functional.hpp<\/a> file.<\/p>\n\r\n\/\/\/ Primary template is an empty struct.\r\n\/\/\/ \\note this is in C++20 ranges now except for the arithmetic types overload \r\n\/\/\/ https:\/\/en.cppreference.com\/w\/cpp\/experimental\/ranges\/iterator\/value_type\r\ntemplate<class I>\r\nstruct value_type { };\r\n\r\n\/\/\/ Specialization for pointers.\r\n\/\/\/ If T is an object type, provides a member type type equal to std::remove_cv_t<T>.\r\n\/\/\/ Otherwise, there is no member type.\r\ntemplate<class T>\r\nstruct value_type<T*> {\r\n using type = std::remove_cv_t<T>;\r\n};\r\n\r\n\/\/\/ Specialization for array types.\r\ntemplate<class I>\r\n requires(std::is_array<I>::value)\r\nstruct value_type<I> : value_type<std::decay_t<I>> { };\r\n\r\n\/\/\/ Specialization for const-qualified types.\r\ntemplate<class T>\r\nstruct value_type<const T> : value_type<std::decay_t<T>> { };\r\n\r\n\/\/\/ Specialization for types that define a public and accessible member type value_type.\r\n\/\/\/ If T::value_type is an object type, provides a member type type equal to T::value_type.\r\n\/\/\/ Otherwise, there is no member type.\r\n\/\/\/ \\todo requires requires\r\ntemplate<class T>\r\n requires requires{ typename T::value_type; }\r\nstruct value_type<T> {\r\n using type = typename T::value_type;\r\n};\r\n\r\n\/\/\/ Specialization for types that define a public and accessible member type element_type.\r\n\/\/\/ (e.g., std::shared_ptr).\r\n\/\/\/ If T::element_type is an object type, provides a member type type equal to std::remove_cv_t<typename T::element_type>.\r\n\/\/\/ Otherwise, there is no member type.\r\ntemplate<class T>\r\n requires requires{ typename T::element_type; }\r\nstruct value_type<T> {\r\n using type = typename T::element_type;\r\n};\r\n\r\n\/\/\/ Helper alias template\r\ntemplate<class T>\r\nusing value_type_t = typename value_type<T>::type;\r\n\r\n\/\/\/ ACPL specialization for arithmetic types\r\ntemplate<class T>\r\n requires(std::is_arithmetic_v<T>)\r\nstruct value_type<T> {\r\n using type = T;\r\n};\r\n\r\n \/\/ Specialization for pointers.\r\n static_assert(std::is_same_v<int, acpl::value_type_t<int*>>);\r\n \/\/ Specialization for array types.\r\n static_assert(std::is_same_v<int, acpl::value_type_t<int[]>>);\r\n \/\/ Specialization for const-qualified types.\r\n static_assert(std::is_same_v<int, acpl::value_type_t<const int*>>);\r\n \/\/ Specialization for types that define a public and accessible member type value_type.\r\n static_assert(std::is_same_v<int, acpl::value_type_t<std::array<int,1>>>);\r\n \/\/ Specialization for types that define a public and accessible member type element_type.\r\n static_assert(std::is_same_v<int, acpl::value_type_t<std::unique_ptr<int>>>);\r\n\r\n \/\/ ACPL specialization for arithmetic types\r\n static_assert(std::is_same_v<int, acpl::value_type_t<int>>);\r\n static_assert(std::is_same_v<int, acpl::value_type_t<const int>>);\r\n<\/pre>\nFor the record, this is my first try:<\/p>\n
\r\ntemplate<typename T>\r\nstruct value_type {\r\n};\r\n\r\ntemplate<typename T>\r\nrequires(std::is_scalar_v<T>)\r\nstruct value_type<T> {\r\n using type = T;\r\n};\r\n\r\ntemplate<typename T>\r\nrequires(not std::is_scalar_v<T>)\r\nstruct value_type<T> {\r\n using type = typename T::value_type;\r\n};\r\n\r\ntemplate<typename T>\r\nusing value_type_t = typename value_type<T>::type;\r\n\r\nstatic_assert(std::is_same_v<int, value_type_t<int>> );\r\nstatic_assert(std::is_same_v<int, value_type_t<std::vector<int>>> );\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"You can create a constrain for a class. And another constrain for the same class. Isn't this crazy stuff? With this we can build something like an identity function for types, like std::type_identity, for value_type. It's something like the C++17 non-member functions size() and empty(). Why must value_type be a member? It can be a […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[17],"class_list":["post-4129","post","type-post","status-publish","format-standard","hentry","category-allgemein","tag-cpp"],"_links":{"self":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/4129","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4129"}],"version-history":[{"count":6,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/4129\/revisions"}],"predecessor-version":[{"id":4135,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/4129\/revisions\/4135"}],"wp:attachment":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4129"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4129"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4129"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}