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#### 8.9.4 Fast enumeration protocol

If you want your own collection object to be usable with fast enumeration, you need to have it implement the method

         - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state
                                               objects: (id *)objects
                                                 count: (unsigned long)len;

where `NSFastEnumerationState` must be defined in your code as follows:

         typedef struct
         {
           unsigned long state;
           id            *itemsPtr;
           unsigned long *mutationsPtr;
           unsigned long extra[5];
         } NSFastEnumerationState;

If no `NSFastEnumerationState` is defined in your code, the compiler will automatically replace `NSFastEnumerationState *` with `struct __objcFastEnumerationState *`, where that type is silently defined by the compiler in an identical way. This can be confusing and we recommend that you define `NSFastEnumerationState` (as shown above) instead.

The method is called repeatedly during a fast enumeration to retrieve batches of objects. Each invocation of the method should retrieve the next batch of objects.

The return value of the method is the number of objects in the current batch; this should not exceed `len`, which is the maximum size of a batch as requested by the caller. The batch itself is returned in the `itemsPtr` field of the `NSFastEnumerationState` struct.

To help with returning the objects, the `objects` array is a C array preallocated by the caller (on the stack) of size `len`. In many cases you can put the objects you want to return in that `objects` array, then do `itemsPtr = objects`. But you don't have to; if your collection already has the objects to return in some form of C array, it could return them from there instead.

The `state` and `extra` fields of the `NSFastEnumerationState` structure allows your collection object to keep track of the state of the enumeration. In a simple array implementation, `state` may keep track of the index of the last object that was returned, and `extra` may be unused.

The `mutationsPtr` field of the `NSFastEnumerationState` is used to keep track of mutations. It should point to a number; before working on each object, the fast enumeration loop will check that this number has not changed. If it has, a mutation has happened and the fast enumeration will abort. So, `mutationsPtr` could be set to point to some sort of version number of your collection, which is increased by one every time there is a change (for example when an object is added or removed). Or, if you are content with less strict mutation checks, it could point to the number of objects in your collection or some other value that can be checked to perform an approximate check that the collection has not been mutated.

Finally, note how we declared the `len` argument and the return value to be of type `unsigned long`. They could also be declared to be of type `unsigned int` and everything would still work.