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

The function-like entities described on this page are algorithm function objects (informally known as 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.

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 input_iterator iterator equals to ranges:: next ( first, n ) and a 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 (; n-- > 0; (void)++first, (void)++result)
            *result = *first;
        return {std::move(first), std::move(result)};
    }
};
inline constexpr copy_n_fn copy_n{};

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())};
    const auto i{std::distance(std::begin(in), res.in)};
    const auto j{std::distance(std::begin(out), res.out)};
    std::cout << "in[" << i << "] = '" << in[i] << "'\n"
              << "out[" << j << "] = '" << out[j] << "'\n";
}

"ABCD"
in[5] = 'F'
out[5] = 'f'

See also