mp-units/docs/getting_started/cpp_compiler_support.md

C++ compiler support (API/ABI)

!!! info

**mp-units** library tries to provide the best user experience possible with the C++ language.
To achieve that, it extensively uses the latest C++ language features.

Even though the library benefits from the latest C++ versions (if available), C++20 is enough
to compile and use all of the library's functionality. Newer features can be hidden behind
some [preprocessor macros](../users_guide/use_cases/wide_compatibility.md#compatibility-macros)
providing a backward-compatible way to use them.

The table below provides the minimum compiler version required to compile the code using a specific C++ feature:

C++ Feature	C++ version	gcc	clang	apple-clang	MSVC
Minimum support	20	12	16	15	None
std::format	20	13	17	None	None
C++ modules	20	14	17	None	None
Static constexpr variables in constexpr functions	23	13	17	None	None
Explicit this parameter	23	14	18	None	None

!!! important

Enabling/disabling features listed above may influence the API of the library and the ABI of
the customers' projects.

std::format

• Provides powerful text formatting capabilities for C++.
• An alternative fmtlib library can be used instead if
  • the C++ language feature is not supported,
  • the customer's project did not switch to std::format yet (even when the compiler supports it).
• To write code with wide compatibility a dedicated macro may be used.
• Tested with __cpp_lib_format feature test macro.
• Related build options:
  • Conan: std_format
  • CMake: MP_UNITS_API_STD_FORMAT

C++ modules

• Provide new way to share declarations and definitions across translation units.
• If used, the library will distribute both "old-style" headers and module interface units
  • associated with the same CMake targets.
• Even with full compiler support, a user may still decide to not pay for C++ modules compilation if they are not needed by the customer's project.
• Feature test macro is not used for testing here because even if the compiler does not support the entire C++ feature (e.g. header units), it is enough to build modules for this library.
• Related build options:
  • Conan: cxx_modules
  • CMake: MP_UNITS_BUILD_CXX_MODULES

!!! note

More requirements for C++ modules support can be found in the
[CMake's documentation](https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html).

Static constexpr variables in constexpr functions

• Allows returning std::string_view from the unit_symbol() and dimension_symbol() functions
  • std::string_view type has a reference semantics so it has to point to a storage with a longer lifetime.
• If this feature is not available, the API returns mp_units::basic_fixed_string<CharT, N> instead.
• Tested as __cpp_constexpr >= 202211L feature test macro.
• Related build options:
  • Conan: string_view_ret
  • CMake: MP_UNITS_API_STRING_VIEW_RET

Explicit this parameter

• This feature removes the need for the usage of the CRTP idiom in the quantity_spec definitions.
• To write code with wide compatibility a dedicated macro may be used.
• Tested with __cpp_explicit_this_parameter feature test macro.
• Related build options:
  • Conan: no_crtp
  • CMake: MP_UNITS_API_NO_CRTP

*[CRTP]: Curiously Recurring Template Parameter