Difference between revisions of "cpp/algorithm/reverse copy"
From cppreference.com
m (→Defect reports: changed footnote position (?)) |
(Added LWG issue #2150 DR (part 5/12), and uses {{mark constexpr since c++20}}.) |
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| Line 3: | Line 3: | ||
{{dcl begin}} | {{dcl begin}} | ||
{{dcl header|algorithm}} | {{dcl header|algorithm}} | ||
| − | {{dcl | + | {{dcl|num=1|notes={{mark constexpr since c++20}}| |
| − | | | + | |
template< class BidirIt, class OutputIt > | template< class BidirIt, class OutputIt > | ||
| − | OutputIt reverse_copy( BidirIt first, BidirIt last, | + | OutputIt reverse_copy( BidirIt first, BidirIt last, |
| − | + | OutputIt d_first ); | |
| − | + | ||
| − | + | ||
| − | + | ||
}} | }} | ||
{{dcl|num=2|since=c++17| | {{dcl|num=2|since=c++17| | ||
template< class ExecutionPolicy, class BidirIt, class ForwardIt > | template< class ExecutionPolicy, class BidirIt, class ForwardIt > | ||
ForwardIt reverse_copy( ExecutionPolicy&& policy, | ForwardIt reverse_copy( ExecutionPolicy&& policy, | ||
| − | BidirIt first, BidirIt last, ForwardIt d_first ); | + | BidirIt first, BidirIt last, |
| + | ForwardIt d_first ); | ||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| − | @1@ Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}}. Copies the elements from the range {{range|first|last}} to another range of {{mathjax-or|\(\scriptsize N\)|N}} elements beginning at {{c|d_first}} (destination range) in such a way that the elements in the destination range are in reverse order. | + | @1@ Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}}. Copies the elements from the range {{range|first|last}} (source range) to another range of {{mathjax-or|\(\scriptsize N\)|N}} elements beginning at {{c|d_first}} (destination range) in such a way that the elements in the destination range are in reverse order. |
| − | + | ||
@@ Behaves as if by executing the assignment {{c|1=*(d_first + N - 1 - i) = *(first + i)}}<ref name="plusminus">{{named req|OutputIterator}} is not required to support binary {{tt|+}} and {{tt|-}}. The usages of {{tt|+}} and {{tt|-}} here are exposition-only: the actual computation does not need to use them.</ref> once for each integer {{c|i}} in {{range|0|N}}. | @@ Behaves as if by executing the assignment {{c|1=*(d_first + N - 1 - i) = *(first + i)}}<ref name="plusminus">{{named req|OutputIterator}} is not required to support binary {{tt|+}} and {{tt|-}}. The usages of {{tt|+}} and {{tt|-}} here are exposition-only: the actual computation does not need to use them.</ref> once for each integer {{c|i}} in {{range|0|N}}. | ||
| + | @@ If source and destination ranges overlap, the behavior is undefined. | ||
| − | + | @2@ Same as {{v|1}}, but executed according to {{c|policy}}. | |
| − | + | @@ {{cpp/algorithm/parallel overload precondition}} | |
| − | @2@ Same as {{v|1}}, but executed according to {{c|policy}}. {{cpp/algorithm/parallel overload precondition}} | + | |
===Parameters=== | ===Parameters=== | ||
| Line 45: | Line 41: | ||
===Exceptions=== | ===Exceptions=== | ||
{{cpp/algorithm/parallel exceptions reporting behavior|singular=yes}} | {{cpp/algorithm/parallel exceptions reporting behavior|singular=yes}} | ||
| − | |||
| − | |||
| − | |||
===Possible implementation=== | ===Possible implementation=== | ||
| Line 61: | Line 54: | ||
} | } | ||
}} | }} | ||
| + | |||
| + | ===Notes=== | ||
| + | Implementations (e.g. [https://github.com/microsoft/STL/blob/main/stl/src/vector_algorithms.cpp MSVC STL]) may enable vectorization when the both iterator types satisfy {{named req|ContiguousIterator}} and have the same value type, and the value type is {{named req|TriviallyCopyable}}. | ||
===Example=== | ===Example=== | ||
| Line 97: | Line 93: | ||
{{dr list begin}} | {{dr list begin}} | ||
