{"id":6036,"date":"2020-11-27T07:09:22","date_gmt":"2020-11-27T07:09:22","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/feature-testing-with-c-20\/"},"modified":"2023-10-20T10:52:32","modified_gmt":"2023-10-20T10:52:32","slug":"feature-testing-with-c-20","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/feature-testing-with-c-20\/","title":{"rendered":"Feature Testing with C++20"},"content":{"rendered":"

When your program’s compilation broke with a brand-new C++20 feature, you often end with a few questions: Did I do something wrong? Did I find a compiler bug? Does my compiler not yet support this feature? Thanks to the feature testing in C++20, the last question is easy to answer.<\/p>\n

<\/p>\n

\"TimelineCpp20CoreLanguage\"<\/p>\n

When I experiment with brand-new C++ features, I check which compiler implements the feature I’m interested in. This is when I visit cppreference.com<\/a>, search for the feature I want to try out, and hope that at least one compiler of the big three (GCC, Clang, MSCV) implements the new feature.<\/p>\n

\"cpp20Support\"<\/p>\n

When I get the answer partially, it is not satisfying. Ultimately, I don’t know who was guilty when the compilation of the brand-new feature failed. There is a more thoughtful way in C++20 to detect if your compiler supports a requested feature.<\/p>\n

Feature Testing<\/h2>\n

The header <version><\/span> allows you to ask your compiler for its C++11 or later support. You can ask for attributes, features of the core language, or the library. <version><\/span> has about 200 macros defined, which expand to a number when the feature is implemented. The number stands for the year and month the feature was added to the C++ standard. These are the numbers for static_assert,<\/span> lambdas, and concepts.<\/p>\n

