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](../how_to_guides/integration/wide_compatibility.md#using-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+ && !19	15-16 && !17.0	195+
std::format	20	13+	17+	16+	195+
C++ modules	20	None	17+	None	None
import std;	23	None	18+	None	195+
Explicit this parameter	23	14+	18+	17+	195+

??? note "Clang-19 unfixable bug"

Unfortunately, Clang-19 does not build **mp-units** because of an
[unfixable bug in the compiler](https://github.com/llvm/llvm-project/pull/118288).

??? note "Apple Clang-17.0 unfixable bug"

Unfortunately, Apple Clang-17.0 (Xcode 16.3, 16.4, 26.0, 26.0.1, 26.1, and 26.2-beta) does not
build **mp-units** because it has the same
[unfixable bug as Clang-19](https://github.com/llvm/llvm-project/pull/118288).

!!! 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).

import std;

• If enabled, the library will obtain all the definitions from the std namespace via import std; instead of the "old-style" header includes.
• Related build options:
  • Conan: import_std
  • CMake: CMAKE_CXX_MODULE_STD

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.
  • Note that some compiler versions do not implement this macro even though they do support the feature well enough. In such cases, compilation with explicit this is enforced.
• Related build options:
  • Conan: no_crtp
  • CMake: MP_UNITS_API_NO_CRTP

*[CRTP]: Curiously Recurring Template Parameter