Difference between revisions of "cpp/algorithm/ranges/copy n"
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m (add return type to the page's title) |
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{{dcl h|Call signature}} | {{dcl h|Call signature}} | ||
{{dcl | since=c++20 | num=1 |1= | {{dcl | since=c++20 | num=1 |1= | ||
| − | template<std::input_iterator I, std::weakly_incrementable O> | + | template< std::input_iterator I, std::weakly_incrementable O > |
requires std::indirectly_copyable<I, O> | requires std::indirectly_copyable<I, O> | ||
| − | + | constexpr ranges::copy_n_result<I, O> | |
| − | + | ranges::copy_n( I first, std::iter_difference_t<I> n, O result ); | |
}} | }} | ||
{{dcl h | Helper type}} | {{dcl h | Helper type}} | ||
{{dcl | since=c++20 | num=2 |1= | {{dcl | since=c++20 | num=2 |1= | ||
| − | template<class I, class O> | + | template< class I, class O > |
using copy_n_result = ranges::in_out_result<I, O>; | using copy_n_result = ranges::in_out_result<I, O>; | ||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| − | @1@ Copies exactly {{tt|n}} values from the range beginning at {{tt|first}} to the range beginning at {{tt|result}}. Effect: for each integer {{ | + | @1@ Copies exactly {{tt|n}} values from the range beginning at {{tt|first}} to the range beginning at {{tt|result}}. Effect: for each integer {{c|1=0 ≤ i < n}}, performs {{c|1=*(result + i) = *(first + i)}}. The behavior is undefined if {{tt|result}} is within the range {{tt|[first, first + n)}}. Then, the {{c|ranges::copy_backward}} might be used instead. |
{{cpp/ranges/niebloid}} | {{cpp/ranges/niebloid}} | ||
| Line 44: | Line 44: | ||
template<std::input_iterator I, std::weakly_incrementable O> | template<std::input_iterator I, std::weakly_incrementable O> | ||
requires std::indirectly_copyable<I, 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; | *result = *first; | ||
| − | + | return {std::move(first), std::move(result)}; | |
| − | + | } | |
}; | }; | ||
Revision as of 23:10, 14 June 2021
| 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> |
(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) |
1) Copies exactly
n values from the range beginning at first to the range beginning at result. Effect: for each integer 0 ≤ i < n, performs *(result + i) = *(first + i). The behavior is undefined if result is within the range [first, first + n). Then, the ranges::copy_backward might be used instead.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.
Contents |
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
The ranges::copy_n_result{first + n, result + n} or more formally, an object 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. Alternativelly, 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 <iterator> #include <iostream> #include <iomanip> #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
| (C++20)(C++20) |
copies a range of elements to a new location (niebloid) |
| (C++20) |
copies a range of elements in backwards order (niebloid) |
| (C++20)(C++20) |
copies a range of elements omitting those that satisfy specific criteria (niebloid) |
| (C++20)(C++20) |
copies a range, replacing elements satisfying specific criteria with another value (niebloid) |
| (C++20) |
creates a copy of a range that is reversed (niebloid) |
| (C++20) |
copies and rotate a range of elements (niebloid) |
| (C++20) |
creates a copy of some range of elements that contains no consecutive duplicates (niebloid) |
| (C++20) |
moves a range of elements to a new location (niebloid) |
| (C++20) |
moves a range of elements to a new location in backwards order (niebloid) |
| (C++11) |
copies a number of elements to a new location (function template) |
