Retain count exhausted after @throw

Question

When I run this code inside a call made from SenTest's STAssertThrowsSpecificNamed:

@throw [[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                  reason:@"---some reason----"
                               userInfo:nil];

I get (with NSZombieEnabled=YES):

*** -[NSException reason]: message sent to deallocated instance 0x100a81d60

The exception is somehow deallocated before STAssertThrowsSpecificNamed has finished processing it.

I can avoid the error by replacing the @throw line above with this code:

NSException *exception = [NSException exceptionWithName:NSInvalidArchiveOperationException
                                                 reason:@"---some reason----"
                                               userInfo:nil];
@throw exception;

I get exactly the same behavior with or without ARC. Without ARC this code avoids the error too:

@throw [[[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                   reason:@"---some reason----"
                                userInfo:nil] retain];

Is this a bug in SenTest? Or a bug in the compiler? Or is my first @throw just incorrect?

Aditya Vaidyam · Answer 1 · 2012-03-08T00:00:46.113

2

@throw releases the object after it's done using it, so use -retain if you want to include it on the same line as @throw.

@throw [[[[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                  reason:@"---some reason----"
                               userInfo:nil] retain] autorelease];

This should do the trick.

EDIT: To check for ARC-specific code, use:

if(__has_feature(objc_arc)) {
    @throw [[[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                       reason:@"---some reason----"
                                     userInfo:nil];
} else {
    @throw [[[[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                      reason:@"---some reason----"
                                   userInfo:nil] retain] autorelease];
}

edited Mar 08 '12 at 00:00

answered Mar 06 '12 at 04:40

Aditya Vaidyam

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And is the second example what you recommend under ARC, where I can't use `retain`? – paulmelnikow Mar 06 '12 at 16:32
The reason this would work is because I see that @throw releases the NSExceptions you create, so if you pass a -retain/-autorelease NSException to it, it will retain, and release when necessary. Retain counts are supposed to be used this way when "giving an object to another object", and with -autorelease, it just means "released if necessary, or leave it the garbage collection". – Aditya Vaidyam Mar 06 '12 at 17:16
Under ARC, you might have to define it as __strong (if I remember correctly, since I try hard to avoid ARC). In ARC, retain counts are indirectly handled by the modern runtime, and weak and strong typing tells it what to do with the object. – Aditya Vaidyam Mar 06 '12 at 17:17
I need a solution that works for each, so for now I'm using the second code block from my original post: `id exc = [NSException exceptionWithName:....]; @throw exc;`. – paulmelnikow Mar 07 '12 at 16:48
It still strikes me as odd that `@throw` is intentionally designed this way. – paulmelnikow Mar 07 '12 at 16:49
You need to change `if(__has_feature...` to `#if __has_feature(` for the above code to compile under ARC, but it still doesn't work (see original post). – paulmelnikow Mar 08 '12 at 19:17

score 1 · Accepted Answer · answered May 18 '13 at 04:21

1

I'm now exclusively using +[NSException raise:format:] under both ARC and manual retain–release.

answered May 18 '13 at 04:21

paulmelnikow

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paulmelnikow · Answer 3 · 2012-03-08T19:20:34.160

1

I'm sticking with this form, for now, which seems to work, both with and without ARC.

id exc = [NSException exceptionWithName:NSInvalidArchiveOperationException
                                 reason:@"---some reason----"
                               userInfo:nil];
@throw exc;

Per Galaxas0's answer, @throw is designed to release the exception after it processes it. This still strikes me as odd, especially under ARC.

In a non-ARC project, use this (also per Galaxas0):

@throw [[[[NSException alloc] initWithName:NSInvalidArchiveOperationException
                                    reason:@"---some reason----"
                                  userInfo:nil] retain] autorelease];

edited Mar 08 '12 at 19:20

answered Mar 08 '12 at 19:11

paulmelnikow

16,895
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1

Why aren't you using `+raise:format:` in NSException instead? I have had no memory issues using that pattern. – Richard J. Ross III Mar 08 '12 at 19:38
I think that's what I was looking for. – paulmelnikow Mar 08 '12 at 20:58
I'm not sure completely, but that's how I *believe* it's designed. Apple suggests the use of the first statement. If my answer (or yours) was helpful, please mark it as the correct answer! :] – Aditya Vaidyam Mar 08 '12 at 23:30
@Galaxas0, it was partially helpful; I upvoted it. See my comments there. – paulmelnikow Mar 08 '12 at 23:45
Thanks then! I'll up vote this post since it's a complement to my post. – Aditya Vaidyam Mar 09 '12 at 00:44

Retain count exhausted after @throw

3 Answers3