std::ranges::copy_n,std::ranges::copy_n_result(3)

NAME
std::ranges::copy_n,std::ranges::copy_n_result - std::ranges::copy_n,std::ranges::copy_n_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::weakly_incrementable O >
   requires std::indirectly_copyable<I, O> (1) (since C++20)
   constexpr copy_n_result<I, O>
       copy_n( I first, std::iter_difference_t<I> n, O result );
   Helper type
   template< class I, class O > (2) (since C++20)
   using copy_n_result = ranges::in_out_result<I, O>;
   1) Copies exactly n values from the range beginning at first to the range beginning
   at result by performing *(result + i) = *(first + i) for each integer in [0, n). The
   behavior is undefined if result is within the range [first, first + n)
   (ranges::copy_backward might be used instead in this case).
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first - the beginning of the range of elements to copy from
   n - number of the elements to copy
   result - the beginning of the destination range

Return value
   ranges::copy_n_result{first + n, result + n} or more formally, a value of type
   ranges::in_out_result that contains an std::input_iterator iterator equals to
   ranges::next(first, n) and a std::weakly_incrementable iterator equals to
   ranges::next(result, n).

Complexity
   Exactly n assignments.

Notes
   In practice, implementations of std::ranges::copy_n may avoid multiple assignments
   and use bulk copy functions such as std::memmove if the value type is
   TriviallyCopyable and the iterator types satisfy contiguous_iterator. Alternatively,
   such copy acceleration can be injected during an optimization phase of a compiler.
   When copying overlapping ranges, std::ranges::copy_n is appropriate when copying to
   the left (beginning of the destination range is outside the source range) while
   std::ranges::copy_backward is appropriate when copying to the right (end of the
   destination range is outside the source range).

Possible implementation
   struct copy_n_fn
   {
       template<std::input_iterator I, std::weakly_incrementable O>
       requires std::indirectly_copyable<I, O>
       constexpr ranges::copy_n_result<I, O>
           operator()(I first, std::iter_difference_t<I> n, O result) const
       {
           for (std::iter_difference_t<I> i {}; i != n; ++i, ++first, ++result)
               *result = *first;
           return {std::move(first), std::move(result)};
       }
   };
   inline constexpr copy_n_fn copy_n {};

Example
// Run this code
 #include <algorithm>
 #include <iomanip>
 #include <iostream>
 #include <iterator>
 #include <string>
 #include <string_view>
 int main()
 {
     const std::string_view in {"ABCDEFGH"};
     std::string out;
     std::ranges::copy_n(in.begin(), 4, std::back_inserter(out));
     std::cout << std::quoted(out) << '\n';
     out = "abcdefgh";
     const auto res = std::ranges::copy_n(in.begin(), 5, out.begin());
     std::cout
         << "*(res.in): '" << *(res.in) << "', distance: "
         << std::distance(std::begin(in), res.in) << '\n'
         << "*(res.out): '" << *(res.out) << "', distance: "
         << std::distance(std::begin(out), res.out) << '\n';
 }

Output:
 "ABCD"
 *(res.in): 'F', distance: 5
 *(res.out): 'f', distance: 5

See also
   ranges::copy
   ranges::copy_if copies a range of elements to a new location
   (C++20) (niebloid)
   (C++20)
   ranges::copy_backward copies a range of elements in backwards order
   (C++20) (niebloid)
   ranges::remove_copy copies a range of elements omitting those that satisfy
   ranges::remove_copy_if specific criteria
   (C++20) (niebloid)
   (C++20)
   ranges::replace_copy copies a range, replacing elements satisfying specific
   ranges::replace_copy_if criteria with another value
   (C++20) (niebloid)
   (C++20)
   ranges::reverse_copy creates a copy of a range that is reversed
   (C++20) (niebloid)
   ranges::rotate_copy copies and rotate a range of elements
   (C++20) (niebloid)
   ranges::unique_copy creates a copy of some range of elements that contains no
   (C++20) consecutive duplicates
                        (niebloid)
   ranges::move moves a range of elements to a new location
   (C++20) (niebloid)
   ranges::move_backward moves a range of elements to a new location in backwards
   (C++20) order
                        (niebloid)
   copy_n copies a number of elements to a new location
   (C++11) (function template)