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)