std::ranges::copy_n() algorithm

od C++20

Simplified

// (1)
constexpr copy_n_result<I, O>
    copy_n( I first, std::iter_difference_t<I> n, O result );

The type of arguments are generic and have following constraints:

• I - std::input_iterator
• O - std::weakly_incrementable

Additionally, (1) has the following constraints:

• std::indirectly_copyable<I, O>

Detailed

// (1)
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 );

With the helper types defined as follows:

template< class I, class O >
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).

Undefined Behaviour

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.

Parameters

first	The beginning of the range to copy from.
n	Number of elements to copy.
result	The beginning of the destination range.

Return value

An object of of type copy_n_result initialized as follows:

{
  first + n,
  result + n
}

or more formally, a value of type ranges::in_out_result that contains:

• std::input_iterator iterator equal to ranges::next(first, n)
• and a std::weakly_incrementable iterator equal to ranges::next(result, n)

Complexity

Exactly n assignments.

Exceptions

(none)

Possible implementation

copy_n(1)

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 {};

Notes

In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy LegacyContiguousIterator.

When copying overlapping ranges, ranges::copy is appropriate when copying to the left (beginning of the destination range is outside the source range), while ranges::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Examples

Main.cpp

#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