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.

Nested types

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)

Type Definition iterator_type Iter iterator_concept std::random_access_iterator_tag if Iter models std::random_access_iterator std::bidirectional_iterator_tag otherwise iterator_category std::random_access_iterator_tag if std::iterator_traits<Iter>::iterator_category models std::derived_from<std::random_access_iterator_tag> std::iterator_traits<Iter>::iterator_category otherwise 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)

Data members

Member	Description
Itercurrent	the underlying iterator (protected member object)

Member functions

(constructor)	constructs a new reverse_iterator (public member function)
operator=	assigns another reverse_iterator (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 reverse_iterator (public member function)

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>) inlineconstexprbool 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;constexprexplicit reverse_iterator(It itr): current(itr){}template<class U> requires (!std::is_same_v<U, It>&&std::convertible_to<const U&, It>)constexprexplicit 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'sstd::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 <<' ';std::cout<<'\n';}

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)