<\/p>\n

\n
__cpp_static_assert  200410L<\/span>\n__cpp_lambdas  200907L<\/span>\n__cpp_concepts 201907L<\/span>\n<\/pre>\n<\/div>\n
The cppreference.com page to feature testing<\/a> put all macros together in a long, long source file.<\/p>\n
<\/p>\n
\n
\/\/ featureTest.cpp\n\/\/ from cppreference.com<\/span>\n\n#if __cplusplus < 201100<\/span>\n#  error \"C++11 or better is required\"<\/span>\n#endif<\/span>\n \n#include <algorithm><\/span>\n#include <cstring><\/span>\n#include <iomanip><\/span>\n#include <iostream><\/span>\n#include <string><\/span>\n \n#ifdef __has_include<\/span>\n# if __has_include(<version>)<\/span>\n#   include <version><\/span>\n# endif<\/span>\n#endif<\/span>\n \n#define COMPILER_FEATURE_VALUE(value) #value<\/span>\n#define COMPILER_FEATURE_ENTRY(name) { #name, COMPILER_FEATURE_VALUE(name) },<\/span>\n \n#ifdef __has_cpp_attribute<\/span>\n# define COMPILER_ATTRIBUTE_VALUE_AS_STRING(s) #s<\/span>\n# define COMPILER_ATTRIBUTE_AS_NUMBER(x) COMPILER_ATTRIBUTE_VALUE_AS_STRING(x)<\/span>\n# define COMPILER_ATTRIBUTE_ENTRY(attr) \\<\/span>\n  { #attr, COMPILER_ATTRIBUTE_AS_NUMBER(__has_cpp_attribute(attr)) },<\/span>\n#else<\/span>\n# define COMPILER_ATTRIBUTE_ENTRY(attr) { #attr, \"_\" },<\/span>\n#endif<\/span>\n \n\/\/ Change these options to print out only necessary info.<\/span>\nstatic<\/span> struct<\/span> PrintOptions {\n    constexpr static<\/span> bool<\/span> titles               =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> attributes           =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> general_features     =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> core_features        =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> lib_features         =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> supported_features   =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> unsupported_features =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> sorted_by_value      =<\/span> 0<\/span>;\n    constexpr static<\/span> bool<\/span> cxx11                =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> cxx14                =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> cxx17                =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> cxx20                =<\/span> 1<\/span>;\n    constexpr static<\/span> bool<\/span> cxx23                =<\/span> 0<\/span>;\n}   print;\n \nstruct<\/span> CompilerFeature {\n    CompilerFeature(const<\/span> char<\/span>*<\/span> name =<\/span> nullptr, const<\/span> char<\/span>*<\/span> value =<\/span> nullptr)\n        :<\/span> name(name), value(value) {}\n    const<\/span> char<\/span>*<\/span> name; const<\/span> char<\/span>*<\/span> value;\n};\n \nstatic<\/span> CompilerFeature cxx[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cplusplus)\nCOMPILER_FEATURE_ENTRY(__cpp_exceptions)\nCOMPILER_FEATURE_ENTRY(__cpp_rtti)\n#if 0<\/span>\nCOMPILER_FEATURE_ENTRY(__GNUC__)<\/span>\nCOMPILER_FEATURE_ENTRY(__GNUC_MINOR__)<\/span>\nCOMPILER_FEATURE_ENTRY(__GNUC_PATCHLEVEL__)<\/span>\nCOMPILER_FEATURE_ENTRY(__GNUG__)<\/span>\nCOMPILER_FEATURE_ENTRY(__clang__)<\/span>\nCOMPILER_FEATURE_ENTRY(__clang_major__)<\/span>\nCOMPILER_FEATURE_ENTRY(__clang_minor__)<\/span>\nCOMPILER_FEATURE_ENTRY(__clang_patchlevel__)<\/span>\n#endif<\/span>\n};\nstatic<\/span> CompilerFeature cxx11[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_alias_templates)\nCOMPILER_FEATURE_ENTRY(__cpp_attributes)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_decltype)\nCOMPILER_FEATURE_ENTRY(__cpp_delegating_constructors)\nCOMPILER_FEATURE_ENTRY(__cpp_inheriting_constructors)\nCOMPILER_FEATURE_ENTRY(__cpp_initializer_lists)\nCOMPILER_FEATURE_ENTRY(__cpp_lambdas)\nCOMPILER_FEATURE_ENTRY(__cpp_nsdmi)\nCOMPILER_FEATURE_ENTRY(__cpp_range_based_for)\nCOMPILER_FEATURE_ENTRY(__cpp_raw_strings)\nCOMPILER_FEATURE_ENTRY(__cpp_ref_qualifiers)\nCOMPILER_FEATURE_ENTRY(__cpp_rvalue_references)\nCOMPILER_FEATURE_ENTRY(__cpp_static_assert)\nCOMPILER_FEATURE_ENTRY(__cpp_threadsafe_static_init)\nCOMPILER_FEATURE_ENTRY(__cpp_unicode_characters)\nCOMPILER_FEATURE_ENTRY(__cpp_unicode_literals)\nCOMPILER_FEATURE_ENTRY(__cpp_user_defined_literals)\nCOMPILER_FEATURE_ENTRY(__cpp_variadic_templates)\n};\nstatic<\/span> CompilerFeature cxx14[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_aggregate_nsdmi)\nCOMPILER_FEATURE_ENTRY(__cpp_binary_literals)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_decltype_auto)\nCOMPILER_FEATURE_ENTRY(__cpp_generic_lambdas)\nCOMPILER_FEATURE_ENTRY(__cpp_init_captures)\nCOMPILER_FEATURE_ENTRY(__cpp_return_type_deduction)\nCOMPILER_FEATURE_ENTRY(__cpp_sized_deallocation)\nCOMPILER_FEATURE_ENTRY(__cpp_variable_templates)\n};\nstatic<\/span> CompilerFeature