Question

I am trying to copy a string that is passed into a method like so:

-(void) parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qualifiedName attributes:(NSDictionary *)attributeDict {
    NSLog( @"elementName, %@:  %i", elementName, [elementName retainCount] ); // rc = 2

    if ( currenttag )
        [currenttag release];

    NSLog( @"currenttag:  %i", [currenttag retainCount] ); // rc = 0

    //currenttag = [[NSString alloc] initWithString:elementName];   // track current element
    [self setCurrenttag:elementName];

    NSLog( @"currenttag:  %i", [currenttag retainCount] ); // rc = 3

    .
    .
    .
    }

setCurrenttag is a synthesized accessor ( @property (copy) ). My understanding was this would create an entirely new object instead just a reference to elementName. The above behaves as though it is keeping a reference to elementName and calling retain. The commented out bit of code shows the same behaviour.

These methods are implementing the NSXMLParserDelegate protocol, but I do need keep a track of certain element names (but not all).

Is there something I am missing concerning NSString objects and memory management on the iphone.

Also, as reference, I am running this on the iPhone simulator with XCode 3.6.

Answer 1

For immutable Foundation classes like NSString, copy simply retains the object. Duplicating an object that’s known to be immutable would be a waste of resources, so it doesn’t happen. This is hinted at in the documentation for the NSCopying protocol. One of the options for implementing the protocol is:

• Implement NSCopying by retaining the
  original instead of creating a new
  copy when the class and its contents
  are immutable

In general, if you know that instances of one of your classes will be immutable, it’s entirely valid to retain the target object rather than duplicate it.