std::ranges::rend
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Defined in header <ranges> | ||
Defined in header <iterator> | ||
inlinenamespace/* unspecified */{ inlineconstexpr/* unspecified */ rend=/* unspecified */; | (since C++20) (customization point object) | |
Call signature | ||
template<class T> requires/* see below */ | (since C++20) | |
Returns a sentinel indicating the end of a reversed range.
IfT is an array type andstd::remove_all_extents_t<std::remove_reference_t<T>> is incomplete, then the call toranges::rend is ill-formed, no diagnostic required.
If the argument is an lvalue orranges::enable_borrowed_range<std::remove_cv_t<T>> istrue, then a call toranges::rend isexpression-equivalent to:
- decay-copy(t.rend())(until C++23)auto(t.rend())(since C++23), if that expression is valid and its type modelsstd::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>.
- Otherwise,decay-copy(rend(t))(until C++23)auto(rend(t))(since C++23), if
Tis a class or enumeration type, that expression is valid and its type modelsstd::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>, where the meaning ofrendis established as if by performingargument-dependent lookup only. - Otherwise,std::make_reverse_iterator(ranges::begin(t)) if bothranges::begin(t) andranges::end(t) are valid expressions, have the same type, and that type modelsstd::bidirectional_iterator.
In all other cases, a call toranges::rend is ill-formed, which can result insubstitution failure whenranges::rend(t) appears in the immediate context of a template instantiation.
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Customization point objects
The nameranges::rend denotes acustomization point object, which is a constfunction object of aliteralsemiregular class type. SeeCustomizationPointObject for details.
[edit]Notes
If the argument is an rvalue (i.e.T is an object type) andranges::enable_borrowed_range<std::remove_cv_t<T>> isfalse, or if it is of an array type of unknown bound, the call toranges::rend is ill-formed, which also results in substitution failure.
Ifranges::rend(std::forward<T>(t)) is valid, thendecltype(ranges::rend(std::forward<T>(t))) anddecltype(ranges::begin(std::forward<T>(t))) modelstd::sentinel_for in all cases, whileT modelsstd::ranges::range.
The C++20 standard requires that if the underlyingrend function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposalP0849R8 to match the implementations.
[edit]Example
#include <algorithm>#include <iostream>#include <ranges>#include <vector> int main(){std::vector<int> v={3,1,4};namespace ranges= std::ranges;if(ranges::find(ranges::rbegin(v), ranges::rend(v),5)!= ranges::rend(v))std::cout<<"found a 5 in vector v!\n"; int a[]={5,10,15};if(ranges::find(ranges::rbegin(a), ranges::rend(a),5)!= ranges::rend(a))std::cout<<"found a 5 in array a!\n";}
Output:
found a 5 in array a!
[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 |
|---|---|---|---|
| P2602R2 | C++20 | there's machinery to prohibit certain non-memberrend found byADL | removed such machinery |
[edit]See also
(C++20) | returns a reverse end iterator to a read-only range (customization point object)[edit] |
(C++20) | returns a reverse iterator to a range (customization point object)[edit] |
(C++14) | returns a reverse end iterator for a container or array (function template)[edit] |
