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4373. function_ref should provide result_type

Section: 22.10.17.6.2 [func.wrap.ref.class] Status: New Submitter: Hewill Kang Opened: 2025-09-12 Last modified: 2025-09-19

Priority: Not Prioritized

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Discussion:

Currently, function, move_only_function, and copyable_function all have a type member result_type, but function_ref does not:

static_assert(is_same_v<          function    <int(int)>::result_type, int>);
static_assert(is_same_v<move_only_function    <int(int)>::result_type, int>);
static_assert(is_same_v< copyable_function    <int(int)>::result_type, int>);
static_assert(is_same_v<          function_ref<int(int)>::result_type, int>); // error

It seems worthwhile to also provide it for the latter, as it is consistent with the other wrappers and allows the user to easily extract the return type.

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.10.17.6.2 [func.wrap.ref.class] as indicated:

   namespace std {
     template<class R, class... ArgTypes>
     class function_ref<R(ArgTypes...) cv noexcept(noex)> {
     public:
       using result_type = R;
   
       // 22.10.17.6.3 [func.wrap.ref.ctor], constructors and assignment operators
       template<class F> function_ref(F*) noexcept;
       […]
     };
   
   }