std::ranges::for_each,std::ranges::for_each_result(3)

NAME
std::ranges::for_each,std::ranges::for_each_result - std::ranges::for_each,std::ranges::for_each_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S, class Proj =
   std::identity,
                        (since
   std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > (1) C++20)
   constexpr for_each_result<I, Fun>
   for_each( I first, S last, Fun f, Proj proj = {} );
   template< ranges::input_range R, class Proj = std::identity,
   std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>, (since
   Proj>> Fun > (2) C++20)
   constexpr for_each_result<ranges::borrowed_iterator_t<R>, Fun>
   for_each( R&& r, Fun f, Proj proj = {} );

Helper types
   template< class I, class F > (3) (since
   using for_each_result = ranges::in_fun_result<I, F>; C++20)
   1) Applies the given function object f to the result of the value projected by each
   iterator in the range [first, last), in order.
   2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as
   first and ranges::end(r) as last.
   For both overloads, if the iterator type is mutable, f may modify the elements of
   the range through the dereferenced iterator. If f returns a result, the result is
   ignored.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - iterator-sentinel pair denoting the range to apply the function to
   r - the range of elements to apply the function to
   f - the function to apply to the projected range
   proj - projection to apply to the elements

Return value
   {std::ranges::next(std::move(first), last), std::move(f)}

Complexity
   Exactly last - first applications of f and proj

Possible implementation
struct for_each_fn {
  template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
           std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
  constexpr ranges::for_each_result<I, Fun>
  operator()(I first, S last, Fun f, Proj proj = {}) const
  {
    for (; first != last; ++first) {
      std::invoke(f, std::invoke(proj, *first));
    }
    return {std::move(first), std::move(f)};
  }
  template<ranges::input_range R, class Proj = std::identity,
           std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>, Proj>> Fun>
  constexpr ranges::for_each_result<ranges::borrowed_iterator_t<R>, Fun>
  operator()(R&& r, Fun f, Proj proj = {}) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), std::move(f), std::ref(proj));
  } }; inline constexpr for_each_fn for_each;

Example
   The following example uses a lambda expression to increment all of the elements of a
   vector and then uses an overloaded operator() in a functor to compute their sum.
   Note that to compute the sum, it is recommended to use the dedicated algorithm
   std::accumulate. // Run this code
 #include <algorithm>
 #include <cassert>
 #include <iostream>
 #include <string>
 #include <utility>
 #include <vector>
 struct Sum
 {
     void operator()(int n) { sum += n; }
     int sum{0};
 };
 int main()
 {
     std::vector<int> nums{3, 4, 2, 8, 15, 267};
     auto print = [](const auto& n) { std::cout << ' ' << n; };
     namespace ranges = std::ranges;
     std::cout << "before:";
     ranges::for_each(std::as_const(nums), print);
     print('\n');
     ranges::for_each(nums, [](int& n){ ++n; });
     // calls Sum::operator() for each number
     auto [i, s] = ranges::for_each(nums.begin(), nums.end(), Sum());
     assert(i == nums.end());
     std::cout << "after: ";
     ranges::for_each(nums.cbegin(), nums.cend(), print);
     std::cout << "\n" "sum: " << s.sum << '\n';
     using pair = std::pair<int, std::string>;
     std::vector<pair> pairs{{1,"one"}, {2,"two"}, {3,"tree"}};
     std::cout << "project the pair::first: ";
     ranges::for_each(pairs, print, [](const pair& p) { return p.first; });
     std::cout << "\n" "project the pair::second:";
     ranges::for_each(pairs, print, &pair::second);
     print('\n');
 }

Output:
 before: 3 4 2 8 15 267
 after: 4 5 3 9 16 268
 sum: 305
 project the pair::first: 1 2 3
 project the pair::second: one two tree

See also
   range-for loop(C++11) executes loop over range
   ranges::transform 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
   (C++20) sequence
                        (niebloid)
   for_each applies a function to a range of elements
                        (function template)