std::reverse_iterator

Defined in header `<iterator>`
template< class Iter > class reverse_iterator;

std::reverse_iterator is an iterator adaptor that reverses the direction of a given iterator, which must be at least a LegacyBidirectionalIterator or model bidirectional_iterator (since C++20). In other words, when provided with a bidirectional iterator, std::reverse_iterator produces a new iterator that moves from the end to the beginning of the sequence defined by the underlying bidirectional iterator.

For a reverse iterator r constructed from an iterator i, the relationship &*r == &*(i-1) is always true (as long as r is dereferenceable); thus a reverse iterator constructed from a one-past-the-end iterator dereferences to the last element in a sequence.

This is the iterator returned by member functions rbegin() and rend() of the standard library containers.

Member types

Member type	Definition
`iterator_type`	`Iter`
`iterator_category`	std::iterator_traits<Iter>::iterator_category
`value_type`	std::iterator_traits<Iter>::value_type
`difference_type`	std::iterator_traits<Iter>::difference_type
`pointer`	std::iterator_traits<Iter>::pointer
`reference`	std::iterator_traits<Iter>::reference

(until C++20)

Member type	Definition
`iterator_type`	`Iter`
`iterator_concept`	If `Iter` models std::random_access_iterator, this is std::random_access_iterator_tag. Otherwise, this is std::bidirectional_iterator_tag
`iterator_category`	If std::iterator_traits<Iter>::iterator_category models std::derived_from<std::random_access_iterator_tag>, this is std::random_access_iterator_tag. Otherwise, this is std::iterator_traits<Iter>::iterator_category
`value_type`	std::iter_value_t<Iter>
`difference_type`	std::iter_difference_t<Iter>
`pointer`	std::iterator_traits<Iter>::pointer
`reference`	std::iter_reference_t<Iter>

(since C++20)

Member types iterator_category, value_type, difference_type, pointer and reference are required to be obtained by inheriting from std::iterator<
std::iterator_traits<Iter>::iterator_category
, std::iterator_traits<Iter>::value_type
, std::iterator_traits<Iter>::difference_type
, std::iterator_traits<Iter>::pointer
, std::iterator_traits<Iter>::reference
>.

(until C++17)

Member functions

(constructor)	constructs a new iterator adaptor (public member function)
operator=	assigns another iterator adaptor (public member function)
base	accesses the underlying iterator (public member function)
operator*operator->	accesses the pointed-to element (public member function)
operator[]	accesses an element by index (public member function)
operator++operator++(int)operator+=operator+operator--operator--(int)operator-=operator-	advances or decrements the iterator (public member function)

Member objects

Member name	Definition
`current` (protected)	the underlying iterator of which `base()` returns a copy

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)	compares the underlying iterators (function template)
operator+	advances the iterator (function template)
operator-	computes the distance between two iterator adaptors (function template)
iter_move (C++20)	casts the result of dereferencing the adjusted underlying iterator to its associated rvalue reference type (function)
iter_swap (C++20)	swaps the objects pointed to by two adjusted underlying iterators (function template)
make_reverse_iterator (C++14)	creates a std::reverse_iterator of type inferred from the argument (function template)

Helper templates

template< class Iterator1, class Iterator2 > requires (!std::sized_sentinel_for<Iterator1, Iterator2>) inline constexpr bool disable_sized_sentinel_for< std::reverse_iterator<Iterator1>, std::reverse_iterator<Iterator2>> = true;		(since C++20)

This partial specialization of std::disable_sized_sentinel_for prevents specializations of reverse_iterator from satisfying sized_sentinel_for if their underlying iterators do not satisfy the concept.

Possible implementation

Below is a partial implementation focusing on the way the inner iterator is saved, calling prev only when the content is fetched via operator*.

template<typename Itr>
class reverse_iterator {
    Itr itr;
public:
    constexpr explicit reverse_iterator(Itr itr): itr(itr) {}
    constexpr auto& operator*() {
        return *std::prev(itr); // <== returns the content of prev
    }
    constexpr auto& operator++() {
        --itr;
        return *this;
    }
    constexpr friend bool operator!=(reverse_iterator<Itr> a, reverse_iterator<Itr> b) {
        return a.itr != b.itr;
    }
};

Notes

std::reverse_iterator does not work with iterators whose dereference returns a reference to a member of *this (so-called "stashing iterators").

Example

Run this code

#include <iostream>
#include <iterator>
 
template<typename T, size_t SIZE>
class Stack {
    T arr[SIZE];
    size_t pos = 0;
public:
    T pop() {
        return arr[--pos];
    }
    Stack& push(const T& t) {
        arr[pos++] = t;
        return *this;
    }
    // we wish that looping on Stack would be in LIFO order
    // thus we use std::reverse_iterator as an adaptor to existing iterators
    // (which are in this case the simple pointers: [arr, arr+pos)
    auto begin() {
        return std::reverse_iterator(arr + pos);
    }
    auto end() {
        return std::reverse_iterator(arr);
    }
};
 
int main() {
    Stack<int, 8> s;
    s.push(5).push(15).push(25).push(35);
    for(int val: s) {
        std::cout << val << ' ';
    }
}

Output:

35 25 15 5

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries