Nested class definition in Abstract Base Class (C++)

Question

Are there any suggestions on how to use a nested class iterator in an ABC in C++ ? Note that, I also want to have a virtual function returning an instance of this class.

More specifically here's my approach:

class ABC {
        typedef iterator<forward_iterator_tag, MyType> MyTypeIter;
        virtual MyTypeIter *begin() = 0;
};

class Foo : ABC {
        MyTypeIter : public ABC::MyTypeIter;
        virtual MyTypeIter *begin();
};

ABC::MyTypeIter *Foo::begin()
{
        Foo::MyTypeIter *ret;
        ret = new Foo::MyTypeIter(...);
        return ret;
}

Is there a better approach than this (e.g. one that does not use pointers) ?

Answer 1

Take a look on how iterators are implemented for std::vector class, for example.

Answer 2

What is your problem? A nested class behaves the same way as a top-level class, so you may return its objects just as you would have returned any other.

Answer 3

I prefer to keep iterators interal to the class and exposing only an interface for iteration.
For example, you can implement a List<> with the methods:

• void prepare_iteration() // reset the internal iterator
• bool step_iteration() // move internal iterator
• DATA_TYPE & read() // return the data by using the iterator
• write( DATA_TYPE & ) // update the data by using the iterator

In this example the iterator can be a simple node pointer and it's never exposed to the user. I find this approach much easier and safer than iterator objects.
(well the 'safer' part needs a lot of discussion)

1. The above interface can be implemented as an abstract class.
2. Your container (or whatever) classes can inherit it and implement it.

I know that's not the answer that you are looking for but it's just an alternative idea to design your classes.