[cfe-dev] ARC and @synthesize of read only property

[Argyrios Kyrtzidis]

On Jul 20, 2011, at 3:19 AM, Jean-Daniel Dupas wrote:

> 
> Le 20 juil. 2011 à 10:57, David Chisnall a écrit :
> 
>> On 19 Jul 2011, at 21:06, Jean-Daniel Dupas wrote:
>> 
>>> Thank you for the explanation. So the retained/autorelease is a new behavior introduced with ARC. AFAIK, the getter did this only for atomic properties in non ARC mode.
>> 
>> You can check the generated IR.  In ARC mode, it calls objc_getProperty() for atomic properties.  For  nonatomic properties, it accesses the ivar directly and calls objc_retainAutoreleaseReturnValue().  If you do something like:
>> 
>> id o = [object synthesizedNonatomicProperty];
>> 
>> Then ARC will translate this into something like:
>> 
>> o = objc_retainAutoreleasedReturnValue([object synthesizedNonatomicProperty]);
>> 
>> This skips the autorelease pool, so the object is now retained in the caller but not present in the autorelease pool.  ARC will then objc_release() it at the end of the scope of o.  This means that you get a lot less redundant autorelease pool churn in ARC mode.
> 
> That make sense. Thanks again.
> 
>> 
>>>> Even in ARC mode, the compiler can't magically guess what you mean.  
>>> 
>>> While it cannot always guess, when the @synthezise directive specify an ivar, it can default to the ivar ownership, as this is the only acceptable value.
>> 
>> Ivars all default to __strong, but it sounds like you actually want unsafe_unretained (which would then simply be objc_retain()'d in the caller in ARC mode).  
> 
> I really have and want a strong ivar. Declaring it as 'unsafe_unretained' will prevent its releasing in the generated object's destructor.
> So the solution in my case it to change the property declaration to tell the compiler it is a retained property.

This changed in r136155, now there will only be an error if the backing ivar is not user declared and there is no ownership attribute on the type.

-Argyrios

> 
> -- Jean-Daniel
> 
> 
> 
> 
> 
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