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Re: Automatic Reference Counting


From: Ondřej Hošek
Subject: Re: Automatic Reference Counting
Date: Thu, 7 Jul 2011 21:52:41 +0200

On Thu, Jul 7, 2011 at 20:23, Ivan Vučica <address@hidden> wrote:
> If calling -retain, -release and -autorelease is not allowed, how is it
> possible to mix ARC with non-ARC code? Does the compiler throw a warning, an
> error, or the thing just silently doesn't work?

The compiler throws an error. The boundary of mixing ARC and non-ARC
code is the translation unit: object files compiled with ARC can be
linked with those compiled without, but mixing ARC and non-ARC code
within the same source file is not possible.

> What happens when -retain, -release and -autorelease are overridden?

The compiler throws an error. If you explicitly require different
retain/release semantics for your class, you can't use ARC within it.
However, ARC code should still be able to use your class (I haven't
tested this, though).

> What happens when non-__unsafe_unretained pointers are stored in structure?

The compiler throws an error here too. Apple says you should be using
Objective-C objects (instead of structs) as containers for other
Objective-C objects anyway. (If you disagree with their sentiment, you
can simply continue using traditional retain-release, which isn't
going away as long as we have oddball classes like NSProxy.)

> Does it silently crash?

Quite unlike a silent crash, the compiler whined loudly when I tried
anything "evil". (Part of the ARC magic happens within the compiler,
part within the runtime, neither of which are known for silent
crashes.)

> Are all pointers __strong by default?

All ObjC pointers, yes.

> What happens with non-Objective-C pointers, such as results of SDL functions?

They remain untouched by ARC. ObjC pointers are detected using
semantic analysis -- a heuristic along the lines of "oh, an asterisk,
I'm taking over" would break everything (including the "strict
superset of C" philosophy).

> Will I have any problems using SDL if ARC is used in same codebase?

My guess is no, but I haven't tried it yet.

Hope that helps (at least partly),
~~ Ondra



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