a glob of nerdishness

Although the NSFastEnumeration Protocol reference is pretty obtuse, the linked article tells all you need to know about how fast enumeration is implemented. It’s not really all that complicated — or pretty, for that matter — but it nonetheless allows for (... in ...) looping over any conforming class. All that said conforming class has to do is give out a pointer to its own internal structures when possible, or, at worst, copy a handful of pointers at a time into some pre-allocated stack space.