Re: Weird behaviour with references to objects...

I'm not sure if my answer is going to be of much help, but I think this has to do with the way PHP handles references and copies of objects--it uses what I understand is a lazy copy mechanism, and thus the results on object manipulations can yield odd results. There was a discussion on a similar problem on PHP-DEV a while back--if you search the forums you should be able to find it. Hope this helps... Marco ------------- php|architect -- The Monthly Magazine For PHP Professionals Come visit us on the web at http://www.phparch.com! On Thu, 2002-11-07 at 21:36, Tim Molendijk wrote: > First of all I would like to say that I know this is a lot of text but I > would be very pleased if you take a little time to read it nevertheless. The > situation described below is not complicated at all... > ===================================== > > Hello all, > > I'm dealing with this really weird problem which occurs when dealing with > references within objects pointing to other objects. I'm trying to find out > what it is now for a few days and I still don't get it. I minimised the code > around the problem and it's weirder than ever. I would really appreciate it > when you could take a look... I'm pretty experienced at PHP but perhaps you > know something that I don't know. > > We have two different classes: Container and Child. Container objects > contain a Child object in its $child property. A Child object has a $parent > property, which contains a *reference* to the Container object by which it > is held. > So $containerObject should be the *same* as $containerObject->child->parent. > To test this Container objects have a $property property. When changing this > property the difference between one object and identical/cloned objects > becomes visible. > > Now take a look at the following code. The indented text is code, the rest > are comments. The code for the class Container and class Child are also > included. Please scroll down or take a look at the attached file. > > begin first code snippet -------------------------------- > <?php > > /* New Container object created. At the same time it sets a Child object in > its $child attribute. (This is indicated by passing TRUE.) */ > $testContainer1 = new Container(TRUE); > /* The Container object is created, the Child object is set and its > reference to the Container object is set. Now it prints > $testContainer1->property and $testContainer->child->parent->property. These > should have an identical value because they should be from the same object > (not cloned/identical, but the same). > This is the result: > property = 'state 1' > child->parent->property = 'state 1' > So this is as expected, no problem... yet. */ > $testContainer1->printState(); > > /* Now $testContainer1's $property property is changed into another value. > /* > $testContainer1->property = 'state 2'; > /* Now $testContainer1->child->parent->property should also have value > 'state 2', but it HAS NOT! > This is the result: > property = 'state 2' > child->parent->property = 'state 1' > Obviously $testContainer1->child->parent->property is not the same object as > $testContainer1, but a clone! */ > $testContainer1->printState(); > > ?> > end first code snippet -------------------------------- > > Well, this is the whole problem... Why on earth isn't it a reference, while > I really did assign it as a reference in the code (see class code). > > Now to make the whole thing even weirder, take a short look at the following > code. It is almost completely the same as above, except that instead of > giving the Container constructor the command to assign a Child object to the > $child property, this command is given manually by calling method load() -- > which is the method that Container constructor uses to load the Child > object. > > begin second code snippet -------------------------------- > <?php > > /* Here the Container object is created again. But now no TRUE is passed > because we don't want the Container constructor method to create and assign > a Child object to the $child property. */ > $testContainer2 = new Container; > /* In the first code snippet load() is called from the Container constructor > method. So in fact nothing has changed, except the fact that load() is > called from the client scope instead of the Container object scope now. */ > $testContainer2->load(); > $testContainer2->printState(); > > $testContainer2->property = 'state 2'; > /* After $property has been modified again, $testContainer2's state is > printed again, and now the result is DIFFERENT!!!: > property = 'loaded state 2' > child->parent->property = 'loaded state 2' > This is the CORRECT result!!! This is what we also expected from the first > code snippet!!! And this while the executed codes are in fact identical!!! > How on earth is this possible?!? */ > $testContainer2->printState(); > > ?> > end second code snippet -------------------------------- > > Well, this is the contradiction I wanted to show you all. I don't see the > logics of it all, but ofcourse I could be missing one obvious thing... > actually I hope so, because I need the construction as in the first code > snippet. > > Below the code for the classes Container en Child. > > begin third code snippet -------------------------------- > <?php > > class Container > { > > var $children; > var $property; > > function Container($load = FALSE) > { > $this->property = 'not loaded'; > if ($load) { > $this->load(); > } > } > > function load() > { > $newChild = new Child(1); > $this->add(&$newChild); > $this->property = 'state 1'; > } > > function add($child) > { > /* Here a reference of $this (the Container object) is assigned to $child's > $parent attribute. */ > $child->parent = &$this; > /* Here $child (the newly created Child object) is assigned to the > container's $child property. */ > $this->child = &$child; > } > > function printState() > { > print 'property = \'' . $this->property . > '\'<BR>' . > 'child->parent->property = \'' . > $this->child->parent->property > . '\'<P>'; > } > > } > > class Child > { > > var $id; > var $parent; > > function Child($id) > { > $this->id = $id; > } > > } > > ?> > end third code snippet -------------------------------- > > Well that's it. I hope someone can help me on this. I'm not a person who > asks for help very quick. I prefer finding it out myself, also because I'm > pretty experienced at PHP. But this is the first time I really can't figure > out what it is... I really NEEEEED you now :) > > Good luck and a lot of thanks in advance, > > Tim. > > P.S. I'm using PHP4, I do not know this code's reliability and behaviour > when running it with previous versions. > > > begin 666 complex.php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`` > ` > end > > > -- > PHP General Mailing List (http://www.php.net/) > To unsubscribe, visit: http://www.php.net/unsub.php >