{"id":103669,"date":"2020-04-13T07:00:00","date_gmt":"2020-04-13T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103669"},"modified":"2020-04-12T20:51:28","modified_gmt":"2020-04-13T03:51:28","slug":"20200413-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200413-00\/?p=103669","title":{"rendered":"The type-dependent type or value that is independent of the type"},"content":{"rendered":"

Some time ago, we saw how to create a type-dependent expression that is always false<\/a>. I noted that it feels weird creating a whole new type just to create a fixed false<\/code> value.<\/p>\n

But maybe we can make it more useful by generalizing it.<\/p>\n

template<typename T, typename...>\r\nusing unconditional_t = T;\r\n\r\ntemplate<typename T, T v, typename...>\r\ninline constexpr T unconditional_v = v;\r\n<\/pre>\n
The unconditional_t<\/code> alias template always represents the type T<\/code>, and the unconditional_v<\/code> variable template always represents the value v<\/code>.<\/p>\n
template<typename Whatever>\r\nvoid f()\r\n{\r\n  \/\/ X is always int\r\n  using X = unconditional_t<int, Whatever>;\r\n\r\n  \/\/ v is always 42\r\n  auto v = unconditional_v<int, 42, Whatever>;\r\n}\r\n<\/pre>\n
Even though the resulting type or value is always the same, it is nevertheless a dependent type, and therefore the evaluation does not occur until template instantiation.<\/p>\n
We can use this to solve our “cannot static_assert(false)<\/code> in a discarded statement” problem:<\/p>\n
auto lambda = [total<\/span>](auto op, auto value) mutable\r\n{\r\n  using Op = decltype(op);<\/span>\r\n  if constexpr<\/b> (std::is_same_v<Op, add_tax_t><\/span>) {\r\n   total += total * value; \/\/ value is the tax rate\r\n   return total;<\/span>\r\n  } else if constexpr<\/b> (std::is_same_v<Op, apply_discount_t><\/span>) {\r\n   total -= std::max(value, total); \/\/ value is the discount\r\n   return total;<\/span>\r\n  } else<\/b> {\r\n   static_assert(unconditional_v<Op, bool, false>,<\/b>\r\n                \"Don't know what you are asking me to do.\");\r\n  }\r\n};\r\n<\/pre>\n
The unconditional_t<\/code> generalizes the alias template std::void_t<...><\/code>: Whereas std::void_t<...><\/code> always evaluates to void<\/code>, the unconditional_t<\/code> lets you pick the type that it resolves to.<\/p>\n
template<typename... Types>\r\nusing void_t = unconditional_t<void, Types...>;\r\n<\/pre>\n
The unconditional_t<\/code> also generalizes the template class std::type_identity<T><\/code>: Whereas std::type_identity<T><\/code> takes only one template type parameter, the unconditional_t<\/code> lets you pass extra parameters, which are evaluated (for SFINAE) but otherwise ignored.<\/p>\n
template<typename T>\r\nusing type_identity = unconditional_t<T>;\r\n<\/pre>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
To delay the evaluation until instantiation.<\/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-103669","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
To delay the evaluation until instantiation.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103669","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=103669"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103669\/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=103669"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103669"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103669"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}