Objective C, NSMutableDictionary reference count

    NSMutableDictionary *attrs = [nodeAttributes objectForKey:UUID];//nodeAttributes is NSMutalbleDictionary
    if (attrs == nil) {
         attrs = [[NSMutableDict开发者_开发百科ionary alloc] init];
         [nodeAttributes setObject:attrs forKey:UUID];
         [attrs release];
    }

I am not sure this code will works...Should I have something like this instead of this

    NSMutableDictionary *attrs = [nodeAttributes objectForKey:UUID];//nodeAttributes is NSMutalbleDictionary
    if (attrs == nil) {
         attrs = [[NSMutableDictionary alloc] init];
         [nodeAttributes setObject:[attrs retain] forKey:UUID];
         [attrs release];
    }

I am not sure if setObject method will increase the reference count...

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NSMutableDictionary's setObject will retain the object (this is documented), so the first bit of code is correct and the second leaks. Style-wise, it may be more clear to a reader to do the following:

if (attrs == nil) {
     attrs = [NSMutableDictionary dictionary];
     [nodeAttributes setObject:attrs forKey:UUID];
}

Although memory-management-wise, your approach is probably better in that it avoids (implicitly) using autorelease.

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The retain count will be incremented in the first case. That's the correct approach.

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Objects are responsible for claiming ownership of the things they own. So yes, setObject:forKey: will retain. This is explained in detail (but still very briefly) in Apple's memory management guide.

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setObject will send a retain message so you don't need to do that.