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

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> count, O d_first );

template< /*execution-policy*/ Ep, std::random_access_iterator I,
          std::random_access_iterator O, std::sized_sentinel_for<O> OutS >
    requires std::indirectly_copyable<I, O>
copy_n_result<I, O>
    copy_n( Ep&& policy, I first, iter_difference_t<I> count,
            O d_first, OutS d_last );

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

For the definition of /*execution-policy*/, see this page.

If count is positive, copies the elements in the source range [first, ranges::next(first, count)) to the destination range. Otherwise do nothing.

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

A ranges::copy_n_result object where:

• The data member in holds the an iterator past the last copied element in the source range, or an iterator to the beginning of the source range if no element is copied.
• The data member out holds and an iterator past the last copy-assigned element in the destination range, or an iterator to the beginning of the destination range if no element is copied.

Complexity

Given

• \(\scriptsize N_1\)N₁ as \(\scriptsize \max(\mathtt{count},0)\)max(count,0), and
• \(\scriptsize N_2\)N₂ as ranges::distance(d_first, d_last):

Exceptions

• If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
• If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

In practice, implementations of 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, ranges::copy_n 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).

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 d_first) const
    {
        for (; n-- > 0; (void)++first, (void)++d_first)
            *d_first = *first;
        
        return {std::move(first), std::move(d_first)};
    }
};

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