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std::reverse_iterator(3) C++ Standard Libary std::reverse_iterator(3) 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 di- rection 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 rela- tionship &*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_itera- tor_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 in- ferred 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 pre- vents specializations of reverse_iterator from satisfying sized_sen- tinel_for if their underlying iterators do not satisfy the concept. Possible implementation Below is a partial implementation focusing on the way the inner it- erator 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) : cur- rent(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 it- erators // (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 creates a std::reverse_iterator of type in- ferred from the (C++14) argument (function template) iterator base class to ease the definition of required types for simple (deprecated in C++17) iterators (class template) http://cppreference.com 2024.06.10 std::reverse_iterator(3)
NAME | Synopsis | Member types | Member functions | Member objects | Non-member functions | Possible implementation | Notes | Example | Output: | See also
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