Disabling in-source builds

In Chapter 1, First Steps with CMake, we talked about in-source builds, and how it is recommended to always specify the build path to be out of source. This not only allows for a cleaner build tree and a simpler .gitignore file, but it also decreases the chances you’ll accidentally overwrite or delete any source files.

To stop the build early you may use the following check:

ch04/09-in-source/CMakeLists.txt

cmake_minimum_required(VERSION 3.26.0)
project(NoInSource CXX)
if(PROJECT_SOURCE_DIR STREQUAL PROJECT_BINARY_DIR)
  message(FATAL_ERROR "In-source builds are not allowed")
endif()
message("Build successful!")

If you would like more information about the STR prefix and variable references, please revisit Chapter 2, The CMake Language.

Notice, however, that no matter what you do in the preceding code, it seems like CMake will still create a CMakeFiles/ directory and a CMakeCache.txt file.

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