We have mentioned that module includes should not be used as function calls since modules could be (accidentally) included multiple times. In this recipe, we will program our own simple include guard, which will warn us if we try to include a module multiple times. The built-in include_guard command is available in CMake since version 3.10 and behaves like #pragma once for C/C++ header files. For this version of CMake, we will discuss and demonstrate how functions and macros can be redefined. We will show how we can check the CMake version and for versions below 3.10, we will use our custom include guard.
The code for this recipe is available at https://github.com/dev-cafe/cmake-cookbook/tree/v1.0/chapter-07/recipe-05. The recipe is valid with CMake version 3.5 (and higher) and has been tested on GNU/Linux, macOS, and Windows.