哋它亢++ named requirements: ReversibleContainer

A ReversibleContainer is a Container that has iterators that meet the requirements of either LegacyBidirectionalIterator or LegacyRandomAccessIterator. Such iterators allow a ReversibleContainer to be iterated over in reverse.

Requirements

A type satisfies ReversibleContainer if it satisfies Container, its iterator type belongs to the bidirectional or random access iterator categories and, given the following types and values, the semantic and complexity requirements in the tables below are satisfied:

Type	Definition
`X`	an ReversibleContainer type
`T`	the `value_type` of `X`
Value	Definition
a	a value of type `X`

Types

Name	Type	Requirements
typename X::reverse_iterator	std::reverse_iterator<X::iterator>	an iterator type whose value type is `T`
typename X::const_reverse_iterator	std::reverse_iterator<X::const_iterator>	a constant iterator type whose value type is `T`

Expressions

The types reverse_iterator and const_reverse_iterator in the following table denote typename X::reverse_iterator and typename X::const_reverse_iterator respectively.

Expression	Type	Semantics	Complexity
a.rbegin()	`reverse_iterator` `const_reverse_iterator` for constant a	reverse_iterator(a.end())	Constant
a.rend()	`reverse_iterator` `const_reverse_iterator` for constant a	reverse_iterator(a.begin())	Constant
a.crbegin()	`const_reverse_iterator`	const_cast<const X&>(a).rbegin()	Constant
a.crend()	`const_reverse_iterator`	const_cast<const X&>(a).rend()	Constant

Standard library

Example

The following example iterates over a vector (which has random-access iterators) in reverse.

Run this code

#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v = {3, 1, 4, 1, 5, 9};
 
    for (std::vector<int>::const_reverse_iterator i{v.crbegin()}; i != v.crend(); ++i)
        std::cout << *i << ' ';
    std::cout << '\n';
}

Output:

9 5 1 4 1 3

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2105	哋它亢++98	typename X::const_reverse_iterator was required to be an iterator type of value type const T	required to be a constant iterator type of value type `T`

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