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std::ranges::subrange<I,S,K>::operator PairLike

template< /* see below */ PairLike >
    requires pair-like-convertible-from<PairLike, const I&, const S&>
constexpr operator PairLike() const;
 (1) (since C++20)
Helper concepts 
template< class T >
concept pair-like =
    !std::is_reference_v<T> && requires(T t) {
        typename std::tuple_size<T>::type; // ensures std::tuple_size<T>
                                           // is complete
        requires std::derived_from<std::tuple_size<T>,
                                   std::integral_constant<std::size_t, 2>>;
        typename std::tuple_element_t<0, std::remove_const_t<T>>;
        typename std::tuple_element_t<1, std::remove_const_t<T>>;
        { std::get<0>(t) } -> std::convertible_to<
                                  const std::tuple_element_t<0, T>&>;
        { std::get<1>(t) } -> std::convertible_to<
                                  const std::tuple_element_t<1, T>&>;
    };
 (2) (until C++23)
(exposition only*)
(3) 
template< class T, class U, class V >
concept pair-like-convertible-from =
    !ranges::range<T> && pair-like<T> &&
    std::constructible_from<T, U, V> &&
    convertible-to-non-slicing<U, std::tuple_element_t<0, T>> &&
    std::convertible_to<V, std::tuple_element_t<1, T>>;
 (until C++23)
(exposition only*)		 
template< class T, class U, class V >
concept pair-like-convertible-from =
    !ranges::range<T> && !std::is_reference_v<T> && pair-like<T> &&
    std::constructible_from<T, U, V> &&
    convertible-to-non-slicing<U, std::tuple_element_t<0, T>> &&
    std::convertible_to<V, std::tuple_element_t<1, T>>;
 (since C++23)
(exposition only*)
1) Converts subrange to a pair-like type (i.e. a type models the helper concept pair-like defined below(until C++23)pair-like(since C++23)). Equivalent to return PairLike(i_, s_);, where i_ and s_ are the stored iterator and sentinel respectively.
PairLike is constrained that std::same_as<std::remove_cvref_t<PairLike>, subrange> is false.
This conversion function has additional constraints imposed by pair-like-convertible (see below).
2) The exposition-only concept pair-like specifies a type is pair-like. Generally, an expression e of a pair-like type can be used for structured binding (i.e. auto const& [x, y] = e; is generally well-formed). This concept is replaced by the library-wide exposition-only concept pair-like.(since C++23)
3) The exposition-only concept pair-like-convertible-from refines pair-like. It
  • rejects reference types and (since C++23)range types,
  • requires that U and V are convertible to the first and second element type of T respectively, and
  • requires the conversion from U (which will be replaced by const I&) to the first element type to be non-slicing (see convertible-to-non-slicing).

Parameters

(none)

Return value

A PairLike value direct-initialized with the stored iterator and sentinel.

Notes

Following types in the standard library are pair-like:

A program-defined type derived from one of these types can be a pair-like type, if

 (until C++23)

Since subrange specializations are range types, conversion to them are not performed via this conversion function.

std::array specializations cannot be converted from subrange, since they are range types.

Example

#include <iostream>
#include <ranges>
#include <string>
#include <utility>
 
using striter = std::string::const_iterator;
 
using legacy_strview = std::pair<striter, striter>;
 
void legacy_print(legacy_strview p)
{
    for (; p.first != p.second; ++p.first)
        std::cout << *p.first << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::string dat{"ABCDE"};
    for (auto v{std::ranges::subrange{dat}}; v; v = {v.begin(), v.end() - 1})
    {
        /*...*/
        legacy_print(legacy_strview{v});
    }
}

Output:

A B C D E 
A B C D 
A B C 
A B 
A

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https://en.cppreference.com/w/cpp/ranges/subrange/operator_PairLike