{{dr list item|wg=lwg|dr=2074|std=C++98|before=for each {{c|i}}, the assignment was<br>{{c|1=*(d_first + N - i) = *(first + i)}}<ref name="plusminus" />|after=corrected to<br>{{c|1=*(d_first + N - 1 - i) = *(first + i)}}<ref name="plusminus" />}} | {{dr list item|wg=lwg|dr=2074|std=C++98|before=for each {{c|i}}, the assignment was<br>{{c|1=*(d_first + N - i) = *(first + i)}}<ref name="plusminus" />|after=corrected to<br>{{c|1=*(d_first + N - 1 - i) = *(first + i)}}<ref name="plusminus" />}} | ||
| + | {{dr list item|wg=lwg|dr=2150|std=C++98|before=only one element was required to be assigned|after=corrected the requirement}} | ||
{{dr list end}} | {{dr list end}} | ||
<references/> | <references/> | ||
Latest revision as of 19:19, 27 March 2024
| Defined in header <algorithm>
|
||
| template< class BidirIt, class OutputIt > OutputIt reverse_copy( BidirIt first, BidirIt last, |
(1) | (constexpr since C++20) |
| template< class ExecutionPolicy, class BidirIt, class ForwardIt > ForwardIt reverse_copy( ExecutionPolicy&& policy, |
(2) | (since C++17) |
1) Given N as std::distance(first, last). Copies the elements from the range
[first, last) (source range) to another range of N elements beginning at d_first (destination range) in such a way that the elements in the destination range are in reverse order. Behaves as if by executing the assignment *(d_first + N - 1 - i) = *(first + i)[1] once for each integer i in
[0, N). If source and destination ranges overlap, the behavior is undefined.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if
|
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. |
(until C++20) |
|
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. |
(since C++20) |
Contents |
[edit] Parameters
| first, last | - | the range of elements to copy |
| d_first | - | the beginning of the destination range |
| Type requirements | ||
-BidirIt must meet the requirements of LegacyBidirectionalIterator.
| ||
-OutputIt must meet the requirements of LegacyOutputIterator.
| ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
| ||
[edit] Return value
Output iterator to the element past the last element copied.
[edit] Complexity
Exactly N assignments.
[edit] Exceptions
The overload with a template parameter named ExecutionPolicy reports errors as follows:
- If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined. - If the algorithm fails to allocate memory, std::bad_alloc is thrown.
[edit] Possible implementation
See also the implementations in libstdc++, libc++, and MSVC STL.
template<class BidirIt, class OutputIt> constexpr // since C++20 OutputIt reverse_copy(BidirIt first, BidirIt last, OutputIt d_first) { for (; first != last; ++d_first) *d_first = *(--last); return d_first; } |
[edit] Notes
Implementations (e.g. MSVC STL) may enable vectorization when the both iterator types satisfy LegacyContiguousIterator and have the same value type, and the value type is TriviallyCopyable.
[edit] Example
Run this code
#include <algorithm> #include <iostream> #include <vector> int main() { auto print = [](const std::vector<int>& v) { for (const auto& value : v) std::cout << value << ' '; std::cout << '\n'; }; std::vector<int> v{1, 2, 3}; print(v); std::vector<int> destination(3); std::reverse_copy(std::begin(v), std::end(v), std::begin(destination)); print(destination); std::reverse_copy(std::rbegin(v), std::rend(v), std::begin(destination)); print(destination); }
Output:
1 2 3 3 2 1 1 2 3
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2074 | C++98 | for each i, the assignment was *(d_first + N - i) = *(first + i)[1] |
corrected to *(d_first + N - 1 - i) = *(first + i)[1] |
| LWG 2150 | C++98 | only one element was required to be assigned | corrected the requirement |
- ↑ 1.0 1.1 1.2 LegacyOutputIterator is not required to support binary
+and-. The usages of+and-here are exposition-only: the actual computation does not need to use them.
[edit] See also
| reverses the order of elements in a range (function template) | |
| (C++20) |
creates a copy of a range that is reversed (niebloid) |
