What is the value of NSEnumerator? - objective-c

Question

when for . . . in . . . is available?
Specifically, when we can write:
NSArray *array;
// array allocated and initialized here.
for (id obj in array) {
// do something to the object here
}
Why would we ever use an NSEnumerator?

Answer 1

NSEnumerator was created before fast enumeration (for/in loop) was available. Think of it as backward-compatibility if you like.
But with NSEnumerator you can enumerate the collection in customized order, e.g. backwards:
NSEnumerator* enu = [array reverseObjectEnumerator];
id object;
while ((object = [enu nextObject])) {
...
}
(Of course, since NSEnumerator also supports for/in loop you can use a better way:
for (id object in [array reverseObjectEnumerator]) {
...
}
)
or define your own iterator class by subclassing NSEnumerator, e.g.
#import <Foundation/Foundation.h>
#interface RangeEnumerator : NSEnumerator {
int cur, len;
}
+(RangeEnumerator*)enumeratorWithLength:(int)length;
-(id)initWithLength:(int)length;
-(id)nextObject;
#end
#implementation RangeEnumerator
-(id)initWithLength:(int)length {
if ((self = [super init]))
len = length;
return self;
}
+(RangeEnumerator*)enumeratorWithLength:(int)length {
return [[(RangeEnumerator*)[self alloc] initWithLength:length] autorelease];
}
-(id)nextObject {
if (cur < len)
return [NSNumber numberWithInt:cur++];
else
return nil;
}
#end
int main () {
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
for (NSNumber* num in [RangeEnumerator enumeratorWithLength:12])
printf("%d\n", [num intValue]);
[pool drain];
return 0;
}