If you plan to design a container class of objects, you often want to access each element of the container. Iterators can do this. Here is a simple recipe for creating iterators for your containers:

• Containers are classes that contain zero, one or more elements.
• An iterator is a class that can be used to access each member of its associated container.
• Iterators (I) are intimately associated with their container classes (C). Thus, iterators are defined as nested public classes (C::I) inside their related container classes.
• Due to C++ access rules, iterators and container classes are often mutual friend classes of each other.
• In C++, iterators are often treated as generalized pointers. The following operations are defined on iterators: *(dereference), ->, ++, ==, !=, The copy, and assignment operators are also defined, if they make sense.
• There must be two container class member functions : C::begin() and C::end() in each container class that has iterators, .
• C::end() returns an iterator pointing “one past” the last element of the container. Often C::end() returns a default iterator. C::begin() returns an iterator pointing to the first element in the container. If the container is empty, C::begin() returns the value of C::end().

The C::begin() member function connects the this pointer for the container with each defined iterator.

To save space below, many normal member functions have been omitted from this executable example.

Iterators come in five categories, remembered using the acronym BRIEF: Bi-directional, Random, Input, Emit/output, and Forward.

• One pass, forward only.
• Can’t be dereferenced, copied or assigned.
• No write access
• No lvalue, can be on the left hand side of an assignment

• One pass, forward only.
• Dereferencing only on the left hand side of an assignment.
• No read access
• Usually only one output iterator instantiated at a time

• Can move forward.
• Iterator values can be stored and retrieved – can be dereferenced on left or right side of an assignment.

• Have all the attributes of Forward iterators, and can move backwards.

• Have all attributes of Bi-Directional iterators plus are able to access entries in the container in any order. Member function at() and operator[] are defined.

The container’s design solely determines the category of iterator. You can not impose an iterator or iterator category onto a container from the outside.

Material for designing iterators can be found in Designing Components with the C++ STL by Ulrich Breymann, Addison-Wesley, ISBN 0-201-67488-2. And again, Nicolai M. Josuttis’s The C++ Standard Template Library, Addison-Wesley, ISBN 0-201-37926-0 is also an excellent source for examples and information on iterators.

Iterator Example

Editorial

I have had a lot of fun designing, creating, and writing this newsletter. But in the last issue, I had gone too far and I was not serving you and for this I offer my apologies. Last month’s editorial comment was the largest part of the newsletter. It did not start out that way, but time and other factors got in the way. I also realized that in my first issue, I promised to be pithy, to the point and technically oriented. Again, that did not happen, except for the excellent technical article by Michael Ball on Compile-Time Template Calculations. Lastly, the technical content was almost buried on page three. All in all, not a good way to go.

So I am now going to make a commitment to you. One that is measurable and one that I can use as a benchmark. The editorial content of the newsletter will be limited to one column in this four-page format (i.e., 12% of the newsletter). Second, the technical articles are going to have pride of place. Each newsletter will start with technical material, not editorial, on page one. The editorial will not appear on page one unless really extraordinary circumstances dictate that it do so. Lastly, some form of the newsletter will appear on the web, hopefully, by the time you read this.

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