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16ce422cdf
Both compilers do not implement the feature test macro, but they do implement `explicit_this`, as mentioned in their corresponding release notes: - https://developer.apple.com/documentation/xcode-release-notes/xcode-16_3-release-notes - https://learn.microsoft.com/en-us/visualstudio/releases/2022/release-notes-v17.2 Resolves #722.
107 lines
5.7 KiB
Markdown
107 lines
5.7 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](../users_guide/use_cases/wide_compatibility.md#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 a specific
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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+ | 194+ :bug:{ title="BEWARE of MSVC Bugs!" } |
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| **`std::format`** | 20 | 13+ | 17+ | 16+ | 194+ |
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| **C++ modules** | 20 | None | 17+ | None | None |
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| **`import std;`** | 23 | None | 18+ | None | None |
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| **Explicit `this` parameter** | 23 | 14+ | 18+ | 17+ | 194+ |
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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 "MSVC bugs"
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MSVC still has a poor C++20 conformance. We had to make many workarounds to our codebase to
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make it compile on this compiler. Usage of such nasty preprocessor macros degrade the
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readability and maintainability of our code. This is why we've applied those patches to
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the main library code but not to unit tests and examples. Those still do not compile on MSVC.
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Here is a list of the most important MSVC bugs:
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- [Syntax error when using non-type template parameters in templated class member function](https://developercommunity.visualstudio.com/t/Syntax-error-when-using-non-type-templat/10729428)
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- [Type always preferred over value when using qualified identifiers](https://developercommunity.visualstudio.com/t/Type-always-prefered-over-value-when-usi/10729382)
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Please upvote them so they get a higher fixing priority at Microsoft.
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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 [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
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a [dedicated macro may be used](../users_guide/use_cases/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 compilation
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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 support
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the entire C++ feature (e.g. header units), it is enough to build modules for this 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](../users_guide/use_cases/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 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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