std::ranges::copy_n, std::ranges::copy_n_result

Since C++20

Header: <algorithm>

  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).

# Declarations

Call signature

template< std::input_iterator I, std::weakly_incrementable O >
requires std::indirectly_copyable<I, O>
constexpr copy_n_result<I, O>
copy_n( I first, std::iter_difference_t<I> n, O result );

(since C++20)

Helper type

template< class I, class O >
using copy_n_result = ranges::in_out_result<I, O>;

(since C++20)

# Parameters

# 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).

# 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).

# Example

#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';
}

# See also