remove CXX_MAX_STANDARD and CXX_MAX_SUPPORTED_STANDARD and deprecate dune_require_cxx_standard
Setting the minimal c++ standard in cmake is now done by a cmake feature-requirement
cxx_std_17 on the
dunecommon library target. This requirement is propagated to all other modules by linking against
dunecommon. The cmake options
DISABLE_CXX_VERSION_CHECK are removed. The cmake function
dune_require_cxx_standard() is now deprecated.
Enforcing a c++-standard is different between compilers, the flags might have different names or are not necessary at all. Sometimes the user needs a higher c++ standard than defined for dune-common, maybe just on a single target. The "cmake-way" of setting the c++-standard requirement is either a target property or a target feature. By using these, the highest required standard is passed to the compiler and there is not mixup with flags and standard requirements.
With this MR, the
dunecommon library target gets the compile-feature
cxx_std_17 that enforces at least c++-language standard 17 to be supported by the compiler. It might be, though, that not all c++-17 library feature are already implemented by the corresponding standard library. This has to be tested separately using individual feature tests.
The change has an implication on other dune modules and targets. The
dunecommon target-properties are only propagated, if a downstream target links against it. The c++-standard is not a global property / global compiler flag anymore.
- How do downstream modules/libraries inherit the c++-standard requirement? Simply by linking against the
- How to enforce in a downstream module a different (higher) c++-standard for a specific target than is set in dune-common?
Mixing c++ standards in dependent libraries is undefined behaviour, and thus not recommended. But a call of
target_compile_features(<target> <PRIVATE|PUBLIC|INTERFACE> cxx_std_[17|20|...])on the target with name
<target>sets an arbitrary standard requirement for the code. This should be combined with a corresponding c++ standard used for all dune modules.
- How to increase the c++ standard for all dune modules? The cmake variable
CMAKE_CXX_STANDARDis used for all targets in a cmake project as default c++ standard. This can only be increased using a
cxx_std_XXXfeature requirement. Thus, one simply has to set this
CMAKE_CXX_STANDARDto the highest standard requested by the dune modules.
- Fix compiler-feature detection