Oh, and if you are implementing something like this, don’t forget to trap
writes to both the copy *and* the original object:
myMutableClass *a=[myMutableClass makeAnObjectWithSomeBuffer:aBuffer];
myMutableClass *b=[a mutableCopy];
[a mutateTheBuffer];
doing [a isEqual:b] should return NO; but if you didn’t trap the write to the
original object and cause the COW copies to actually do the copy then it will
erroniously return YES (and all sorts of other havoc will likely ensue…)
On Apr 27, 2014, at 4:06 AM, Maxthon Chan <xcvi...@me.com> wrote:
> Immutables copied are just retained and returned, and mutableCopied are COW.
>
> Mutables copied generates COW.
>
> That would be the fastest way implementing that.
>
> * COW = copy on write
>
> On Apr 27, 2014, at 17:06, John McCall <rjmcc...@apple.com> wrote:
>
>> On Apr 27, 2014, at 2:00 AM, Dave <d...@looktowindward.com> wrote:
>>> A long time back, I remember reading something about the way in which copy
>>> and mutableCopy are implemented. I think basically it said that:
>>>
>>> someObject = [someOtherObject copy];
>>>
>>> or
>>>
>>> someObject = [someOtherObject mutableCopy];
>>>
>>> Wouldn’t necessarily allocate any extra data storage. I’ve been searching
>>> for it to refresh my memory, but I can’t see to find it anywhere. Does
>>> anyone know if this document or something like it exists somewhere?
>>
>> I don’t know if there’s a document, and like a lot of things with ObjC the
>> actual guarantees are pretty weak, but implementations of these methods on
>> immutable types have been turning into essentially “return [self retain];"
>> for quite some time. But that’s when the actual dynamic type is
>> guaranteed-immutable, e.g. something constructed as an NSArray, not just
>> something that’s immutable by convention, e.g. an NSMutableArray that you’re
>> passing around as an NSArray.
>>
>> John.
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