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Posted by Ray Hauge on 12/07/05 02:10
I am uncertain on this, but I believe that the $this variable is already
just a reference to the class you are calling it from. Then passing the
reference by-reference to the addParent() method of the Child class
could be what is causing your issue. I'd be curious to see what would
happen if you took out the pass-by-reference and instead pass-by-value
for the addParent() method. Then again, that doesn't particularly sound
correct either.
This link might help. They cover a lot of advanced reference usage for PHP.
http://www.onlamp.com/pub/a/php/2002/09/12/php_foundations.html
Alan Pinstein wrote:
> So.. I am having PHP5 memory management problems.
>
> They are similar to those described in this thread:
>
> http://aspn.activestate.com/ASPN/Mail/Message/php-Dev/1555640
>
> (so maybe this question belongs on php-dev but I figured I'd try here
> first... seems like a userland question)
>
> Basically I have an object model to represent db objects, and I am
> bulk-loading the objects via some PHP scripts. Sadly the scripts
> consume unbounded memory because of this problem.
>
> I have done a lot of programming in C++ and Obj-C and the normal way
> to handle circular references is to have parents "retain" (keep ref-
> counted links) to their kids, and have the kids have "weak
> references" (non-ref-counted) links to their parents. This way, when
> the parent is no longer used, it will automatically 0-out the ref
> counts to all children it links too and things GC correctly.
>
> Now, how to do this in PHP?
>
> Well, there are no "documented" weak references. However, I figured
> out by trial that if you obtain a php-reference to an object, it
> doesn't bump the refcount.
>
> Question #1: Is the fact that references to objects in the form
> $objRef = &$obj don't bump the refcount of $obj an intended behavior
> that can be counted on? If so, cool!
>
> So, now that we have a way to do weak references, we should be able
> to implement a reasonable memory management scheme for parent-child
> objects.
>
> Normally from the client side the interface should look something like:
>
> $parent = new Parent();
> $child = new Child();
> $parent->addChild($child);
>
> Where parent can have 0,n children and child can have 0,1 parent.
>
> And all of parent's internal links to child should be refcounted, and
> the internal links from child to parent are weak (not ref-counted).
>
> So based on the above discovery about references, I tried to
> implement this as such:
>
> class Parent
> {
> public $children = array();
>
> // add a child to our list. We want a ref-counted link here.
> function addChild($child)
> {
> $this->children[] = $child; // refCounted desired in
> parent->child link
> $child->setParent($this);
> }
> }
>
> class Child
> {
> public $parent;
>
> // set the parent object. We want a non-ref-counted link here.
> function setParent(&$parent)
> {
> // refCount NOT desired in child->parent link
> $this->parent = &$parent;
> }
> }
>
> Now, you'd expect this would work, but it doesn't. On a hunch, I
> changed the client code to:
>
> $parent = new Parent();
> $child = new Child();
> $parent->addChild($child);
> $child->setParent($child); // new line here - you can
> successfully create a reference to the object when not passed in as
> $this
>
> Now, this works! However, it's not practical. The setParent call
> should work from within the parent object....
>
> So what I figured out is that $this is a "pseudo variable" according
> to the docs, but I don't know what that means. Empirically I have
> figured out that it means you cannot create a reference to it.
>
> Is this a feature or a bug? What's the workaround?
>
> This is a serious problem for PHP scripts that need to do things that
> require large amounts of memory.
>
> Please advise.
>
> Thanks,
> Alan
>
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