std::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.

[edit] Nested types

1. ↑ ^{1.0 1.1} The definition is provided by the base std::iterator specialization until C++17.

[edit] Data members

[edit] Member functions

[edit] Non-member functions

[edit] Helper templates

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.

[edit] 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.
};

[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 is MSVC STL's std::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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