Prevent in source build

[ClausKlein]

/Users/clausklein/Workspace/cpp/beman-project/exemplar
bash-5.2$ cmake -S .
-- The CXX compiler identification is AppleClang 16.0.0.16000026
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found GTest: /usr/local/lib/cmake/GTest/GTestConfig.cmake (found version "1.17.0")
-- Looking for __cpp_lib_ranges
-- Looking for __cpp_lib_ranges - not found
CMake Warning at examples/CMakeLists.txt:38 (message):
  Missing range support! Skip: identity_as_default_projection


-- Examples to be built: identity_direct_usage
-- Configuring done (24.5s)
-- Generating done (0.5s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/beman-project/exemplar
bash-5.2$ git status
On branch feature/test-installed-package
Your branch is up to date with 'claus/feature/test-installed-package'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	CMakeCache.txt
	CTestTestfile.cmake
	DartConfiguration.tcl
	build.ninja
	cmake_install.cmake
	examples/CTestTestfile.cmake
	examples/cmake_install.cmake
	src/beman/exemplar/CMakeFiles/
	src/beman/exemplar/CTestTestfile.cmake
	src/beman/exemplar/beman.exemplar-config-version.cmake
	src/beman/exemplar/beman.exemplar-config.cmake
	src/beman/exemplar/beman.exemplar_verify_interface_header_sets/
	src/beman/exemplar/cmake_install.cmake
	tests/beman/exemplar/CTestTestfile.cmake
	tests/beman/exemplar/beman.exemplar.tests.identity[1]_include.cmake
	tests/beman/exemplar/cmake_install.cmake

nothing added to commit but untracked files present (use "git add" to track)
bash-5.2$ 

[ClausKlein]

that would prevent this:

if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
  message(
    FATAL_ERROR
      "In-source builds are not supported. "
      "Please read the BUILDING document before trying to build this project. "
      "You may need to delete 'CMakeCache.txt' and 'CMakeFiles/' first."
  )
endif()

[camio]

Developers who choose to bypass the provided CMake presets and invoke cmake manually are typically advanced users who understand that in-source builds are a bad practice. There's a trade-off between adding guardrails and keeping the build scripts lean. In this case, I'd prefer to trust the user's expertise and avoid adding boilerplate for non-standard workflows.

[ClausKlein]

Do you know how hard it is to delete the generated files again if you do a cmake -L . accidentally in project root instead of binary directory?

I had a lot of dislocations when I tried to add /CMakeFiles /CMakeCache.txt to .gitignore file?

[wermos]

FWIW, LLVM has CMake logic in its repo to prevent in-source builds.

[camio]

@nickelpro do you have an opinion here as our build lead?

[JeffGarland]

I could go either way on this one, but I'd like to decide and move on by either making the change or closing the issue. If we don't have a resolution in the next few weeks I'll be inclined to stick with status quo and just close this.