CODEWITHSUNDEEP
c++iterator
sid
sid

Reputation: 165

C++ reverse iterator based on existing random access iterator?

I have implemented a (non-const) random access iterator for my custom container class by inheriting from the very basic std::iterator template class (see below). All that was required to do this was to pass in the required types.

I have not been able to find much information on how to set up a reverse iterator, but I'm pretty sure there's a method where I can use the existing iterator with a few new typedefs and define rbegin() and rend(). And that's where I'm stuck. Could someone help?

template <class T, class A = std::allocator<T>>
class ring {
public:
    typedef ring<T, A> self_type;
    typedef T value_type;
    typedef A alloc_type;
    typedef ptrdiff_t size_type;
    typedef typename alloc_type::difference_type    difference_type;
    typedef typename alloc_type::pointer            pointer;
    typedef typename alloc_type::reference          reference;
    typedef typename alloc_type::const_pointer      const_pointer;
    typedef typename alloc_type::const_reference    const_reference;

    class iterator;  // we implement iterator as nested class, so no need to typedef it

    iterator begin() { return iterator(this, 0); }
    iterator end() { return iterator(this, size()); }  // the current size (which is one past the end of the array)

    class iterator : public std::iterator<std::random_access_iterator_tag, value_type, difference_type, pointer, reference> {
    private:
        self_type *ring_;  // typedefs defined in ring class apply here
        size_type offset_;
    public:
        iterator(self_type *r, size_type o) : ring_{r}, offset_{o} {}
        reference operator* () const { return (*ring_)[offset_]; }
        pointer operator-> () const { return &(operator*()); }
        iterator& operator++ () {
            ++offset_;
            return *this;
        }
        friend bool operator== (const iterator& it1, const iterator& it2) { return ((it1.ring_ == it2.ring_ && it1.offset_ == it2.offset_)); }
        friend bool operator!= (const iterator& it1, const iterator& it2) { return (!(it1 == it2)); }
        iterator& operator-- () {
            --offset_;
            return *this;
        }
        iterator operator++ (int) {
            iterator clone(*this);  // make a duplicate
            ++offset_;
            return clone;  // return the duplicate
        }
        iterator operator-- (int) {  // has to be return by value
            iterator clone(*this);
            --offset_;
            return clone;
        }
        iterator& operator+=(size_type n) {
            offset_ += n;
            return *this;
        }       
        iterator& operator-=(size_type n) {
            offset_ -= n;
            return *this;
        }
        ...
        reference operator[] (size_type n) const { return (*ring_)[n]; }
    };
};

Upvotes: 2

Views: 632

Answers (1)

L. F.
L. F.

Reputation: 20619

You need to define the reverse_iterator type alias and the rbegin and rend methods. The reverse iterator should be implemented with std::reverse_iterator (defined in header <iterator>)

using       reverse_iterator = std::reverse_iterator<      iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;

      reverse_iterator rbegin()       { return end  (); }
const_reverse_iterator rbegin() const { return end  (); }
      reverse_iterator rend  ()       { return begin(); }
const_reverse_iterator rend  () const { return begin(); }

const_reverse_iterator crbegin() const { return rbegin(); }
const_reverse_iterator crend  () const { return rend  (); }

And that's everything. No magic.

Upvotes: 3

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