Question

- (void)foo
{
    @try {
         for (id o in os){
             @autoreleasepool {
                 for (id o2 in moreOs){
                       // create a lot of autoreleased objects.
                 }
                //exception raised
            }
        }

    }
    @catch (NSException *exception) {
     // handle the exception   
    }
}

1. Will the compiler rewrite the above code to drain the pool in the event of an exception or will it leak?

2. If the compiler does rewrite it, how will it do it?

Answer 1

To address your question, no those objects created in that pool will effectively be leaked until some autoreleasepool further down the stack is drained. There may be no pool further down the stack except the main autoreleasepool for your app which may not get drained for some time.

Try the following to force the drain to happen when this method returns if an exception is caught.

- (void)foo
{
    @autoreleasepool {
        @try {
            for (id o in os){
                @autoreleasepool {
                    for (id o2 in moreOs){
                        // create a lot of autoreleased objects.
                    }
                    //exception raised
                }
            }

        }
        @catch (NSException *exception) {
            // handle the exception   
        }
    }
}

According to documentation, when the outermost autoreleasepool is drained it will drain any nested ones as well.

If you drain an autorelease pool that is not the top of the stack, all (unreleased) autorelease pools above it on the stack are drained (and all their objects sent appropriate release messages). If you neglect to send drain to an autorelease pool when you are finished with it (something not recommended), the pool is drained when one of the autorelease pools in which it nests is drained.

From the “Scope of Autorelease Pools and Implications of Nested Autorelease Pools” section in Advanced Memory Management Programming Guide