Subject: Re: Is "-init" really needed?



> On Aug 10, 2017, at 9:44 AM, Alastair Houghton <[email protected]>
> wrote:
>
> On 10 Aug 2017, at 15:24, Jeremy Hughes <[email protected]
> <mailto:[email protected]>> wrote:
>>
>>> On 10 Aug 2017, at 15:15, Alastair Houghton <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>> On 10 Aug 2017, at 15:09, Charles Srstka <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>>
>>>> They’re equivalent syntactically, but performance-wise, +array and friends
>>>> will cause the object to be put into an autorelease pool. Therefore, +new
>>>> is better for performance.
>>>
>>> Not with ARC they don’t. The ARC logic circumvents the autorelease pool in
>>> that case.
>>
>> Are you sure?
>
> Yes, I’m sure. At the call site, ARC causes the compiler to emit a call to
> objc_retainAutoreleasedReturnValue() or
> objc_unsafeClaimAutoreleasedReturnValue(), while in the method itself, ARC
> will use objc_autoreleaseReturnValue() or
> objc_retainAutoreleaseReturnValue(). The latter looks at the code for the
> call site and, assuming it matches, it will *not* do the autorelease and will
> set a flag that causes objc_retainAutoreleasedReturnValue() to eliminate the
> retain.

The ...

frameworks (and thus, the implementation of +array) are not built using
ARC. The -autorelease method is called manually, and the object is put in the
autorelease pool. You can see this for yourself by making a small test app that
calls [NSMutableArray array] and running it in Instruments, where the
autorelease will be clearly visible.

Charles

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