C++11 introduced variadic templates and template parameter packs.<\/p>\n
template<typename... Args>\r\nstruct S;\r\n<\/pre>\nPassing template parameter packs around is a bit of a hassle, because the dots “soak up” parameters, which make them hard to pass to other templated types. You also can’t “save” parameter packs in another type:<\/p>\n
template<typename... Args>\r\nstruct S\r\n{\r\n \/\/ doesn't compile: cannot create a type that\r\n \/\/ is itself a parameter pack\r\n using TheArgs = Args...;\r\n};\r\n<\/pre>\nOne workaround for this is to capture the types into a
std::tuple<\/code>.<\/p>\ntemplate<typename... Args>\r\nstruct S\r\n{\r\n using Tuple = std::tuple<Args...>;\r\n};\r\n<\/pre>\nYou can then pass the
Tuple<\/code> around, and if you need to extract the types, you can pull them out of the tuple.<\/p>\nMy first thought about how to extract the types was to use
std::get<\/code>:<\/p>\n\/\/ code in italics is wrong\r\ntemplate<typename... Args>\r\nstruct Traits\r\n{\r\n using Tuple = std::tuple<Args...>;\r\n static constexpr auto Size = sizeof...(Args);\r\n template <std::size_t N>\r\n using Nth = decltype(std::get<N>(std::declval<Tuple>()));<\/i>\r\n using First = Nth<0>;\r\n using Last = Nth<Size - 1>;\r\n};\r\n<\/pre>\nThis doesn’t work because
std::get<\/code> returns a reference:<\/p>\nvoid f()\r\n{\r\n whatis<Traits<int, double>::First>();\r\n}\r\n\r\n call void whatis<int&&>()\r\n<\/pre>\nThis behavior is presumably so you can modify the tuple:<\/p>\n
std::tuple<int> tuple;\r\nstd::get<0>(tuple) = 2;\r\n<\/pre>\nFortunately, there’s another way to extract the type from a tuple:
std::tuple_element<\/code>.<\/p>\ntemplate<typename... Args>\r\nstruct Traits\r\n{\r\n using Tuple = std::tuple<Args...>;\r\n static constexpr auto Size = sizeof...(Args);\r\n template <std::size_t N>\r\n using Nth = typename std::tuple_element<N, Tuple>::type;<\/span>\r\n using First = Nth<0>;\r\n using Last = Nth<Size - 1>;\r\n};\r\n<\/pre>\nThis provides a simpler way to extract the last type from a template parameter pack than writing some horrible recursive template metaprogram, which is what some people did instead of hiding the pack inside a tuple.<\/p>\n","protected":false},"excerpt":{"rendered":"
So you can pull them back out.<\/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-103810","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
So you can pull them back out.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103810","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=103810"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103810\/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=103810"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103810"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103810"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}