Re: Arc and **
Re: Arc and **
- Subject: Re: Arc and **
- From: John McCall <email@hidden>
- Date: Wed, 05 Dec 2012 15:44:36 -0800
On Dec 4, 2012, at 10:44 PM, Gerriet M. Denkmann <email@hidden> wrote:
> On 5 Dec 2012, at 12:59, Greg Parker <email@hidden> wrote:
>> On Dec 4, 2012, at 8:56 PM, Gerriet M. Denkmann <email@hidden> wrote:
>>> I have (using Arc) a method which works fine:
>>> NSString *explanation;
>>> [ self doSomeThingAndExplain: &explanation ];
>>>
>>> Now I decided that sometimes I don't need this explanation. So I changed it to:
>>>
>>> NSString **explanatioP = urgent ? NULL : &explanation; // <-- "no explicit ownership...
>>> [ self doSomeThingAndExplain: explanatioP]; // passing address of non-local object...
>>>
>>> But now the compiler gets really upset (errors written as comments above).
>>>
>>> What Arc-magic is needed to get this to compile?
>>
>> You need to add explicit ownership to your ** declarations. ARC needs to know whether explanationP points to to a strong variable or a weak variable or an autoreleasing variable.
>>
>> This is one way:
>> -(void) doSomethingAndExplain:(__strong NSString **)explanationP;
>>
>> __strong NSString **explanationP = urgent ? NULL : &explanation;
>> [self doSomethingAndExplain:explanationP];
>>
>> This is another way:
>> -(void) doSomethingAndExplain:(NSString **)explanationP;
>> // NSString** parameter is implicitly __autoreleasing NSString **
>>
>> __autoreleasing NSString **explanationP = urgent ? NULL : &explanation;
>> [self doSomethingAndExplain:explanationP];
>>
>> The first version may have better performance because it avoids the autorelease pool.
>
> Another way (I don't know how efficient it is though) is:
> [self doSomethingAndExplain: urgent ? NULL : &explanation ];
>
> Does this use autorelease pool ?
Yes. By default, an argument of type id* is assumed to be __autoreleasing id *,
because that's the common convention for out-parameters in Cocoa.
It is generally more efficient to make your out-parameters __strong id*, but
of course that requires a bit more work from MRC callers, which is why it's
not the standard convention.
John.
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