{"id":103902,"date":"2020-06-24T07:00:00","date_gmt":"2020-06-24T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103886"},"modified":"2020-07-02T06:50:42","modified_gmt":"2020-07-02T13:50:42","slug":"20200624-00-2","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200624-00\/?p=103902","title":{"rendered":"Mundane std::tuple tricks: Selecting via an index sequence, part 2"},"content":{"rendered":"

Last time<\/a>, we developed the select_<\/code>tuple<\/code> function which takes a tuple and an index sequence and produces a new tuple that selects the elements based on the index sequence. Here’s what we had:<\/p>\n

\/\/ Don't use this; see discussion.\r\ntemplate<typename Tuple, std::size_t... Ints>\r\nauto select_tuple(Tuple&& tuple, std::index_sequence<Ints...>)\r\n{\r\n return std::make_tuple(\r\n    std::get<Ints>(std::forward<Tuple>(tuple))...);\r\n}\r\n<\/pre>\n
The idea is that you can do something like<\/p>\n
std::tuple<int, char, float> t{ 1, 'x', 2.0 };\r\nauto t2 = select_tuple(t, std::index_sequence<0, 2>{});\r\n<\/pre>\n
and the result is that t2<\/code> is a std::tuple<int, float>{ 1, 2.0 }<\/code>.<\/p>\n
But there’s a problem with this function.<\/p>\n
Here’s a riddle: When does std::make_tuple<T>()<\/code> return something that isn’t a std::tuple<T><\/code>?<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
std::make_tuple<T><\/code><\/th>\nProduces std::tuple<T><\/code><\/th>\n<\/tr>\n
int\u00a0\u00a0\u00a0<\/code><\/td>\nint<\/code><\/td>\n<\/tr>\n
const int\u00a0\u00a0\u00a0<\/code><\/td>\n<\/tr>\n
int&\u00a0\u00a0<\/code><\/td>\n<\/tr>\n
int&&\u00a0<\/code><\/td>\n<\/tr>\n
std::reference_wrapper<\u00a0\u00a0\u00a0\u00a0\u00a0 int\u00a0 ><\/code><\/td>\nint&<\/code><\/td>\n<\/tr>\n
std::reference_wrapper<const int\u00a0 ><\/code><\/td>\n<\/tr>\n
std::reference_wrapper<\u00a0\u00a0\u00a0\u00a0\u00a0 int&\u00a0><\/code><\/td>\n<\/tr>\n
std::reference_wrapper<\u00a0\u00a0\u00a0\u00a0\u00a0 int&&><\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Answer: When T<\/code> is subject to decay or decays to a reference_<\/code>wrapper<\/code>.<\/p>\n
Decay is a term in the C++ standard that refers to the changes of type that typically occur when something is passed by value to a function:<\/p>\n
    \n
  • References decay to the underlying type.<\/li>\n
  • cv-qualifiers (const<\/code> and volatile<\/code>) are removed.<\/li>\n
  • Arrays decay to pointers.<\/li>\n
  • Function decay to function pointers.<\/li>\n<\/ul>\n
    But make_<\/code>tuple<\/code> adds an additional wrinkle: If the decayed type is a reference_<\/code>wrapper<\/code>, then the result is the underlying reference.<\/p>\n
    We don’t want any of these transformations to occur. If you select a type from a tuple that is a reference, then you want the resulting tuple to have the same reference type.<\/p>\n
    So we can’t use make_<\/code>tuple<\/code>. We’ll specify the desired tuple type explicitly.<\/p>\n
    template<typename Tuple, std::size_t... Ints>\r\nauto select_tuple(Tuple&& tuple, std::index_sequence<Ints...>)\r\n{\r\n return std::tuple<std::tuple_element_t<Ints, Tuple>...>(\r\n    std::get<Ints>(std::forward<Tuple>(tuple))...);\r\n}\r\n<\/pre>\n
    or alternatively<\/p>\n
    template<typename Tuple, std::size_t... Ints>\r\nstd::tuple<std::tuple_element_t<Ints, Tuple>...>\r\nselect_tuple(Tuple&& tuple, std::index_sequence<Ints...>)\r\n{\r\n return { std::get<Ints>(std::forward<Tuple>(tuple))... };\r\n}\r\n<\/pre>\n
    Okay, now that we have this helper function, we can do a bunch of fancy tuple manipulation.<\/p>\n
    Which we’ll do next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Preserving references.<\/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-103902","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
    Preserving references.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103902","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=103902"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103902\/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=103902"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103902"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103902"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}