On Nov 12, 2012, at 09:19 , Jerry Krinock <[email protected]> wrote:
> I'm debugging a crash in a large project which evidence indicates is caused
> by a retain/release imbalance. The project is written with manual
> retain/release, not ARC.
>
> The project is built in Xcode 4.5.2, and when I 'Analyze', I get no warnings
> pertaining to memory management. So the problem must be some edge case which
> is not caught by 'Analyze'. Further, I think that ARC is built upon
> 'Analyze', and therefore if I converted this project to ARC, it would still
> crash.
>
> Am I correct?
I don't think so. The analyzer is only able to detect retain/release problems
basic on a *static* analysis of the source code (supplemented perhaps by
guesses about what certain code patterns suggest about run-time behavior).
Committing a memory management error is hardly an edge case -- it's
ridiculously easy to do. In particular, the analyzer doesn't cross file
boundaries, so a retain in one file that's supposed to be matched by a release
in another file isn't something the analyzer can detect.
ARC works by propagating correctness. It implements correct memory management
in each local scope, and follows correctness-preserving rules when
transitioning into and out of scopes. In theory, if you follow ARC's rules, you
can't write code with incorrect memory management. So, converting the project
to ARC may very well solve the problem. In practice, the only real danger
points are bridging points into and out of ARC control, and a very small number
of cases where ARC deliberately doesn't preserve 100% correctness.
For example, ARC doesn't fully manage the 'self' pointer, for performance
reasons. You can write code like this:
…
someObjectInAnIvar = …
[someObjectInAnIvar dontDoThisAtHome];
…
- (void) dontDoThisAtHome {
someObjectInAnIvar = nil;
…
}
Assuming there's no other reference keeping the object alive, then it will get
dealloc'ed in 'dontDoThisAtHome', immediately invalidating 'self'.
This example came up on one of the lists a few months ago, because if the ivar
happens to be __weak, then counterintuitively ARC throws incaution to the winds
and safely retains the object for the length of the method invocation.
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