{"id":109942,"date":"2024-06-28T07:00:00","date_gmt":"2024-06-28T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109942"},"modified":"2024-06-24T08:39:10","modified_gmt":"2024-06-24T15:39:10","slug":"20240628-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240628-00\/?p=109942","title":{"rendered":"Writing a remove_all_pointers<\/CODE> type trait, part 2"},"content":{"rendered":"

Last time, we wrote a remove_all_pointers<\/code> type trait<\/a>, but I noted that even though we found a solution, we weren’t finished yet.<\/p>\n

We can bring back the one-liner by using a different trick to delay the recursion: Don’t ask for the type<\/code> until we know we really are recursing.<\/p>\n

The sketch is<\/p>\n

template<typename T>\r\nstruct dummy { using type = T; };\r\n\r\ntemplate<typename T>\r\nstruct remove_all_pointers\r\n{\r\n    if (std::is_pointer_v<T>) {\r\n        using type_holder =\r\n            remove_all_pointers<\r\n                std::remove_pointer_t<T>\r\n            >;\r\n    } else {\r\n        using type_holder = dummy<T>;\r\n    }\r\n    using type =\r\n        typename type_holder::type;\r\n};\r\n<\/pre>\n
We first define a type_holder<\/code> to be a type which has a type<\/code> member type that holds our answer. If T<\/code> is a pointer, then the type holder is the recursive call. Otherwise, the type holder is a dummy type whose sole purpose is to have a type<\/code> member type that produces T<\/code> again.<\/p>\n
We can now pack up that if<\/code> into a std::conditional<\/code>.<\/p>\n
template<typename T>\r\nstruct remove_all_pointers\r\n{\r\n    using type_holder =\r\n        std::conditional_t<\r\n            std::is_pointer_v<T>,\r\n            remove_all_pointers<\r\n                std::remove_pointer_t<T>\r\n            >,\r\n            dummy<T>>;\r\n    using type =\r\n        typename type_holder::type;\r\n};\r\n<\/pre>\n
It turns out that we don’t need to define a dummy<\/code>: The C++ standard library comes with one built in! It’s called std::type_identity<T><\/code>, available starting in C++20. (We looked at std::type_identity<T><\/code> a little while ago<\/a>.)<\/p>\n
template<typename T>\r\nstruct remove_all_pointers\r\n{\r\n    using type_holder =\r\n        std::conditional_t<\r\n            std::is_pointer_v<T>,\r\n            remove_all_pointers<\r\n                std::remove_pointer_t<T>\r\n            >,\r\n            std::type_identity<T>>;\r\n    using type =\r\n        typename type_holder::type;\r\n};\r\n<\/pre>\n
Now we can inline the type_holder<\/code>.<\/p>\n
template<typename T>\r\nstruct remove_all_pointers\r\n{\r\n    using type =\r\n        typename std::conditional_t<\r\n            std::is_pointer_v<T>,\r\n            remove_all_pointers<\r\n                std::remove_pointer_t<T>\r\n            >,\r\n            std::type_identity<T>>::type;\r\n};\r\n<\/pre>\n
Or even better, just derive from the type_holder<\/code>!<\/p>\n
template<typename T>\r\nstruct remove_all_pointers :\r\n        std::conditional_t<\r\n            std::is_pointer_v<T>,\r\n            remove_all_pointers<\r\n                std::remove_pointer_t<T>\r\n            >,\r\n            std::type_identity<T>>\r\n{\r\n};\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Factoring out the type resolution to after the dangerous part.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-109942","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Factoring out the type resolution to after the dangerous part.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=109942"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109942\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=109942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109942"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}