cxx14lib[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_lib_chrono_udls)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_complex_udls)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_exchange_function)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_generic_associative_lookup)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_integer_sequence)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_integral_constant_callable)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_final)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_null_pointer)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_make_reverse_iterator)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_make_unique)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_null_iterators)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_quoted_string_io)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_result_of_sfinae)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_robust_nonmodifying_seq_ops)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shared_timed_mutex)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_string_udls)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_transformation_trait_aliases)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_transparent_operators)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_tuple_element_t<\/span>)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_tuples_by_type)\n};\n \nstatic<\/span> CompilerFeature cxx17[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_aggregate_bases)\nCOMPILER_FEATURE_ENTRY(__cpp_aligned_new)\nCOMPILER_FEATURE_ENTRY(__cpp_capture_star_this)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_deduction_guides)\nCOMPILER_FEATURE_ENTRY(__cpp_enumerator_attributes)\nCOMPILER_FEATURE_ENTRY(__cpp_fold_expressions)\nCOMPILER_FEATURE_ENTRY(__cpp_guaranteed_copy_elision)\nCOMPILER_FEATURE_ENTRY(__cpp_hex_float)\nCOMPILER_FEATURE_ENTRY(__cpp_if_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_inheriting_constructors)\nCOMPILER_FEATURE_ENTRY(__cpp_inline_variables)\nCOMPILER_FEATURE_ENTRY(__cpp_namespace_attributes)\nCOMPILER_FEATURE_ENTRY(__cpp_noexcept_function_type)\nCOMPILER_FEATURE_ENTRY(__cpp_nontype_template_args)\nCOMPILER_FEATURE_ENTRY(__cpp_nontype_template_parameter_auto)\nCOMPILER_FEATURE_ENTRY(__cpp_range_based_for)\nCOMPILER_FEATURE_ENTRY(__cpp_static_assert)\nCOMPILER_FEATURE_ENTRY(__cpp_structured_bindings)\nCOMPILER_FEATURE_ENTRY(__cpp_template_template_args)\nCOMPILER_FEATURE_ENTRY(__cpp_variadic_using)\n};\nstatic<\/span> CompilerFeature cxx17lib[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_lib_addressof_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_allocator_traits_is_always_equal)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_any)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_apply)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_array_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_as_const)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_is_always_lock_free)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_bool_constant)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_boyer_moore_searcher)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_byte)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_chrono)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_clamp)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_enable_shared_from_this)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_execution)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_filesystem)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_gcd_lcm)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_hardware_interference_size)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_has_unique_object_representations)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_hypot)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_incomplete_container_elements)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_invoke)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_aggregate)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_invocable)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_swappable)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_launder)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_logical_traits)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_make_from_tuple)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_map_try_emplace)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_math_special_functions)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_memory_resource)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_node_extract)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_nonmember_container_access)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_not_fn)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_optional)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_parallel_algorithm)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_raw_memory_algorithms)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_sample)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_scoped_lock)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shared_mutex)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shared_ptr_arrays)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shared_ptr_weak_type)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_string_view)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_to_chars)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_transparent_operators)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_type_trait_variable_templates)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_uncaught_exceptions)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_unordered_map_try_emplace)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_variant)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_void_t<\/span>)\n};\n \nstatic<\/span> CompilerFeature cxx20[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_aggregate_paren_init)\nCOMPILER_FEATURE_ENTRY(__cpp_char8_t<\/span>)\nCOMPILER_FEATURE_ENTRY(__cpp_concepts)\nCOMPILER_FEATURE_ENTRY(__cpp_conditional_explicit)\nCOMPILER_FEATURE_ENTRY(__cpp_consteval)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr_dynamic_alloc)\nCOMPILER_FEATURE_ENTRY(__cpp_constexpr_in_decltype)\nCOMPILER_FEATURE_ENTRY(__cpp_constinit)\nCOMPILER_FEATURE_ENTRY(__cpp_deduction_guides)\nCOMPILER_FEATURE_ENTRY(__cpp_designated_initializers)\nCOMPILER_FEATURE_ENTRY(__cpp_generic_lambdas)\nCOMPILER_FEATURE_ENTRY(__cpp_impl_coroutine)\nCOMPILER_FEATURE_ENTRY(__cpp_impl_destroying_delete)\nCOMPILER_FEATURE_ENTRY(__cpp_impl_three_way_comparison)\nCOMPILER_FEATURE_ENTRY(__cpp_init_captures)\nCOMPILER_FEATURE_ENTRY(__cpp_modules)\nCOMPILER_FEATURE_ENTRY(__cpp_nontype_template_args)\nCOMPILER_FEATURE_ENTRY(__cpp_using_enum)\n};\nstatic<\/span> CompilerFeature cxx20lib[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_lib_array_constexpr)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_assume_aligned)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_flag_test)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_float)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_lock_free_type_aliases)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_ref)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_shared_ptr)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_value_initialization)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_atomic_wait)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_barrier)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_bind_front)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_bit_cast)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_bitops)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_bounded_array_traits)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_char8_t<\/span>)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_chrono)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_concepts)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_algorithms)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_complex)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_dynamic_alloc)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_functional)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_iterator)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_memory)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_numeric)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_string)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_string_view)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_tuple)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_utility)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_constexpr_vector)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_coroutine)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_destroying_delete)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_endian)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_erase_if)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_execution)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_format)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_generic_unordered_lookup)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_int_pow2)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_integer_comparison_functions)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_interpolate)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_constant_evaluated)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_layout_compatible)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_nothrow_convertible)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_is_pointer_interconvertible)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_jthread)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_latch)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_list_remove_return_type)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_math_constants)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_polymorphic_allocator)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_ranges)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_remove_cvref)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_semaphore)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shared_ptr_arrays)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_shift)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_smart_ptr_for_overwrite)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_source_location)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_span)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_ssize)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_starts_ends_with)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_string_view)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_syncbuf)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_three_way_comparison)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_to_address)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_to_array)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_type_identity)\nCOMPILER_FEATURE_ENTRY(__cpp_lib_unwrap_ref)\n};\n \nstatic<\/span> CompilerFeature cxx23[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_cxx23_stub) \/\/< Populate eventually<\/span>\n};\nstatic<\/span> CompilerFeature cxx23lib[] =<\/span> {\nCOMPILER_FEATURE_ENTRY(__cpp_lib_cxx23_stub) \/\/< Populate eventually<\/span>\n};\n \nstatic<\/span> CompilerFeature attributes[] =<\/span> {\nCOMPILER_ATTRIBUTE_ENTRY(carries_dependency)\nCOMPILER_ATTRIBUTE_ENTRY(deprecated)\nCOMPILER_ATTRIBUTE_ENTRY(fallthrough)\nCOMPILER_ATTRIBUTE_ENTRY(likely)\nCOMPILER_ATTRIBUTE_ENTRY(maybe_unused)\nCOMPILER_ATTRIBUTE_ENTRY(nodiscard)\nCOMPILER_ATTRIBUTE_ENTRY(noreturn)\nCOMPILER_ATTRIBUTE_ENTRY(no_unique_address)\nCOMPILER_ATTRIBUTE_ENTRY(unlikely)\n};\n \nconstexpr bool<\/span> is_feature_supported<\/span>(const<\/span> CompilerFeature&<\/span> x) {\n    return<\/span> x.value[0<\/span>] !=<\/span> '_'<\/span> &&<\/span> x.value[0<\/span>] !=<\/span> '0'<\/span> ;\n}\n \ninline<\/span> void<\/span> print_compiler_feature<\/span>(const<\/span> CompilerFeature&<\/span> x) {\n    constexpr static<\/span> int<\/span> max_name_length =<\/span> 44<\/span>; \/\/< Update if necessary<\/span>\n    std::<\/span>string value{ is_feature_supported(x) ?<\/span> x.value :<\/span> \"------\"<\/span> };\n    if<\/span> (value.back() ==<\/span> 'L'<\/span>) value.pop_back(); \/\/~ 201603L -> 201603<\/span>\n    \/\/ value.insert(4, 1, '-'); \/\/~ 201603 -> 2016-03<\/span>\n    if<\/span> ( (print.supported_features &&<\/span> is_feature_supported(x))\n        ||<\/span> (print.unsupported_features &&<\/span> !<\/span>is_feature_supported(x))) {\n            std::<\/span>cout <<<\/span> std::<\/span>left <<<\/span> std::<\/span>setw(max_name_length)\n                      <<<\/span> x.name <<<\/span> \" \"<\/span> <<<\/span> value <<<\/span> '\\n'<\/span>;\n    }\n}\n \ntemplate<\/span><<\/span>size_t<\/span> N><\/span>\ninline<\/span> void<\/span> show(char<\/span> const<\/span>*<\/span> title, CompilerFeature (&<\/span>features)[N]) {\n    if<\/span> (print.titles) {\n        std::<\/span>cout <<<\/span> '\\n'<\/span> <<<\/span> std::<\/span>left <<<\/span> title <<<\/span> '\\n'<\/span>;\n    }\n    if<\/span> (print.sorted_by_value) {\n        std::<\/span>sort(std::<\/span>begin(features), std::<\/span>end(features),\n            [](CompilerFeature const<\/span>&<\/span> lhs, CompilerFeature const<\/span>&<\/span> rhs) {\n                return<\/span> std::<\/span>strcmp(lhs.value, rhs.value) <<\/span> 0<\/span>;\n            });\n    }\n    for<\/span> (const<\/span> CompilerFeature&<\/span> x :<\/span> features) {\n        print_compiler_feature(x);\n    }\n}\n \nint<\/span> main() {\n    if<\/span> (print.general_features) show(\"C++ GENERAL\"<\/span>, cxx);\n    if<\/span> (print.cxx11 &&<\/span> print.core_features) show(\"C++11 CORE\"<\/span>, cxx11);\n    if<\/span> (print.cxx14 &&<\/span> print.core_features) show(\"C++14 CORE\"<\/span>, cxx14);\n    if<\/span> (print.cxx14 &&<\/span> print.lib_features ) show(\"C++14 LIB\"<\/span> , cxx14lib);\n    if<\/span> (print.cxx17 &&<\/span> print.core_features) show(\"C++17 CORE\"<\/span>, cxx17);\n    if<\/span> (print.cxx17 &&<\/span> print.lib_features ) show(\"C++17 LIB\"<\/span> , cxx17lib);\n    if<\/span> (print.cxx20 &&<\/span> print.core_features) show(\"C++20 CORE\"<\/span>, cxx20);\n    if<\/span> (print.cxx20 &&<\/span> print.lib_features ) show(\"C++20 LIB\"<\/span> , cxx20lib);\n    if<\/span> (print.cxx23 &&<\/span> print.core_features) show(\"C++23 CORE\"<\/span>, cxx23);\n    if<\/span> (print.cxx23 &&<\/span> print.lib_features ) show(\"C++23 LIB\"<\/span> , cxx23lib);\n    if<\/span> (print.attributes) show(\"ATTRIBUTES\"<\/span>, attributes);\n}\n<\/pre>\n<\/div>\n
Of course, the length of the source file is overwhelming. When you want to know more about each macro, visit the page to feature testing<\/a>. In particular, they provide each macro with a link that can use to get more information about a feature. For example, here is the table on attributes:<\/p>\n
\"attributes\"<\/p>\n
To finish my presentation of the <version><\/code> header and its macros, I execute the program on the brand-new GCC, Clang, and MSVC compiler. I used Compiler Explorer<\/a> for the GCC and Clang compilers. On Windows, I had to enable preprocessor macros with the flag \/Zc:__cplusplus<\/code>. Additionally, I compiled on all three platforms with C++20 support.<\/p>\n
For obvious reasons, I only display the support of the C++20 core language.<\/p>\n