Re: Very interesting ordering caveat with NSArray arrayWithObjects:
Re: Very interesting ordering caveat with NSArray arrayWithObjects:
- Subject: Re: Very interesting ordering caveat with NSArray arrayWithObjects:
- From: Michael Ash <email@hidden>
- Date: Sat, 4 Apr 2009 00:29:35 -0400
On Fri, Apr 3, 2009 at 8:30 PM, Eric Hermanson <email@hidden> wrote:
> Some (or most) people might be aware of this caveat, but I was not, so I'll
> share it.
>
> Consider this code:
>
> NSArray *array = [NSArray arrayWithObjects:[MyCounterClass newObject],
> [MyCounterClass newObject], nil];
>
> where [MyCounterClass newObject] is a static method that returns a new
> autoreleased instance that simply stores an incrementing int32 counter in
> its instance variable, e.g.
>
> self.oid = SomeStaticCounter++; // (or ideally,
> OSAtomicIncrement32Barrier(&SomeStaticCounter);
>
> Now, one would expect that the array would contain:
>
> element 1: MyCounterInstance.oid=1
> element 2: MyCounterInstance.oid=2
>
> However, this is NOT the case. Either the compiler or the runtime executes
> the SECOND call to [MyCounterClass newObject] FIRST, so the array actually
> contains:
>
> element 1: MyCounterInstance.oid=2
> element 2: MyCounterInstance.oid=1
>
> NSArray arrayWithObjects: is of course correctly putting the objects into
> the array in the correct natural ordering, but the objects are CREATED on
> the stack in the oppose order. Maybe most people knew that, I did not. So
> the (or a) workaround is:
>
> MyCounterClass *object1 = [MyCounterClass newObject];
> MyCounterClass *object2 = [MyCounterClass newObject];
> NSArray *array = [NSArray arrayWithObjects: object1, object2, nil];
This is actually a "feature" of C, which ObjC inherits. C does not
define an order of operations except across "sequence points", which
are basically semicolons, although C defines some others too.
Different parts of a statement are executed in an arbitrary order.
Basically, the compiler can decide which order suits it best. As such,
conforming C (and thus ObjC) code must never rely on the order of
execution of function arguments, arithmetic subexpressions, or
anything else of that nature. In any given statement, there should
never be two parts of the statement that have interdependent side
effects.
Wikipedia has a decent discussion of this concept along with some
illuminating examples:
http://en.wikipedia.org/wiki/Sequence_point
Mike
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