std::ranges::in_fun_result(3)

NAME
std::ranges::in_fun_result - std::ranges::in_fun_result

Synopsis
   Defined in header <algorithm>
   template< class I, class F > (since C++20)
   struct in_fun_result;
   ranges::in_fun_result is a class template that provides a way to store an iterator
   and a function object 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 - the type of the iterator that the ranges::in_fun_result stores.
   F - the type of the function object that the ranges::in_fun_result stores.
   Data members
   Member name Definition
   in a value (that is supposed to be an iterator) of type I.
               (public member object)
   fun a value (that is supposed to be a function object) of type F.
               (public member object)
   All these members are declared with [[no_unique_address]] attribute.

Member functions
std::ranges::in_fun_result::operator in_fun_result<I2, F2>
   template<class I2, class F2>
   requires std::convertible_to<const I&, I2> && std::convertible_to<const F&, F2> (1)
   constexpr operator in_fun_result<I2, F2>() const &;
   template<class I2, class F2>
   requires std::convertible_to<I, I2> && std::convertible_to<F, F2> (2)
   constexpr operator in_fun_result<I2, F2>() &&;
   Converts *this to the result by constructing every data member of the result from
   the corresponding member of *this.
   1) Equivalent to return {in, fun};.
   2) Equivalent to return {std::move(in), std::move(fun)};.

Standard library
   The following standard library functions use ranges::in_fun_result as the return
   type:
         Algorithm functions
   ranges::for_each applies a function to a range of elements
   (C++20) (niebloid)
   ranges::for_each_n applies a function object to the first N elements of a sequence
   (C++20) (niebloid)

Synopsis
 namespace std::ranges
 {
     template<class I, class F>
     struct in_fun_result
     {
         [[no_unique_address]] I in;
         [[no_unique_address]] F fun;
         template<class I2, class F2>
         requires std::convertible_to<const I&, I2> && std::convertible_to<const F&, F2>
         constexpr operator in_fun_result<I2, F2>() const &
         {
             return {in, fun};
         }
         template<class I2, class F2>
         requires std::convertible_to<I, I2> && std::convertible_to<F, F2>
         constexpr operator in_fun_result<I2, F2>() &&
         {
             return {std::move(in), std::move(fun)};
         }
     };
 }

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 <iostream>
 #include <iterator>
 #include <ranges>
 int main()
 {
     int v[]{1, 2, 3};
     const std::ranges::in_fun_result res1 = std::ranges::for_each_n(
         v, std::size(v),
         [](int& x) { return x = -x; } // negating lambda
     );
     assert(res1.in == std::end(v));
     const std::ranges::in_fun_result res2 = std::ranges::for_each(
         std::begin(v),
         res1.in,
         [](int x) { std::cout << x << ' '; } // printing lambda
     );
     std::cout << "│ ";
     std::ranges::for_each(v, res1.fun); // uses negating lambda
     std::ranges::for_each(v, res2.fun); // uses printing lambda
     std::cout << '\n';
 }

Output:
 -1 -2 -3 │ 1 2 3

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)