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< h1 id = "cpp-compiler-support" > C++ compiler support (API/ABI)< a class = "headerlink" href = "#cpp-compiler-support" title = "Permanent link" > ¶ </ a ></ h1 >
< div class = "admonition info" >
< p class = "admonition-title" > Info</ p >
< p >< strong > mp-units</ strong > library tries to provide the best user experience possible with the C++ language.
To achieve that, it extensively uses the latest C++ language features.</ p >
< p > 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 < a href = "../../users_guide/use_cases/wide_compatibility/#compatibility-macros" > preprocessor macros</ a >
providing a backward-compatible way to use them.</ p >
</ div >
< p > The table below provides the minimum compiler version required to compile the code using a specific
C++ feature:</ p >
< table >
< thead >
< tr >
< th > C++ Feature</ th >
< th style = "text-align: center;" > C++ version</ th >
< th style = "text-align: center;" > gcc</ th >
< th style = "text-align: center;" > clang</ th >
< th style = "text-align: center;" > apple-clang</ th >
< th style = "text-align: center;" > MSVC</ th >
</ tr >
</ thead >
< tbody >
< tr >
< td >< strong > Minimum support</ strong ></ td >
< td style = "text-align: center;" > 20</ td >
< td style = "text-align: center;" > 12</ td >
< td style = "text-align: center;" > 16</ td >
< td style = "text-align: center;" > 15</ td >
< td style = "text-align: center;" > 194 < img alt = "🐛" class = "twemoji" src = "https://cdn.jsdelivr.net/gh/jdecked/twemoji@15.1.0/assets/svg/1f41b.svg" title = "BEWARE of MSVC Bugs!" /></ td >
</ tr >
< tr >
< td >< strong >< code > std::format</ code ></ strong ></ td >
< td style = "text-align: center;" > 20</ td >
< td style = "text-align: center;" > 13</ td >
< td style = "text-align: center;" > 17</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > 194</ td >
</ tr >
< tr >
< td >< strong > C++ modules</ strong ></ td >
< td style = "text-align: center;" > 20</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > 17</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > None</ td >
</ tr >
< tr >
< td >< strong >< code > import std;</ code ></ strong ></ td >
< td style = "text-align: center;" > 23</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > 18</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > None</ td >
</ tr >
< tr >
< td >< strong > Static < code > constexpr</ code > variables in < code > constexpr</ code > functions</ strong ></ td >
< td style = "text-align: center;" > 23</ td >
< td style = "text-align: center;" > 13</ td >
< td style = "text-align: center;" > 17</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > None</ td >
</ tr >
< tr >
< td >< strong > Explicit < code > this</ code > parameter</ strong ></ td >
< td style = "text-align: center;" > 23</ td >
< td style = "text-align: center;" > 14</ td >
< td style = "text-align: center;" > 18</ td >
< td style = "text-align: center;" > None</ td >
< td style = "text-align: center;" > None</ td >
</ tr >
</ tbody >
</ table >
< details class = "note" >
< summary > MSVC bugs</ summary >
< p > MSVC still has a poor C++20 conformance. We had to make many workarounds to our codebase to
make it compile on this compiler. Usage of such nasty preprocessor macros degrade the
readability and maintainability of our code. This is why we've applied those patches to
the main library code but not to unit tests and examples. Those still do not compile on MSVC.</ p >
< p > Here is a list of the most important MSVC bugs:</ p >
< ul >
< li >< a href = "https://developercommunity.visualstudio.com/t/Discrepancy-in-Behavior-of-operator-an/10732445" > Discrepancy in Behavior of operator< em > = and operator</ em > for Multiplying int and double at compile time</ a ></ li >
< li >< a href = "https://developercommunity.visualstudio.com/t/Syntax-error-when-using-non-type-templat/10729428" > Syntax error when using non-type template parameters in templated class member function</ a ></ li >
< li >< a href = "https://developercommunity.visualstudio.com/t/Type-always-prefered-over-value-when-usi/10729382" > Type always preferred over value when using qualified identifiers</ a ></ li >
</ ul >
< p > Please upvote them so they get a higher fixing priority at Microsoft.</ p >
</ details >
< div class = "admonition important" >
< p class = "admonition-title" > Important</ p >
< p > Enabling/disabling features listed above may influence the API of the library and the ABI of
the customers' projects.</ p >
</ div >
< h2 id = "stdformat" >< code > std::format</ code >< a class = "headerlink" href = "#stdformat" title = "Permanent link" > ¶ </ a ></ h2 >
< ul >
< li > Provides < a href = "../../users_guide/framework_basics/text_output/#text-formatting" > powerful text formatting capabilities</ a >
for C++.</ li >
< li > An alternative < a href = "https://github.com/fmtlib/fmt" > fmtlib</ a > library can be used instead if< ul >
