NSMapTableKeyCallBacks
typedef struct { unsigned (*hash)(NSMapTable *table, const void *); BOOL (*isEqual)(NSMapTable *table, const void *, const void *); void (*retain)(NSMapTable *table, const void *); void (*release)(NSMapTable *table, void *); NSString *(*describe)(NSMapTable *table, const void *); const void *notAKeyMarker; } NSMapTableKeyCallBacks;