std::reverse_iterator - cppreference.com (original) (raw)
| | | | | ------------------------------------------------ | | | | 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.
Contents
- 1 Nested types
- 2 Data members
- 3 Member functions
- 4 Non-member functions
- 5 Helper templates
- 6 Possible implementation
- 7 Notes
- 8 Example
- 9 See also
[edit] Nested types
- ↑ 1.0 1.1 The definition is provided by the base std::iterator specialization until C++17.
[edit] Data members
| Member | Description |
|---|---|
| Iter current | the underlying iterator(protected member object) |
[edit] Member functions
| (constructor) | constructs a new reverse_iterator (public member function) [edit] |
|---|---|
| operator= | assigns another reverse_iterator (public member function) [edit] |
| base | accesses the underlying iterator (public member function) [edit] |
| operator*operator-> | accesses the pointed-to element (public member function) [edit] |
| operator[] | accesses an element by index (public member function) [edit] |
| operator++operator++(int)operator+=operator+operator--operator--(int)operator-=operator- | advances or decrements the reverse_iterator (public member function) [edit] |
[edit] Non-member functions
| operator==operator!=operator<operator<=operator>operator>=operator<=>(C++20) | compares the underlying iterators (function template) [edit] |
|---|---|
| operator+ | advances the iterator (function template) [edit] |
| operator- | computes the distance between two iterator adaptors (function template) [edit] |
| iter_move(C++20) | casts the result of dereferencing the adjusted underlying iterator to its associated rvalue reference type (function) [edit] |
| iter_swap(C++20) | swaps the objects pointed to by two adjusted underlying iterators (function template) [edit] |
| make_reverse_iterator(C++14) | creates a std::reverse_iterator of type inferred from the argument (function template) [edit] |
[edit] Helper templates
| template< class Iterator1, class Iterator2 > requires (<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.
[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 reverse_iterator
{
protected:
It current = It();
public:
reverse_iterator() = default;
constexpr explicit reverse_iterator(It itr) : current(itr) {}
template
requires (<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 #include #include 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:
[edit] See also
| | creates a std::reverse_iterator of type inferred from the argument (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | base class to ease the definition of required types for simple iterators (class template) [edit] |