I'm trying to implement a reverse-iterator adaptor for my iterator and const_iterator classes with a little bit of trouble. If anyone could guide me through this, that would be greatly appreciated!
The idea is that I should be able to create a reverse-iterator from my rbegin() and rend() function calls
reverse_iterator rbegin();
reverse_iterator rend();
const_reverse_iterator rbegin() const;
const_reverse_iterator rend() const;
I'm using the following typedef's in the class:
typedef btree_iterator<T> iterator;
typedef const_btree_iterator<T> const_iterator;
typedef reverse_btree_iterator<iterator> reverse_iterator;
typedef reverse_btree_iterator<const_iterator> const_reverse_iterator;
As you can see, I would like to be able to create reverse-iterators using templates, giving the reverse_iterator class either an iterator or const_iterator.
Unfortunately, it is this bit I'm stuck on...
Below is the class definition that I currently have, with errors.
template <typename I> class reverse_btree_iterator {
typedef ptrdiff_t difference_type;
typedef bidirectional_iterator_tag iterator_category;
public:
reverse_btree_iterator() : base_(I()) {}
template <typename T> reverse_btree_iterator(const btree_iterator<T>& rhs) : base_(rhs) {}
I base() { return base_; }
I::reference operator*() const;
I::pointer operator->() const;
I& operator++();
I operator++(int);
I& operator--();
I operator--(int);
bool operator==(const I& other) const;
bool operator!=(const I& other) const;
private:
I base_;
};
I've never used templates like this before, so it is very likely I'm completely misunderstanding how they can be used...
Since I can be an iterator or a const_iterator, the typedef of reference and pointer vary between the two classes. The lines that aren't compiling are these:
I::reference operator*() const;
I::pointer operator->() const;
I'm not sure how else I can make the one reverse_iterator class work for both iterator and const_iterator if I'm not able to do I::reference and I::pointer. I also tried adding template in front of those, since they are defined in the iterator class (for example) as:
typedef T* pointer;
typedef T& reference;
Iterator adaptors are a separate type of iterators with special behavior. They simplify the work with containers and are very useful in standard algorithms.
A reverse iterator is made from a bidirectional, or random access iterator which it keeps as a member which can be accessed through base() . To iterate backwards use rbegin() and rend() as the iterators for the end of the collection, and the start of the collection respectively.
A const iterator points to an element of constant type which means the element which is being pointed to by a const_iterator can't be modified. Though we can still update the iterator (i.e., the iterator can be incremented or decremented but the element it points to can not be changed).
reference and pointer are dependent names, so you have to use
typename I::reference operator*() const;
typename I::pointer operator->() const;
In addition, the constructor should accept just the I
.
However, there is no need to write this class at all. The standard library has reverse_iterator for this. Or if you are not happy with that, there's also Boost.ReverseIterator.
All it takes is just
typedef std::reverse_iterator<iterator> reverse_iterator;
typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
In addition, you forgot to provide comparison operators with other reverse iterators of the same type. This is a reverse iterator requirement.
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