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

Jean-Daniel Dupas devlists at shadowlab.org
Wed Jul 20 03:19:24 PDT 2011

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.

-- Jean-Daniel

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