C++ 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
typenameX::reverse_iterator	std::reverse_iterator<X::iterator>	an iterator type whose value type is `T`
typenameX::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 typenameX::reverse_iterator and typenameX::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

Library types

The following standard library types satisfy ReversibleContainer requirements:

array (C++11)	fixed-sized inplace contiguous array (class template)
deque	double-ended queue (class template)
list	doubly-linked list (class template)
vector	resizable contiguous array (class template)
inplace_vector (C++26)	resizable, fixed capacity, inplace contiguous array (class template)
map	collection of key-value pairs, sorted by keys, keys are unique (class template)
multimap	collection of key-value pairs, sorted by keys (class template)
set	collection of unique keys, sorted by keys (class template)
multiset	collection of keys, sorted by keys (class template)

Example

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

#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 C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2105	C++98	typenameX::const_reverse_iterator was required to be an iterator type of value type constT	required to be a constant iterator type of value type `T`

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