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An enumerator is an object that returns successive elements of a sequence, such as an array.

Usually used to access the contents of collections; see FoundationCollections for a general overview.


For a general discussion about the utility of NSEnumerators and enumerators/iterators in general, refer to the OOP classic Design Patterns by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides. Specifically, check out the Iterator design pattern on page 257. Caveat Emptor! The code samples in this book are in C++ with a bit of Smalltalk. Nevertheless, if you want to learn about general software design, this is an excellent book to read, and there is quite a bit of conceptual discussion that is beneficial.

Could an overview be provided for those of us who will never, ever have an opportunity to read this excellent work? – RobRix

Browse around:

Use NSEnumerator to loop through an array via a while loop to perform some operation on or with each object in the array. If the array is empty, nothing happens in the while loop.

Example Code:

NSArray * myArray = [NSArray arrayWithObjects:@”One”, @”Two”, @”Three”, nil]; NSEnumerator * myArrayEnumerator = [myArray objectEnumerator]; NSString *thisObject; while (thisObject = [myArrayEnumerator nextObject]) { NSLog(@”thisObject: %@”, thisObject); }


thisObject: One thisObject: Two thisObject: Three

Can someone explain to me why you would ever bother to use an NSEnumerator to run over an array since all the tests I have seen show that they are far slower than the C equivilent…

NSArray *myArray = [NSArray arrayWithObjects:@”One”, @”Two”, @”Three”, nil]; unsigned count; unsigned arrayCount = [myArray count]; for (count = 0; count < arrayCount; count++) { NSLog (@”thisObject: %@”,[myArray objectAtIndex:0]); }


There are a couple of reasons. One is speed. Yes, that’s right. NSEnumerator can be faster than a for loop. See NSEnumeratorSpeed for test code and results. The reason is that NSEnumerator can take advantage of knowing NSArray’s internal structure and optimize its use of it for linear access, whereas continually calling -objectAtIndex: can’t.

Another reason is to write more generic code. If your code uses NSEnumerator, then it will work not only with arrays, but also with sets, dictionaries and anything else that responds to -objectEnumerator without having to change anything.

It also avoids off-by-one errors.

They’re also great to use if you don’t know the type of the thing you are enumerating, only that it is enumerable. For example, imagine a method that operates on a collection of objects. If you only need to enumerate it, and don’t need random access, a set or an array will do just fine, so the contract of the method could be that the collection parameter is just something that responds to objectEnumerator. Jon H.