[question] CMakeToolchain: When use cached_variables vs variables

The documentation only explains CMakeToolchain.variables and CMakeToolchain.cached_variables features, one is better for multiple configuration and another is single configuration:

https://docs.conan.io/en/latest/reference/conanfile/tools/cmake/cmaketoolchain.html#variables

But it’s not clear when each one should be used, based on real examples.

So far, we are migrating recipes in CCI and there is a doubt about which one should be adopted by default when using CMakeToolchain.

• I’ve read the CONTRIBUTING guide.

[SpaceIm]

• CMakeToolchain.variables are variables that are defined directly in the generated conan_toolchain.cmake file (with set(VAR_NAME "var_value")

From what I’ve seen, they are passed as set(VAR_NAME "var_value" CACHE <type> "Variable VAR_NAME conan-toolchain defined") (https://github.com/conan-io/docs/issues/2689). So the new type cache_variables is quite misleading (it’s cache_variables in CMakePresets terminology AFAIK). If they were not passed as CACHE variables, they couldn’t override options until CMake >=3.13 policies. It’s what happens to BUILD_SHARED_LIBS: https://github.com/conan-io/conan/issues/11840

From my experimentations:

• CMAKE_POLICY_DEFAULT_CMPxxxx must be passed as cache_variables.
• Anything else can be passed as variables.
• if BUILD_SHARED_LIBS or CMAKE_POSITION_INDEPENDENT_CODE are turned as option() in upstream CMakeLists and cmake_minimum_required() is lower than 3.13 , you have to either:
  • inject CMAKE_POLICY_DEFAULT_CMP0077 (https://github.com/conan-io/conan/issues/11840)
  • pass BUILD_SHARED_LIBS/CMAKE_POSITION_INDEPENDENT_CODE as variables (or cache_variables).
• if BUILD_SHARED_LIBS or CMAKE_POSITION_INDEPENDENT_CODE are turned as CACHE variables in upstream CMakeLists, you have to pass BUILD_SHARED_LIBS/CMAKE_POSITION_INDEPENDENT_CODE as variables (or cache_variables).
• if upstream CMakeLists is illformed, things can go wrong, specially for variables (and it may not be obvious to see it), for example:
  • cmake_minimum_required() missing.
  • project() before cmake_minimum_required().
  • option() or CACHE variables between cmake_minimum_required() & project() (pernicious, you may not see that recipe options injected through conan toolchain are not honored).

[uilianries]

@jwillikers Sorry! 😓