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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: 2025-10-22

Priority: 3

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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.

[2025-10-03; Hewill comments]

Additionally, given that some third-party implementations also provide a mechanism for extracting the return type, for example, type_safe::function_ref provides a public member type alias return_type, and llvm::function_ref can work with llvm::function_traits to obtain the return type, providing such a member type alias for std::function_ref meets user potential demands.

[2025-10-22; Reflector poll.]

Set priority to 3 after reflector poll.

Alternative (with incoming paper targeting LEWG) of deprecating result_type in existing wrappers was suggested.

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;
       […]
     };
   
   }