On Jan 18, 2015, at 12:45 , Trygve Inda <cocoa...@xericdesign.com> wrote:
>
> NSMutableDictionary* collection; // keys are myID, values are MyObject
>
> for (MyObject* object in [[self collection] allValues])
> {
> [collection removeObjectForKey:[object myID]];
> }
What Ken said, plus …
— If the collection of values is (as a matter of implementation) mutable, then
I certainly wouldn’t want to rely on it not changing during the enumeration. If
it is (as a matter of implementation) immutable, then it’s cost-free to copy.
So the correct strategy is to copy it.
— It makes me uncomfortable to see you iterating through values, not keys.
There is no API contract regarding uniqueness of the value objects, in the case
that different keys refer to the same value object. Furthermore, your loop is
logically incorrect in that case.
I know that might not matter here. Apparently, in the code fragment you showed,
1 unique key == 1 unique object. But even if that’s so it seems more correct to
iterate over “allKeys” instead of “allObjects”.
Also, every time I do something like this, I wonder if it’s semantically
preferable to use a block enumeration, which for a dictionary would have to be
‘keysOfEntriesPassingTest:’, followed by ‘removeObjectsForKeys:’. [I’m assuming
your actual intent is to remove only some of the objects, because otherwise
you’d use ‘removeAllObjects’ instead of a loop.]
I believe we found out, when the block enumeration API was introduced, that
these don’t currently do anything particularly optimized, it seems to me that
we can always hope that there will be such an optimization one day, so that the
keysOf/remove pattern could be much more efficient for large dictionaries.
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