{"id":103981,"date":"2020-07-14T07:00:00","date_gmt":"2020-07-14T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103981"},"modified":"2022-06-01T19:29:49","modified_gmt":"2022-06-02T02:29:49","slug":"20200714-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200714-00\/?p=103981","title":{"rendered":"Deconstructing function pointers in a C++ template, the noexcept complication"},"content":{"rendered":"

Last time, we put together a little traits class to decompose a function pointer into its components<\/a>. But one thing missing from our class is the noexcept<\/code> qualifier.<\/p>\n

For the remainder of the discussion, I’ve removed the First\u00adArg<\/code> and Last\u00adArg<\/code> type aliases, since I came to the conclusion that they aren’t really needed. What’s left is this:<\/p>\n

template<typename R, typename... Args>\r\nstruct FunctionTraitsBase\r\n{\r\n  using RetType = R;\r\n  using ArgTypes = std::tuple<Args...>;\r\n  static constexpr std::size_t ArgCount = sizeof...(Args);\r\n  template<std::size_t N>\r\n  using NthArg = std::tuple_element_t<N, ArgTypes>;\r\n};\r\n\r\ntemplate<typename F> struct FunctionTraits;\r\n\r\ntemplate<typename R, typename... Args>\r\nstruct FunctionTraits<R(*)(Args...)>\r\n    : FunctionTraitsBase<R, Args...>\r\n{\r\n  using Pointer = R(*)(Args...);\r\n};\r\n<\/pre>\n
But it falls apart when we give it a noexcept<\/code> function pointer. (Note that noexcept<\/code> did not become part of the function pointer type until C++17.)<\/p>\n
void f()\r\n{\r\n  using T = int(*)() noexcept;\r\n  using R = FunctionTraits<T>::RetType; \/\/ error\r\n}\r\n<\/pre>\n
There is no match for T<\/code> because none of our specializations support noexcept<\/code> function pointers.<\/p>\n
So let’s add noexcept<\/code> to our signatures. Let’s try this version, which takes advantage of the fact that noexcept<\/code> takes a Boolean parameter that says whether the noexcept<\/code> applies. Saying noexcept<\/code> with no parameters is shorthand for noexcept(true)<\/code>, and omitting noexcept<\/code> is the same as noexcept(false)<\/code>.<\/p>\n
template<typename R, typename... Args, bool Nonthrowing>\r\nstruct FunctionTraits<R(*)(Args...) noexcept(Nonthrowing)>\r\n    : FunctionTraitsBase<R, Args...>\r\n{\r\n  using Pointer = R(*)(Args...) noexcept(Nonthrowing);\r\n  static constexpr bool IsNoexcept = Nonthrowing;\r\n};\r\n<\/pre>\n
The Microsoft compiler doesn’t like it:<\/p>\n
\/\/ MSVC\r\nerror C2057: expecting constant expression\r\n    struct FunctionTraits<R(*)(Args...) noexcept(Nonthrowing)>\r\n                                                 ^^^^^^^^^^^\r\nerror C27027: 'Nonthrowing': template parameter not used or deducible\r\n<\/pre>\n
icc also doesn’t like it, but for a different reason: It’s perfectly happy to match the partial specialization to a non-noexcept<\/code> function, but thinks it doesn’t apply to a noexcept<\/code> function.<\/p>\n
    \/\/ icc is okay with this\r\n    using Test1 = FunctionTraits<int(*)(float) noexcept>;\r\n\r\n    \/\/ but not this. \"error: incomplete type is not allowed\"\r\n    using Test2 = FunctionTraits<int(*)(float) noexcept>;\r\n<\/pre>\n
On the other hand, gcc and clang are okay with it and deduce Nonthrowing<\/code> appropriately. I’m not sure who is right. (I didn’t check icc.)<\/p>\n
Well that’s a bummer. The parameter to noexcept<\/code> is not deducible by the Microsoft compiler. We’ll just have to add a separate specialization.<\/p>\n
template<typename R, typename... Args>\r\nstruct FunctionTraits<R(*)(Args...)>\r\n    : FunctionTraitsBase<R, Args...>\r\n{\r\n  using Pointer = R(*)(Args...);\r\n  constexpr static bool IsNoexcept = false;\r\n};\r\n\r\ntemplate<typename R, typename... Args>\r\nstruct FunctionTraits<R(*)(Args...) noexcept>\r\n    : FunctionTraitsBase<R, Args...>\r\n{\r\n  using Pointer = R(*)(Args...);\r\n  constexpr static bool IsNoexcept = true;\r\n};\r\n<\/pre>\n
Okay, so that takes care of the noexcept<\/code> wrinkle. We’ll look at another attribute next time.<\/p>\n
Update<\/b>: Paragraph [temp.deduct.type]\/8 of the C++ specification lists the deducible contexts, and the noexcept<\/code> specifier is not on the list. Therefore, MSVC is correct to reject it, and gcc and clang’s behavior are nonstandard extensions. This was tracked as Core Working Group  issue number CWG2355<\/a>, with a vote to revise the standard  passing in January 2022<\/a> and accepted on May 21, 2022. MSVC implemented the language change in February 2020.<\/p>\n","protected":false},"excerpt":{"rendered":"
Trying to infer the noexcept qualifier.<\/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-103981","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Trying to infer the noexcept qualifier.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103981","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=103981"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103981\/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=103981"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103981"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103981"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}