std::reverse_iterator is an iterator adaptor that reverses the direction of a given iterator, which must be at least aLegacyBidirectionalIterator or modelbidirectional_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 iteratorr constructed from an iteratori, the relationship&*r==&*(i-1) is alwaystrue (as long asr isdereferenceable); 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 functionsrbegin() andrend() of thestandard library containers.

[edit]Nested types

1. ↑^1.01.1The definition is provided by the basestd::iterator specialization until C++17.

[edit]Data members

[edit]Member functions

[edit]Non-member functions

[edit]Helper templates

This partial specialization ofstd::disable_sized_sentinel_for prevents specializations ofreverse_iterator from satisfyingsized_sentinel_for if their underlying iterators do not satisfy the concept.

[edit]Possible implementation

Below is a partial implementation focusing on the way the inner iterator is stored, callingstd::prev only when the content is fetched viaoperator*.

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.};

[edit]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 isMSVC STL'sstd::filesystem::path::iterator.

[edit]Example

#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';}

35 25 15 5

[edit]See also

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