Re: copy vs mutableCopy?
Re: copy vs mutableCopy?
- Subject: Re: copy vs mutableCopy?
- From: "A.M." <email@hidden>
- Date: Tue, 11 Jul 2006 11:56:59 -0400 (EDT)
On Tue, July 11, 2006 11:50 am, Greg Titus wrote:
> On Jul 11, 2006, at 8:50 AM, Rob Ross wrote:
>
>> Ok, I know the difference between a mutable and immutable object.
>> And I know the difference between a shallow copy and a deep copy.
>>
>>
>> But I am having trouble understanding the difference between
>> implementing copy vs mutable copy, since it seems you can make mutable
>> copies of immutable objects, and immutable copies of mutable objects.
>> I'm just a bit confused how this works in practice.
>>
>>
>> I've read this:
>>
>>
>> http://developer.apple.com/documentation/Cocoa/Conceptual/
>> MemoryMgmt/Tasks/ImplementCopy.html
>>
>>
>> but it doesn't really tell me the difference between the two methods.
>
> The difference is in the intent of the caller. You make a mutableCopy
> when you want to make a copy that you can later modify for yourself, and
> make a copy when you want to make a copy to hold onto unchanged.
>
> For many objects, the implementation of the two methods can be
> identical. But knowing the intent of the caller allows certain
> optimizations, such as returning the same immutable object (just retained
> again) from a call to -copy, because you know that the caller just wants
> something to hold onto unchanged, and the existing object can not change.
Also, mutable classes are sometimes subclasses of their immutable
brethren, so a -copy might fall through to the superclass. A lot of
classes are inherently mutable, so copy will return the mutable copy
anyway. Note that the NSCopying protocol only requires -copyWithZone: so
it has no notion of mutability.
-M
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