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In computer programming, an iterator is an object that progressively provides access to each item of a collection, in order.[1][2][3]
A collection may provide multiple iterators via its interface that provide items in different orders, such as forwards and backwards.
An iterator is often implemented in terms of the structure underlying a collection implementation and is often tightly coupled to the collection to enable the operational semantics of the iterator.
An iterator is behaviorally similar to a database cursor.
Iterators date to the CLU programming language in 1974.
- ^ Gatcomb, Joshua (Jun 16, 2005). "Understanding and Using Iterators". Perl.com. Retrieved 2012-08-08.
A user-defined iterator usually takes the form of a code reference that, when executed, calculates the next item in a list and returns it. When the iterator reaches the end of the list, it returns an agreed-upon value.
- ^ Watt, Stephen M. (September 16, 2006). "A Technique for Generic Iteration and Its Optimization" (PDF). The University of Western Ontario, Department of Computer Science. Retrieved 2012-08-08.
Iterators were introduced as constructs to allow looping over abstract data structures without revealing their internal representation.
- ^ Alex Allain. "STL Iterators". Cprogramming.com - Your resource for C and C++. Retrieved 2012-08-08.
You can think of an iterator as pointing to an item that is part of a larger container of items.