std::ranges::copy_n, std::ranges::copy_n_result
| Defined in header <algorithm>
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| Call signature |
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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 ); |
(1) | (since C++20) |
| Helper type |
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template< class I, class O > using copy_n_result = ranges::in_out_result<I, O>; |
(2) | (since C++20) |
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 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
| 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 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{};
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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";
}
Output:
"ABCD"
in[5] = 'F'
out[5] = 'f'
See also
(C++20)(C++20) |
copies a range of elements to a new location (algorithm function object) |
(C++20) |
copies a range of elements in backwards order (algorithm function object) |
(C++20)(C++20) |
copies a range of elements omitting those that satisfy specific criteria (algorithm function object) |
(C++20)(C++20) |
copies a range, replacing elements satisfying specific criteria with another value (algorithm function object) |
(C++20) |
creates a copy of a range that is reversed (algorithm function object) |
(C++20) |
copies and rotate a range of elements (algorithm function object) |
(C++20) |
creates a copy of some range of elements that contains no consecutive duplicates (algorithm function object) |
(C++20) |
moves a range of elements to a new location (algorithm function object) |
(C++20) |
moves a range of elements to a new location in backwards order (algorithm function object) |
(C++11) |
copies a number of elements to a new location (function template) |