exe ${install_dir}/lib/alib.lib)