Re: [Boost-users] Iterators and subclasses

Subject: Re: [Boost-users] Iterators and subclasses
From: Jeff Flinn (TriumphSprint2000_at_[hidden])
Date: 2009-10-12 15:01:19

Swerts, Benjamin wrote:
> Hello all,
> 
> I'm trying to create a base class that provides 2 iterators (begin/end).
> These will be used to iterate over data structures that will be provided
> by subclasses. These data structures are completely unrelated. Normally
> this can be solved using virtual functions but the problem is that
> iterators are normally returned by value so I cannot go that route.
> 
> My concrete problem:
> 
> struct Point { int x, y; };
> class PointIterator : public boost::iterator_facade<PointIterator, const
> Point, boost::random_access_traversal_tag, Point>;
> class PointCollection
> {
>   public:
>     typedef PointIterator const_iterator;
>     const_iterator begin();
>     const_iterator end();
> };
> 
>>From the PointCollection base class I want to derive concrete
> implementations:
> 
> class Collection1 : public PointCollection
> {
>   public:
>     Collection1(char* data, size_t size) : data_(data), size_(size) {};
>   private:
>     char* data_;
>     size_t size_;
> };
> 
> Collection1 would return a Point with x = data_[index] and y = index *
> 1000, for example. Other subclasses might contain 2 vectors or just
> generated values.
> In my first attempt I tried to subclass the iterator too and make it
> return the correct Point when the iterator is dereferenced. As the
> iterators are returned by value, however, the subclass was sliced right
> off.
> An iterator-less solution is easy:
> 
> class PointCollection
> {
>   public:
>     virtual Point getPoint(size_t index) const = 0;
>     virtual size_t size() const = 0;
> };

Can't your iterator just hold on to a "container" base class ref, an 
incrementable index and call the above virtual method in the iterator's 
deref implementation?

Jeff