22 General utilities library [utilities]

22.14 Formatting [format]

22.14.7 Formatting of ranges [format.range]

22.14.7.1 Variable template format_kind [format.range.fmtkind]

template<ranges::input_range R>
    requires same_as<R, remove_cvref_t<R>>
  constexpr range_format format_kind<R> = see below;

A program that instantiates the primary template of format_kind is ill-formed.

For a type R, format_kind<R> is defined as follows:

(2.1)
If same_as<remove_cvref_t<ranges::range_reference_t<R>>, R> is true, format_kind<R> is range_format::disabled.

[Note 1:
This prevents constraint recursion for ranges whose reference type is the same range type.

For example, std::filesystem::path is a range of std::filesystem::path.
— end note]
(2.2)
Otherwise, if the qualified-id R::key_type is valid and denotes a type:
- (2.2.1)
  If the qualified-id R::mapped_type is valid and denotes a type, let U be remove_cvref_t<ranges::range_reference_t<R>>.
  
  If either U is a specialization of pair or U is a specialization of tuple and tuple_size_v<U> == 2, format_kind<R> is range_format::map.
- (2.2.2)
  Otherwise, format_kind<R> is range_format::set.
(2.3)
Otherwise, format_kind<R> is range_format::sequence.

Remarks: Pursuant to [namespace.std], users may specialize format_kind for cv-unqualified program-defined types that model ranges::input_range.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const range_format.