Re: NSAutoreleasePool drain/dealloc

[Richard Frith-Macdonald]

> On 6 Mar 2018, at 17:08, Ivan Vučica <address@hidden> wrote:
> 
> Hi,
> 
> I was explaining refcounting and NSAutoreleasePool to someone, and I thought 
> referencing GNUstep might be useful to explain the correct mental model of 
> the behavior.
> 
> But I'm confused about -drain in the non-ARC implementation:
>   
> https://github.com/gnustep/libs-base/blob/b8185fa6c13172436c070ab3bf60b3f12bce433b/Source/NSAutoreleasePool.m#L546
> 
> Won't equating drain and dealloc mean that pools will misbehave, and that 
> after [pool drain], the incorrect pool will get populated (and later drained)?
> 
> Am I correctly interpreting that this happens? If so, is it correct that this 
> happens?
> 
> NSAutoreleasePool * outerPool = [NSAutoreleasePool new];
> [[NSObject new] autorelease]; // object 0 added to outerPool
> 
> NSAutoreleasePool * innerPool = [NSAutoreleasePool new];
> [[NSObject new] autorelease]; // object 1 added to innerPool
> [innerPool drain]; // object 1 released; outerPool is the closest pool
> [[NSObject new] autorelease]; // object 2 added to outerPool
> [innerPool release]; // object 2 released, object 0 released; new pool 
> created as the closest pool
> 
> [outerPool release]; // no objects released; new pool created as the closest 
> pool
> 
> Unless I am missing something, object 0 would be released early here?

According to Apple, the -drain method is a synonym for -release (or -dealloc 
since you don't retain autorelease pools).
So yes, if youi drain a pool the next time an object is autoreleased it goes 
into the parent pool of the one you drained.
Your code above should crash at the point where you call [innerPool release] 
since you are sending the -release message to a deallocated object.

I think the -drain method name is unintuitive.  To me it sounds like it ought 
to do the same as the gnustep-specific -emptyPool method (a more efficient 
equivalent to draining/releasing the pool and immediately creating a new one).