< li > the C++ language feature is not supported,</ li >
< li > the customer's project did not switch to < code > std::format</ code > yet (even when the compiler
supports it).</ li >
</ ul >
</ li >
< li > To write code with wide compatibility
a < a href = "../../users_guide/use_cases/wide_compatibility/#mp_units_std_fmt" > dedicated macro may be used</ a > .</ li >
< li > Tested with < code > __cpp_lib_format</ code > < a href = "https://en.cppreference.com/w/cpp/feature_test" > feature test macro</ a > .</ li >
< li > Related build options:< ul >
< li > Conan: < a href = "../installation_and_usage/#std_format" > std_format</ a ></ li >
< li > CMake: < a href = "../installation_and_usage/#MP_UNITS_API_STD_FORMAT" > MP_UNITS_API_STD_FORMAT</ a ></ li >
</ ul >
</ li >
</ ul >
< h2 id = "c-modules" > C++ modules< a class = "headerlink" href = "#c-modules" title = "Permanent link" > ¶ </ a ></ h2 >
< ul >
< li > Provide new way to share declarations and definitions across translation units.</ li >
< li > If used, the library will distribute both "old-style" headers and module interface units< ul >
< li > associated with the same CMake targets.</ li >
</ ul >
</ li >
< li > 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.</ li >
< li > 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.</ li >
< li > Related build options:< ul >
< li > Conan: < a href = "../installation_and_usage/#cxx_modules" > cxx_modules</ a ></ li >
< li > CMake: < a href = "../installation_and_usage/#MP_UNITS_BUILD_CXX_MODULES" > MP_UNITS_BUILD_CXX_MODULES</ a ></ li >
</ ul >
</ li >
</ ul >
< div class = "admonition note" >
< p class = "admonition-title" > Note</ p >
< p > More requirements for C++ modules support can be found in the
< a href = "https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html" > CMake's documentation</ a > .</ p >
</ div >
< h2 id = "import-std" >< code > import std;</ code >< a class = "headerlink" href = "#import-std" title = "Permanent link" > ¶ </ a ></ h2 >
< ul >
< li > If enabled, the library will obtain all the definitions from the < code > std</ code > namespace via
< code > import std;</ code > instead of the "old-style" header includes.</ li >
< li > Related build options:< ul >
< li > Conan: < a href = "../installation_and_usage/#import_std" > import_std</ a ></ li >
< li > CMake: < a href = "https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_MODULE_STD.html" > CMAKE_CXX_MODULE_STD</ a ></ li >
</ ul >
</ li >
</ ul >
< h2 id = "static-constexpr-variables-in-constexpr-functions" > Static < code > constexpr</ code > variables in < code > constexpr</ code > functions< a class = "headerlink" href = "#static-constexpr-variables-in-constexpr-functions" title = "Permanent link" > ¶ </ a ></ h2 >
< ul >
< li > Allows returning < code > std::string_view</ code > from the
< a href = "../../users_guide/framework_basics/text_output/#unit_symbol" >< code > unit_symbol()</ code ></ a >
and < a href = "../../users_guide/framework_basics/text_output/#dimension_symbol" >< code > dimension_symbol()</ code ></ a >
functions< ul >
< li >< code > std::string_view</ code > type has a reference semantics so it has to point to a storage with
a longer lifetime.</ li >
</ ul >
</ li >
< li > If this feature is not available, the API returns < code > mp_units::basic_fixed_string< CharT, N> </ code > instead.</ li >
< li > Tested as < code > __cpp_constexpr > = 202211L</ code > < a href = "https://en.cppreference.com/w/cpp/feature_test" > feature test macro</ a > .</ li >
< li > Related build options:< ul >
< li > Conan: < a href = "../installation_and_usage/#string_view_ret" > string_view_ret</ a ></ li >
< li > CMake: < a href = "../installation_and_usage/#MP_UNITS_API_STRING_VIEW_RET" > MP_UNITS_API_STRING_VIEW_RET</ a ></ li >
</ ul >
</ li >
</ ul >
< h2 id = "explicit-this-parameter" > Explicit < code > this</ code > parameter< a class = "headerlink" href = "#explicit-this-parameter" title = "Permanent link" > ¶ </ a ></ h2 >
< ul >
< li > This feature removes the need for the usage of the < abbr title = "Curiously Recurring Template Parameter" > CRTP</ abbr > idiom in the
< a href = "../../users_guide/framework_basics/systems_of_quantities/#defining-quantities" >< code > quantity_spec</ code > definitions</ a > .</ li >
< li > To write code with wide compatibility
a < a href = "../../users_guide/use_cases/wide_compatibility/#QUANTITY_SPEC" > dedicated macro may be used</ a > .</ li >
< li > Tested with < code > __cpp_explicit_this_parameter</ code > < a href = "https://en.cppreference.com/w/cpp/feature_test" > feature test macro</ a > .</ li >
< li > Related build options:< ul >
< li > Conan: < a href = "../installation_and_usage/#no_crtp" > no_crtp</ a ></ li >
< li > CMake: < a href = "../installation_and_usage/#MP_UNITS_API_NO_CRTP" > MP_UNITS_API_NO_CRTP</ a ></ li >
</ ul >
</ li >
</ ul >
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