CMake detect if project is top level

2 August, 2022

CMake can detect if a project is “top level” that is, NOT via FetchContent using PROJECT_IS_TOP_LEVEL.

cmake_minimum_required(VERSION 3.21)
project(child)

if(PROJECT_IS_TOP_LEVEL)
  message(STATUS "${PROJECT_NAME} directly building, not FetchContent")
endif()

Note that directory property PARENT_DIRECTORY and PROJECT_IS_TOP_LEVEL properties are NOT useful for detecting if the child project is being used as an ExternalProject.

For CMake < 3.21, “PROJECT_IS_TOP_LEVEL” can be set like:

if(CMAKE_VERSION VERSION_LESS 3.21)
  get_property(not_top DIRECTORY PROPERTY PARENT_DIRECTORY)
  if(NOT not_top)
    set(PROJECT_IS_TOP_LEVEL true)
  endif()
endif()