std::ranges::in_value_result(3)

NAME
std::ranges::in_value_result - std::ranges::in_value_result

Synopsis
   Defined in header <algorithm>
   template< class I, class T > (since C++23)
   struct in_value_result;
   ranges::in_value_result is a class template that provides a way to store an iterator
   and a value as a single unit.
   This class template has no base classes or declared members other than those shown
   below. Thus it is suitable for use with structured bindings.
   All special member functions of this class template are implicitly declared, which
   makes specializations be aggregate classes, and propagate triviality,
   potentially-throwing-ness, and constexpr-ness of corresponding operations on data
   members.

Template parameters
   I, T - the types of the objects that the ranges::in_value_result stores.
   Data members
   Member name Definition
   in a value (that is supposed to be an iterator) of type I.
               (public member object)
   value a value (that is supposed to be a stored value) of type T.
               (public member object)
   All these members are declared with [[no_unique_address]] attribute.

Member functions
std::ranges::in_value_result::operator in_value_result<I2, T2>
   template<class I2, class T2>
   requires convertible_to<const I&, I2> && convertible_to<const T&, T2> (1)
   constexpr operator in_value_result<I2, T2>() const &;
   template<class I2, class T2>
   requires convertible_to<I, I2> && convertible_to<T, T2> (2)
   constexpr operator in_value_result<I2, T2>() &&;
   Converts *this to the result by constructing every data member of the result from
   the corresponding member of *this.
   1) Equivalent to return {in, value};.
   2) Equivalent to return {std::move(in), std::move(value)};.

Standard library
   The following standard library functions use ranges::in_value_result as the return
   type:
         Algorithm functions
   ranges::fold_left_with_iter left-folds a range of elements, and returns a pair
   (C++23) (iterator, value)
                        (niebloid)
                        left-folds a range of elements using the first
   ranges::fold_left_first_with_iter element as an initial value, and returns a pair
   (C++23) (iterator, optional)
                        (niebloid)

Synopsis
 namespace std::ranges
 {
     template<class I, class T>
     struct in_value_result
     {
         [[no_unique_address]] I in;
         [[no_unique_address]] T value;
         template<class I2, class T2>
         requires convertible_to<const I&, I2> && convertible_to<const T&, T2>
         constexpr operator in_value_result<I2, T2>() const &
         {
             return {in, value};
         }
         template<class I2, class T2>
         requires convertible_to<I, I2> && convertible_to<T, T2>
         constexpr operator in_value_result<I2, T2>() &&
         {
             return {std::move(in), std::move(value)};
         }
     };
 }

Notes
   Each standard library algorithm that uses this family of return types declares a new
   alias type, e.g. using merge_result = in_in_out_result<I1, I2, O>;.
   The names for such aliases are formed by adding the suffix "_result" to the
   algorithm's name. So, the return type of std::ranges::merge can be named as
   std::ranges::merge_result.
   Unlike std::pair and std::tuple, this class template has data members of meaningful
   names.

Example
// Run this code
 #include <algorithm>
 #include <cassert>
 #include <functional>
 int main()
 {
     const auto v = {1, 2, 3};
     const auto ret = std::ranges::fold_left_with_iter(v, 4, std::plus<>());
     assert(ret.in == v.end());
     assert(ret.value == 1 + 2 + 3 + 4);
 }

See also
   pair implements binary tuple, i.e. a pair of values
           (class template)
   tuple implements fixed size container, which holds elements of possibly different
   (C++11) types
           (class template)