std::reverse_iterator

 

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std::reverse_iterator

Member functions

reverse_iterator::reverse_iterator

reverse_iterator::operator=

reverse_iterator::base

reverse_iterator::operator*reverse_iterator::operator->

reverse_iterator::operator[]

reverse_iterator::operator++reverse_iterator::operator+reverse_iterator::operator+=reverse_iterator::operator--reverse_iterator::operator-reverse_iterator::operator-=

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

operator+

operator-

iter_move (C++20)

iter_swap (C++20)

make_reverse_iterator (C++14)

 

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 stored, calling std::prev only when the content is fetched via operator*.

template<class It> class reverse_iterator { protected: It current = It(); public: reverse_iterator() = default; constexpr explicit reverse_iterator(It itr) : current(itr) {} template<class U> requires (!std::is_same_v<U, It> && std::convertible_to<const U&, It>) constexpr explicit reverse_iterator(const U& other) : current(other.base()) {}   constexpr decltype(auto) operator*() const { return *std::prev(current); // <== returns the content of prev }   constexpr reverse_iterator& operator++() { --current; return *this; } constexpr reverse_iterator operator++(int) { auto tmp = *this; ++(*this); return tmp; }   constexpr reverse_iterator& operator--() { ++current; return *this; } constexpr reverse_iterator operator--(int) { auto tmp = *this; --(*this); return tmp; }   constexpr It base() const { return current; }   // Other member functions, friend functions, and member typedefs are not shown here. };

Notes

std::reverse_iterator does not work with iterators whose dereference returns a reference to a member of *this (so-called "stashing iterators"). An example of a stashing iterator is MSVC STL's std::filesystem::path::iterator.

Example

Run this code

#include <cstddef>
#include <iostream>
#include <iterator>
 
template <typename T, std::size_t SIZE>
class Stack
{
    T arr[SIZE];
    std::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

See also

make_reverse_iterator (C++14)	creates a std::reverse_iterator of type inferred from the argument (function template)
iterator (deprecated in C++17)	base class to ease the definition of required types for simple iterators (class template)