How to clone and integrate external (from git) cmake project into local one

Question

I faced with a problem when I was trying to use Google Test.

There are lot of manuals on how to use ExternalProject_Add for the adding gtest into the project, however most of these describe a method based on downloading zip archive with gtest and build it.

As we know gtest is github-hosted and cmake-based project. So I'd like to find native cmake way.

If this would be a header-only project, I'd write something like:

cmake_minimum_required(VERSION 2.8.8)
include(ExternalProject)
find_package(Git REQUIRED)

ExternalProject_Add(
    gtest
    PREFIX ${CMAKE_CURRENT_SOURCE_DIR}/ext
    GIT_REPOSITORY https://github.com/google/googletest.git
    TIMEOUT 10
    UPDATE_COMMAND ${GIT_EXECUTABLE} pull
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND ""
    LOG_DOWNLOAD ON
)

ExternalProject_Get_Property(gtest source_dir)
set(GTEST_INCLUDE_DIR ${source_dir}/googletest/include CACHE INTERNAL "Path to include folder for GTest")
set(GTEST_ROOT_DIR ${source_dir}/googletest CACHE INTERNAL "Path to source folder for GTest")
include_directories(${INCLUDE_DIRECTORIES} ${GTEST_INCLUDE_DIR} ${GTEST_ROOT_DIR})
message(${GTEST_INCLUDE_DIR})

and add this script from my cmake project like:

 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake.modules/")
 include(AddGTest)
....
 add_dependencies(${PROJECT_NAME} gtest)

However this requires a build step.

How should this be implemented?

1. By adding BUILD_COMMAND into ExternaProject_Add and linking with produced libs?
2. Or by including gtest's cmakelists into my project, something like this:

add_subdirectory (${CMAKE_SOURCE_DIR}\ext\src\gtest\googletest\CMakeLists.txt)

this is not correct way because on the moment of the project load the folder does not exist.

3. Any other ways?

What is a correct/prefer way?

Solution

I would go with the first approach. You don't need to specify a build command because cmake is used by default. This could look like:

cmake_minimum_required(VERSION 3.0)
project(GTestProject)

include(ExternalProject)

set(EXTERNAL_INSTALL_LOCATION ${CMAKE_BINARY_DIR}/external)

ExternalProject_Add(googletest
    GIT_REPOSITORY https://github.com/google/googletest
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNAL_INSTALL_LOCATION}
)

include_directories(${EXTERNAL_INSTALL_LOCATION}/include)
link_directories(${EXTERNAL_INSTALL_LOCATION}/lib)

add_executable(FirstTest main.cpp)
add_dependencies(FirstTest googletest)
target_link_libraries(FirstTest gtest gtest_main pthread)

I don't know if this is the correct/preferred way if there even is one. If you wanted to implement your second approach you would have to download the code with execute_process first.

Comments

[amigo421]

Thank you, this is what i want to hear - "cmake does this by default" , didn't find this in manual, and looked for build command

[Craig Scott]

This article documents a way that the gtest manual now also recommends. CMake 3.11 added the FetchContent module which extends the approach further and uses gtest as one of its examples.

[ataraxis]

@CraigScott Your answers are always the most enlightening ones. This is worth a own answer instead of just a comment. I really love your book by the way!