A comma is a sequence yet the order in arrayWithObjects is
indeterminate. It must be the var arg causing the ordering mix.
Sent from my iPhone
On Apr 4, 2009, at 12:29 AM, Michael Ash <michael....@gmail.com> wrote:
On Fri, Apr 3, 2009 at 8:30 PM, Eric Hermanson <zmons...@mac.com>
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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