CMake: How to build external projects and include their targets

Question

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)

This post has a reasonable answer:

CMakeLists.txt.in:

cmake_minimum_required(VERSION 2.8.2)

project(googletest-download NONE)

include(ExternalProject)
ExternalProject_Add(googletest
  GIT_REPOSITORY    https://github.com/google/googletest.git
  GIT_TAG           master
  SOURCE_DIR        "${CMAKE_BINARY_DIR}/googletest-src"
  BINARY_DIR        "${CMAKE_BINARY_DIR}/googletest-build"
  CONFIGURE_COMMAND ""
  BUILD_COMMAND     ""
  INSTALL_COMMAND   ""
  TEST_COMMAND      ""
)

CMakeLists.txt:

# Download and unpack googletest at configure time
configure_file(CMakeLists.txt.in
               googletest-download/CMakeLists.txt)
execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download )
execute_process(COMMAND ${CMAKE_COMMAND} --build .
  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download )

# Prevent GoogleTest from overriding our compiler/linker options
# when building with Visual Studio
set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)

# Add googletest directly to our build. This adds
# the following targets: gtest, gtest_main, gmock
# and gmock_main
add_subdirectory(${CMAKE_BINARY_DIR}/googletest-src
                 ${CMAKE_BINARY_DIR}/googletest-build)

# The gtest/gmock targets carry header search path
# dependencies automatically when using CMake 2.8.11 or
# later. Otherwise we have to add them here ourselves.
if (CMAKE_VERSION VERSION_LESS 2.8.11)
  include_directories("${gtest_SOURCE_DIR}/include"
                      "${gmock_SOURCE_DIR}/include")
endif()

# Now simply link your own targets against gtest, gmock,
# etc. as appropriate

However it does seem quite hacky. I'd like to propose an alternative solution - use Git submodules.

cd MyProject/dependencies/gtest
git submodule add https://github.com/google/googletest.git
cd googletest
git checkout release-1.8.0
cd ../../..
git add *
git commit -m "Add googletest"

Then in MyProject/dependencies/gtest/CMakeList.txt you can do something like:

cmake_minimum_required(VERSION 3.3)

if(TARGET gtest) # To avoid diamond dependencies; may not be necessary depending on you project.
    return()
endif()

add_subdirectory("googletest")

I haven't tried this extensively yet but it seems cleaner.

Edit: There is a downside to this approach: The subdirectory might run install() commands that you don't want. This post has an approach to disable them but it was buggy and didn't work for me.

Edit 2: If you use add_subdirectory("googletest" EXCLUDE_FROM_ALL) it seems means the install() commands in the subdirectory aren't used by default.

Edit: CMake now has builtin support for this. See new answer which uses FetchContent.

You can also force the build of the dependent target in a secondary make process

See my answer on a related topic.

I was searching for similar solution. The replies here and the Tutorial on top is informative. I studied posts/blogs referred here to build mine successful. I am posting complete CMakeLists.txt worked for me. I guess, this would be helpful as a basic template for beginners.

"CMakeLists.txt"

cmake_minimum_required(VERSION 3.10.2)

# Target Project
project (ClientProgram)

# Begin: Including Sources and Headers
include_directories(include)
file (GLOB SOURCES "src/*.c")
# End: Including Sources and Headers


# Begin: Generate executables
add_executable (ClientProgram ${SOURCES})
# End: Generate executables


# This Project Depends on External Project(s) 
include (ExternalProject)

# Begin: External Third Party Library
set (libTLS ThirdPartyTlsLibrary)
ExternalProject_Add (${libTLS}
PREFIX          ${CMAKE_CURRENT_BINARY_DIR}/${libTLS}
# Begin: Download Archive from Web Server
URL             http://myproject.com/MyLibrary.tgz
URL_HASH        SHA1=<expected_sha1sum_of_above_tgz_file>
DOWNLOAD_NO_PROGRESS ON
# End: Download Archive from Web Server

# Begin: Download Source from GIT Repository
#    GIT_REPOSITORY  https://github.com/<project>.git
#    GIT_TAG         <Refer github.com releases -> Tags>
#    GIT_SHALLOW     ON
# End: Download Source from GIT Repository

# Begin: CMAKE Comamnd Argiments
CMAKE_ARGS      -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_CURRENT_BINARY_DIR}/${libTLS}
CMAKE_ARGS      -DUSE_SHARED_LIBRARY:BOOL=ON
# End: CMAKE Comamnd Argiments    
)

# The above ExternalProject_Add(...) construct wil take care of \
# 1. Downloading sources
# 2. Building Object files
# 3. Install under DCMAKE_INSTALL_PREFIX Directory

# Acquire Installation Directory of 
ExternalProject_Get_Property (${libTLS} install_dir)

# Begin: Importing Headers & Library of Third Party built using ExternalProject_Add(...)
# Include PATH that has headers required by Target Project
include_directories (${install_dir}/include)

# Import librarues from External Project required by Target Project
add_library (lmytls SHARED IMPORTED)
set_target_properties (lmytls PROPERTIES IMPORTED_LOCATION ${install_dir}/lib/libmytls.so)
add_library (lmyxdot509 SHARED IMPORTED)
set_target_properties(lmyxdot509 PROPERTIES IMPORTED_LOCATION ${install_dir}/lib/libmyxdot509.so)

# End: Importing Headers & Library of Third Party built using ExternalProject_Add(...)
# End: External Third Party Library

# Begin: Target Project depends on Third Party Component
add_dependencies(ClientProgram ${libTLS})
# End: Target Project depends on Third Party Component

# Refer libraries added above used by Target Project
target_link_libraries (ClientProgram lmytls lmyxdot509)