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Re: Copy and release

On 22 Oct 2004, at 13:24, Paul Sargent wrote:
Nicko van Someren wrote:
...
If you make immutable copies of immutable NSString, NSDate, NSArray or NSDictionary objects (to pick a few important examples) you simply get the same object with the reference count incremented. This saves some data copying for simple objects and by the time you get to complex objects like dictionaries the cost of copying can get pretty high.

A question I've had is when a container is said to be immutable, is it still fair game to change the contents of the objects that are contained?
For example: An NSArray containing NSMutableString objects, or an NSData object. Can you / Should you change the contents of the NSMutableStrings in the NSArray or the Memory block pointed to by the NSData?
Should items contained in immultable container classes be considered immutable?

I would submit that if I have a mutable object and I make a change to it then it's still the same object, it's just that its state has changed. Thus an immutable array of mutable strings can reasonably have its strings altered. If you think of the case of I have a fixed list of who my parents are but their addresses might change this might make more sense.

It is worth noting that the NSDictionary class cluster deals with the problem that the keys might be mutable by stating that the keys must support the copying protocol and the keys actually get copied when added to the dictionary whereas the values are merely retained. If the keys are immutable then the copying may well be very efficient (as for immutable NSString objects) but there is still a copy message sent. It's actually the dictionary case for which the copyAndRelease call would have most effect since copying a mutable dictionary re-copies all the keys and if you're keying off anything the requires a full copy the conversion from a mutable dictionary to an immutable one can be very costly.

Nicko

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References: 
 >Copy and release (From: Nicko van Someren <email@hidden>)
 >Re: Copy and release (From: Evan Schoenberg <email@hidden>)
 >Re: Copy and release (From: Nicko van Someren <email@hidden>)
 >Re: Copy and release (From: Paul Sargent <email@hidden>)



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