Question

I have a general question about objects, memory, and retaining.

I am not a complete newb (I have apps for sale on the iTunes Store) but something must have slipped past me. I work with the Google Docs API, and yesterday I was downloading a spreadsheet feed, and enumerating the results, namely, the Spreadsheet Record Feed.

After enumerating and adding objects to a dictionary, the dictionary got added to an array before the loop went to the next Record. So after 5 times through, the array had 5 objects, one dictionary per record, with values from each. Weirdly, at the end, the Array had 5 copies of the same information. Each time through the loop, the dictionary changed, like it was supposed to, but when I inserted it into the array, the other objects in the array changed to match.

I looked through some stuff on StackOverflow and found a suggestion to try this:

[array insertObject:[dictionary copy] atIndex:0];

That fixed it. Adding the copy method made everything work like normal.

I’m just wondering why.

Answer 1

When you insert an object (let’s call it X) into an array, what actually gets placed into the array is a copy of the pointer to X, and not a copy of X itself. X is sent a retain message so that the array can hold on to it, but X is not sent a copy message. This means that changes to X later on will affect the object “stored” in the array.

WARNING: The suggested solution results in a substantial memory leak, since the copied data is never released. A better solution is to autorelease the copied array, so that it will be released when the array is released.

[array insertObject:[[dictionary copy] autorelease] atIndex:0];

Or, for iPhone code (in which autorelease can be bad, especially in loops):

NSDictionary * newDict = [dictionary copy];
[array insertObject:newDict atIndex:0];
[newDict release];

UPDATE: Be sure to read Ole Begemann’s answer as well. He makes an excellent point about deep vs. shallow copies of the dictionary.