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;
}