NAME

std::reverse_iterator - std::reverse_iterator

Synopsis

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.

range-rbegin-rend.svg

Member types

Member type Definition
iterator_type Iter
iterator_category std::iterator_traits<Iter>::iterator_category (until
value_type std::iterator_traits<Iter>::value_type C++20)
difference_type std::iterator_traits<Iter>::difference_type
pointer std::iterator_traits<Iter>::pointer
reference std::iterator_traits<Iter>::reference
Member type Definition
iterator_type Iter
If Iter models std::random_access_iterator, this is
iterator_concept std::random_access_iterator_tag. Otherwise, this is
std::bidirectional_iterator_tag
If std::iterator_traits<Iter>::iterator_category models (since
iterator_category std::derived_from<std::random_access_iterator_tag>, this is C++20)
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>

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 (until C++17)
, std::iterator_traits<Iter>::difference_type
, std::iterator_traits<Iter>::pointer
, std::iterator_traits<Iter>::reference
>.

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* accesses the pointed-to element
operator-> (public member function)
operator[] accesses an element by index
(public member function)
operator++
operator++(int)
operator+=
operator+ advances or decrements the iterator
operator-- (public member function)
operator--(int)
operator-=
operator-

Member objects

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

Non-member functions

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

Helper templates

template< class Iterator1, class Iterator2 >

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

std::reverse_iterator<Iterator2>> = true;

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