Iterators in C++ (stl) vs Java, is there a conceptual difference?

Question

I\'m returning to c++ after being away for a bit and trying to dust off the old melon.

In Java Iterator is an interface to a container having methods: hasNext

Answer 1

Perhaps a bit more theoretical. Mathematically, collections in C++ can be described as a half-open interval of iterators, namely one iterator pointing to the start of the collection and one iterator pointing just behind the last element.

This convention opens up a host of possibilities. The way algorithms work in C++, they can all be applied to subsequences of a larger collection. To make such a thing work in Java, you have to create a wrapper around an existing collection that returns a different iterator.

Another important aspect of iterators has already been mentioned by Frank. There are different concepts of iterators. Java iterators correspond to C++' input iterators, i.e. they are read-only iterators that can only be incremented one step at a time and can't go backwards.

On the other extreme, you have C pointers which correspond exactly to C++' concept of a random access iterator.

All in all, C++ offers a much richer and purer concept that can be applied to a much wider variety of tasks than either C pointers or Java iterators.

Answer 2

A pointer to an array element is indeed an iterator into the array.

As you say, in Java, an iterator has more knowledge of the underlying container than in C++. C++ iterators are general, and a pair of iterators can denote any range: this can be a sub-range of a container, a range over multiple containers (see http://www.justsoftwaresolutions.co.uk/articles/pair_iterators.pdf or http://www.boost.org/doc/libs/1_36_0/libs/iterator/doc/zip_iterator.html) or even a range of numbers (see http://www.boost.org/doc/libs/1_36_0/libs/iterator/doc/counting_iterator.html)

The iterator categories identify what you can and can't do with a given iterator.

Answer 3

C++ iterators are a generalization of the pointer concept; they make it applicable to a wider range of situations. It means that they can be used to do such things as define arbitrary ranges.

Java iterators are relatively dumb enumerators (though not so bad as C#'s; at least Java has ListIterator and can be used to mutate the collection).

Answer 4

C++ library (the part formerly known as STL) iterators are designed to be compatible with pointers. Java, without pointer arithmetic, had the freedom to be more programmer-friendly.

In C++ you end up having to use a pair of iterators. In Java you either use an iterator or a collection. Iterators are supposed to be the glue between algorithm and data structure. Code written for 1.5+ rarely need mention iterators, unless it is implementing a particular algorithm or data structure (which the vary majority of programmers have no need to do). As Java goes for dynamic polymorphism subsets and the like are much easier to handle.

Answer 5

To me the fundamental difference is that Java Iterators point between items, whereas C++ STL iterators point at items.

Answer 6

Yes, there is a large conceptual difference. C++ utilizes different "classes" of iterators. Some are used for random access (unlike Java), some are used for forward access (like java). While even others are used for writing data (for use with, say, transform).

See the iterators concept in the C++ Documentation:

• Input Iterator
• Output Iterator
• Forward Iterator
• Bidirectional Iterator
• Random Access Iterator

These are far more interesting and powerful compared to Java/C#'s puny iterators. Hopefully these conventions will be codified using C++0x's Concepts.