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
here is what happens when you run it:
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
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
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
. This allows you to determine whether the iteration finished because a
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
must not make any changes to the collection object; if they do, it is a hard error and the fast enumeration terminates by invoking
, a runtime function that normally aborts the program but which can be customized by Foundation libraries via
to do something different, such as raising an exception.