Mailing Lists: Apple Mailing Lists
Image of Mac OS face in stamp
 
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Copy and release

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?

Not necessarily. Your design should specify if this is the case, otherwise it's most likely assumed that the immutability attribute applies only to the container, not it's contents. This is consistent with the rest of Foundation's container behaviour, and that of other common containers such as found in the STL (where everything, most notably copies, are shallow in so far as containers holding pointers are concerned).

Having said all that, you really shouldn't assume anything about the mutability of any class, but rather call the appropriate method (retain, copy or mutableCopy), as I'm about to explain in answer to your next question.

P.S. Sorry for taking this further off-topic. As far as the OP goes, the main effeciency point I've seen when dealing with mutable Vs. immutable is that a copy of an immutable object can just be a reference to the same object, but a mutable copy needs to be a full copy. Things is, why do you need a copy? If it's to change it, then it needs to be mutable, and you need a full copy. If it's just to call it by a different name, then regardless of wether it's a mutable object or not you just need to pass a reference around.

Perhaps, but it's an issue of generality - your code needn't know necessarily whether the object it's been given is mutable or not (since it could be any subclass of your parameter type, and either way one or the other is going to have to thus satisfy any dynamic typing requirements you set, short of using isMemberOfClass), but it usually knows what behaviour it wants from that object, whether mutability or immutability. For example, a container allows it's contents to be mutable, since that's a necessary behaviour - you don't want to have to rebuild your container every time you change one item. But a socket class, for example, would want an immutable copy of the original address it's given to connect to, since you don't want it to change for the life of that particular instance. So the container would retain it's objects, the socket would copy it's parameter (which might boil down to a retain, if the given NSString is immutable anyway; that's just an efficiency thing, as mentioned by another poster).

From the converse point of view, if you want a mutable object because you know you'll change it, you mightn't want to limit your callers to providing a mutable version of the object... ultimately it's easier to call mutableCopy once in your target method than in some, most or even all your callers. As with the immutable case just mentioned, if the given object is already of the correct mutability you'll just end up retaining it.

Ultimately Foundation could survive without explicit mutability conversion, but it would then require stronger typing, and ObjC is a fantastic language for exactly the opposite reason.

Wade Tregaskis (AIM, Yahoo & Skype: wadetregaskis, ICQ: 40056898, MSN & email: email@hidden, Jabber: email@hidden
-- Sed quis custodiet ipsos custodes?

_______________________________________________
Do not post admin requests to the list. They will be ignored.
Cocoa-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/cocoa-dev/email@hidden

This email sent to email@hidden
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>)



Visit the Apple Store online or at retail locations.
1-800-MY-APPLE

Contact Apple | Terms of Use | Privacy Policy

Copyright © 2007 Apple Inc. All rights reserved.