{"id":9797,"date":"2024-08-05T09:02:39","date_gmt":"2024-08-05T09:02:39","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=9797"},"modified":"2024-08-05T09:02:39","modified_gmt":"2024-08-05T09:02:39","slug":"feature-testing-macros","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/feature-testing-macros\/","title":{"rendered":"Feature Testing Macros"},"content":{"rendered":"\n

The feature testing macros is a relatively unknown feature in C + + 20. They give you the definitive answer to which C++ feature your compiler supports.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

When you try out the latest C++ features, you often get errors. Now the question is: who is to blame?<\/p>\n\n\n\n

    \n
  1. Did you make a mistake?<\/li>\n\n\n\n
  2. Does your compiler support this feature?<\/li>\n\n\n\n
  3. Did you find a compiler bug?<\/li>\n<\/ol>\n\n\n\n

    Option 3 occurs rarely, so you have only one question to answer: Does your compiler support this feature?<\/p>\n\n\n\n

    This question is easy to answer thanks to the cppreference and the feature testing macros.<\/p>\n\n\n\n

    C++ Compiler Support<\/h2>\n\n\n\n

    As so often, cppreference.com\/compiler_support<\/a> answers questions about the core language and library, including the C++11 standards up to C++ 26.<\/p>\n\n\n\n

    The following tables give you an overview of the core language and library support for the C++26 standard.<\/p>\n\n\n\n

    C++26 Core Language<\/h3>\n\n\n\n
    \"\"<\/figure>\n\n\n\n

    C++26 Library<\/h3>\n\n\n\n
    \"\"<\/figure>\n\n\n\n

    Are the answers not specific enough? Let me jump to the feature testing macros in C++20.<\/p>\n\n\n\n

    Feature Testing macros<\/h2>\n\n\n\n

    <\/p>\n\n\n\n

    The header <version><\/code> 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><\/code> has more than 300 macros defined, which expand to a number when the feature is implemented. The number represents the year and month the feature was added to the C++ draft standard. These are the numbers for static_assert, lambdas, and concepts<\/code>.<\/p>\n\n\n\n__cpp_static_assert 200410L\n__cpp_lambdas 200907L\n__cpp_concepts 201907L\n\n\n\n

    The page feature testing <\/a>macros shows all macros.<\/p>\n\n\n\n

    The following program, adopted from the cppreference, displays all C++20 core language macros. The flag \/Zc:__cplusplus<\/code> enables the feature testing macro on Windows.<\/p>\n\n\n\n

    \/\/ featureTesting20.cpp<\/span>\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 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};\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    \n    if<\/span> (print.cxx20 &&<\/span> print.core_features) show("C++20 CORE"<\/span>, cxx20);\n   \n}\n<\/pre><\/div>\n\n\n\n
    The following screenshots from the Compiler Explorer show the C++20 support of GCC 8.1, 10.1, and 14.1 compilers.<\/p>\n\n\n\n
      \n
    • GCC 8.1<\/strong><\/li>\n<\/ul>\n\n\n\n
      \"\"<\/figure>\n\n\n\n
        \n
      • GCC 10.1<\/strong><\/li>\n<\/ul>\n\n\n\n
        \"\"<\/figure>\n\n\n\n
          \n
        • GCC 14.1<\/strong><\/li>\n<\/ul>\n\n\n\n
          \"\"<\/figure>\n\n\n\n
           What’s next?<\/h2>\n\n\n\n
           With my next post, I continue my journey through C + + 23.<\/p>\n","protected":false},"excerpt":{"rendered":"
          The feature testing macros is a relatively unknown feature in C + + 20. They give you the definitive answer to which C++ feature your compiler supports. When you try out the latest C++ features, you often get errors. Now the question is: who is to blame? Option 3 occurs rarely, so you have only […]<\/p>\n","protected":false},"author":21,"featured_media":9800,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[375],"tags":[],"class_list":["post-9797","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c-20"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/9797","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=9797"}],"version-history":[{"count":23,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/9797\/revisions"}],"predecessor-version":[{"id":9828,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/9797\/revisions\/9828"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/9800"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=9797"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=9797"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=9797"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}