mirror of
https://github.com/mpusz/mp-units.git
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Mateusz Pusz
96965634f5
mp-units' minimum-supported MSVC 195 (VS 2026) has solid `import std;`
support, but enabling `-o import_std=True` against the build hit two
issues that this change fixes.
(1) Module scanning was only enabled when `cxx_modules=True`.
Without `CMAKE_CXX_SCAN_FOR_MODULES=ON`, CMake doesn't link the implicit
std module BMI into non-module translation units that do `import std;`,
producing C2230 "could not find module 'std'". Enable scanning whenever
`import_std=True` as well. Same bug fixed in `test_package/conanfile.py`.
(2) The std module BMI is built with mismatched compile flags.
CMake materializes the implicit std module BMI as `__cmake_cxx_std_NN`,
a separate internal target that does not link mp-units and therefore
does not inherit mp-units' target-level options. On MSVC this caused
C5050 (`_UTF8` defined on consumer but not on module command line) and
on Clang 21 there is a parallel `-Wreserved-module-identifier` issue
already partially handled in `src/CMakeLists.txt` but in a directory
scope that did not actually reach the BMI either.
Consolidate the workarounds into one `src/cmake/import-std-setup.cmake`
snippet that applies the flags project-wide via `add_compile_options`,
self-guarded by `if(NOT CMAKE_CXX_MODULE_STD) return()`. It is included
from every entry point so all delivery paths converge:
- top-level `CMakeLists.txt` for the dev build
- bundled `mp-unitsConfig.cmake` for `find_package` consumers
- via `cmake_build_modules` in `package_info()` for Conan-generated
configs (the `cxx_modules=False` consumer path)
Other pieces:
- `test/static/unit_magnitude_test.cpp` was the only test in the repo
missing the `MP_UNITS_IMPORT_STD` gate around `#include <type_traits>`,
causing duplicate `std::integral_constant` definitions in this
configuration.
- Compiler support table announces MSVC 195+ for `import std;`.
- Installation docs gain a consumer-side setup note covering the
CMake cache variables required, what mp-units handles automatically
via the snippet, and the `add_subdirectory`-vendored caveat.
Verified: `conan build . -pr msvc195 -o '&:cxx_modules=False'
-o '&:import_std=True' -s compiler.cppstd=23 -c
user.mp-units.build:all=True` -> 40/40 tests pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
102 lines
4.9 KiB
Markdown
102 lines
4.9 KiB
Markdown
# C++ compiler support (API/ABI) { #cpp-compiler-support }
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!!! info
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**mp-units** library tries to provide the best user experience possible with the C++ language.
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To achieve that, it extensively uses the latest C++ language features.
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Even though the library benefits from the latest C++ versions (if available), C++20 is enough
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to compile and use all of the library's functionality. Newer features can be hidden behind
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some [preprocessor macros](../how_to_guides/integration/wide_compatibility.md#using-compatibility-macros)
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providing a backward-compatible way to use them.
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The table below provides the minimum compiler version required to compile the code using
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a specific C++ feature:
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| C++ Feature | C++ version | gcc | clang | apple-clang | MSVC |
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|-------------------------------|:-----------:|:----:|:----------:|:--------------:|:----:|
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| **Minimum support** | 20 | 12+ | 16+ && !19 | 15-16 && !17.0 | 195+ |
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| **`std::format`** | 20 | 13+ | 17+ | 16+ | 195+ |
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| **C++ modules** | 20 | None | 17+ | None | None |
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| **`import std;`** | 23 | None | 18+ | None | 195+ |
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| **Explicit `this` parameter** | 23 | 14+ | 18+ | 17+ | 195+ |
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??? note "Clang-19 unfixable bug"
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Unfortunately, Clang-19 does not build **mp-units** because of an
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[unfixable bug in the compiler](https://github.com/llvm/llvm-project/pull/118288).
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??? note "Apple Clang-17.0 unfixable bug"
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Unfortunately, Apple Clang-17.0 (Xcode 16.3, 16.4, 26.0, 26.0.1, 26.1, and 26.2-beta) does not
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build **mp-units** because it has the same
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[unfixable bug as Clang-19](https://github.com/llvm/llvm-project/pull/118288).
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!!! important
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Enabling/disabling features listed above may influence the API of the library and the ABI of
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the customers' projects.
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## `std::format`
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- Provides
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[powerful text formatting capabilities](../users_guide/framework_basics/text_output.md#text-formatting)
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for C++.
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- An alternative [fmtlib](https://github.com/fmtlib/fmt) library can be used instead if
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- the C++ language feature is not supported,
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- the customer's project did not switch to `std::format` yet (even when the compiler
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supports it).
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- To write code with wide compatibility a
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[dedicated macro may be used](../how_to_guides/integration/wide_compatibility.md#mp_units_std_fmt).
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- Tested with `__cpp_lib_format` [feature test macro](https://en.cppreference.com/w/cpp/feature_test).
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- Related build options:
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- Conan: [std_format](installation_and_usage.md#std_format)
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- CMake: [MP_UNITS_API_STD_FORMAT](installation_and_usage.md#MP_UNITS_API_STD_FORMAT)
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## C++ modules
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- Provide new way to share declarations and definitions across translation units.
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- If used, the library will distribute both "old-style" headers and module interface units
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- associated with the same CMake targets.
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- Even with full compiler support, a user may still decide to not pay for C++ modules
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compilation if they are not needed by the customer's project.
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- Feature test macro is not used for testing here because even if the compiler does not
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support the entire C++ feature (e.g. header units), it is enough to build modules for this
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library.
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- Related build options:
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- Conan: [cxx_modules](installation_and_usage.md#cxx_modules)
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- CMake: [MP_UNITS_BUILD_CXX_MODULES](installation_and_usage.md#MP_UNITS_BUILD_CXX_MODULES)
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!!! note
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More requirements for C++ modules support can be found in the
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[CMake's documentation](https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html).
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## `import std;`
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- If enabled, the library will obtain all the definitions from the `std` namespace via
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`import std;` instead of the "old-style" header includes.
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- Related build options:
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- Conan: [import_std](installation_and_usage.md#import_std)
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- CMake: [CMAKE_CXX_MODULE_STD](https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_MODULE_STD.html)
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## Explicit `this` parameter
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- This feature removes the need for the usage of the CRTP idiom in the
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[`quantity_spec` definitions](../users_guide/framework_basics/systems_of_quantities.md#defining-quantities).
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- To write code with wide compatibility
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a [dedicated macro may be used](../how_to_guides/integration/wide_compatibility.md#QUANTITY_SPEC).
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- Tested with `__cpp_explicit_this_parameter` [feature test macro](https://en.cppreference.com/w/cpp/feature_test).
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- Note that some compiler versions do not implement this macro even though they do
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support the
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feature well enough. In such cases, compilation with explicit `this` is enforced.
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- Related build options:
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- Conan: [no_crtp](installation_and_usage.md#no_crtp)
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- CMake: [MP_UNITS_API_NO_CRTP](installation_and_usage.md#MP_UNITS_API_NO_CRTP)
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*[CRTP]: Curiously Recurring Template Parameter
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