Re: NSAutoreleasePool drain/dealloc

[David Chisnall]

On 6 Mar 2018, at 17:24, Richard Frith-Macdonald 
 wrote:
> 
> 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).

-drain was introduced with GC, where -release was optimised away in either the 
compiler or the runtime and never delivered.  It allowed the GC implementation 
to use autorelease pools as a hint that there were a lot of short-lived objects 
to delete and have the same code work in non-GC mode.  ARC fixed this a lot 
better by introducing the @autoreleasepool syntax, and should be used in all 
new code.

David


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Re: NSAutoreleasePool drain/dealloc

[Ivan Vučica]

On Tue, Mar 6, 2018 at 5:24 PM, Richard Frith-Macdonald <
richard.frith-macdon...@theengagehub.com> wrote:

>
>
> 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.
>

Thank you! I guess I never read that part of the NSAutoreleasePool docs.


> 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).
>

That's what I assumed it was doing: releasing the members of the pool,
while not releasing the pool itself.

Thank you!
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Re: NSAutoreleasePool drain/dealloc

[Richard Frith-Macdonald]

> On 6 Mar 2018, at 17:08, Ivan Vučica  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).



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