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Posted by Alan Pinstein on 12/08/05 04:30
On Dec 7, 2005, at 12:36 AM, Curt Zirzow wrote:
> My original statement was to show how the the php4 = &$o is
> different.
Oh, well, sure I believe that! :)
> In php5 variables are just containers that point to objects, so
> when you make
> a variable a reference to another variable all you are doing is
> saying these variables are the same thing.
>
That's good to know... one thing that's difficult about PHP is that
everything's opaque and not well documented. In C/C++ I know what
things actually are b/c you can see the typedefs. In php I just don't
know how objects are represented, what references are, etc.
So, knowing that an "object" variable is really just a "special
container object" that points to the real instance, this is useful.
That isn't clear from the php docs. Thanks!
> php5's objects dont know any such thing as a reference, they just
> know of instances of themselves. The variables ($o, $a, $b)
> existance is just a container for the instance of the object. So in
> the case when I do a:
>
> $b = &$o;
>
> All that is happening is the container is identical, so when I say:
>
> $o = null;
>
> Since $b is the same thing as $o , $b is set to null as well and
> thus, there are now more variable (containers) that reference to
> the instance of the object, thus the object will get destroyed,
> but.. if i say we have two containers:
>
> $o = new stcClass;
> $b = $o;
>
> Now the instance of that 'new StdClass' is contained in two vars,
> when I set $o to null, $b still exists since it doesn't know about
> $o whats so ever, and the instance of the stdClass still exists.
Yes, this makes sense, too. It's tricky, with so many levels of
indirection. So at the core level, you've got the "real" object
instance. Then, you've got N container "object variables" that point
to the real instance. PHP refcounts the number of object variables
pointing at the real instance. Then, on top of that, you have
references, which are "aliases" to "object variables" and thus don't
affect the ref count. So good this all makes sense and agrees with
the behavior I see.
> I guess it comes down to objects are treated the same way as you
> would expect these results:
>
> <?php
>
> $i = 1; /* aka new object */
> $k = $i;
>
> $i = null;
> var_dump($i); /* null */
> var_dump($k); /* int(1) */
>
> $i = 1; /* aka new object */
> $j = &$i;
>
> $i = null;
> var_dump($i); /* null */
> var_dump($j); /* null */
>
Yes...
>> The sample code below shows that indeed, in practice, on 5.0.4, that
>> & will create another reference (ie a weak reference) to an object
>> WITHOUT incrementing the refcount....
>
> I'm not sure how you mean a weak reference, and well a refcount is
> rather meaning less in php userland.
So, this gets interesting. I don't know if you're familiar with the
circular-reference problem. But if you have two instances that have
references to each other, even once you remove all references to the
objects, they will not be GC'd since they have a mutual deadlock on
each other:
$a = new MyObj;
$b = new MyObj;
$a->setB($b); // does $this->b = $b;
$b->setA($a); // does $this->a = $a;
$a = NULL;
$b = NULL;
The actual instances pointed to by $a and $b WILL NOT GET FREED HERE
as you would *wish*. However this is expected behavior.
Only by changing MyObj to store "weak references", that is references
to the objects that are NOT reference-counted, can you get the GC to
free the instances.
function setB(&$B) { $this->b = &$a; }
function setA(&$B) { $this->a = &$b; }
Now, the instances will be freed when the $a and $b are null'd out.
So, while I now feel more confident of how references act with
respect to objects (which is, they act the same as they do for any
variable), I still am not sure what "$this->this" is and why it
worked so magically.
Thanks for the explanations! I feel better about this now.
Alan
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