8.9.3 Fast Enumeration Details

Here is a more technical description with the gory details. Consider the code

for (object expression in collection expression)
{
  statements
}

here is what happens when you run it:

• collection expression is evaluated exactly once and the result is used as the collection object to iterate over. This means it is safe to write code such as for (object in [NSDictionary keyEnumerator]) ....
• the iteration is implemented by the compiler by repeatedly getting batches of objects from the collection object using the fast enumeration protocol (see below), then iterating over all objects in the batch. This is faster than a normal enumeration where objects are retrieved one by one (hence the name “fast enumeration”).
• if there are no objects in the collection, then object expression is set to nil and the loop immediately terminates.
• if there are objects in the collection, then for each object in the collection (in the order they are returned) object expression is set to the object, then statements are executed.
• statements can contain break and continue commands, which will abort the iteration or skip to the next loop iteration as expected.
• when the iteration ends because there are no more objects to iterate over, object expression is set to nil. This allows you to determine whether the iteration finished because a break command was used (in which case object expression will remain set to the last object that was iterated over) or because it iterated over all the objects (in which case object expression will be set to nil).
• statements must not make any changes to the collection object; if they do, it is a hard error and the fast enumeration terminates by invoking objc_enumerationMutation, a runtime function that normally aborts the program but which can be customized by Foundation libraries via objc_set_mutation_handler to do something different, such as raising an exception.