for(;;) vs. for(:) in Objective-C Performance and Practice

Question

Is there any different between for(;;) and for(:) in terms of performance in Objective-C? And what are good practices to use for(;;) or for(:)?

Answer 1

I'll assume that in each case you are enumerating a collection of objects, as only the C for(;;) form allows enumeration of primitive types. The for(in) construction uses a protocol called NSFastEnumeration to fill a buffer with objects to use in future iterations, and uses a cursor to track which object it's got up to. That makes it faster than:

NSEnumerator *e = [collection objectEnumerator];
while (id o = [e nextObject]) {
  //...
}

which requires one message-send per iteration, and it's faster than:

for (NSInteger i=0; i < [collection count]; i++) {
  id o = [collection objectAtIndex: i];
  //...
}

which also requires one message-send per iteration[*]. The for(in) construct only requires a message send every time the buffer runs dry, which might be once every 8 iterations or so.

Notice that there's also block-based looping with [collection enumerateObjectsUsingBlock: ^(id obj, int idx, BOOL *stop){/*...*/}]; which has different properties again. Particularly the version of this construction that takes options can be told to execute the blocks concurrently.

[*]Observant readers will notice that this actually requires two message sends per iteration, as the termination condition will be evaluated every time through; however it's so easy to reduce that to one message send that we'll treat it as one for this discussion.