How to configure ExternalProject during main project configuration?

Question

The CMake\'s ExternalProject allows to define how to an external project is going to be downloaded, configured, built and installed. All whose steps are going to be performed at

Answer 1

If you don't want to build the project at configure-time^†, but just want to download it, use FetchContent. FetchContent_Declare uses many of the same arguments as ExternalProject_Add, except it doesn't allow building the project.

The documentation has a great example on how to use this:

FetchContent_Declare(
  googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  GIT_TAG        release-1.8.0
)

FetchContent_GetProperties(googletest)
if(NOT googletest_POPULATED)
  FetchContent_Populate(googletest)
  add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR})
endif()

This requires CMake 3.11 or later. For prior versions, you can download the FetchContent.cmake module from the CMake repository along with the FetchContent directory, ensuring you comply with the BSD 3-Clause license.

^{† Building at configure-time has some serious drawbacks. For example, users of your library can't control the build process unless you set it up very carefully. A package manager is a better solution}