Prevent a circular reference

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Hi,
I need your help. If I add the drums to the cymbals that already belong to drums then I create a circular reference. When calling getDescription() a line like this can crash a Web Server. I want to safeguard against such mistakes. My strategy would be to check during the add() method to see whether the instrument being added already contains a reference to the one calling the method.My pseudo-code is:

Check if drums has children;
If has children then compare drums with cymbals:
If are equals then return true else return false;

My questions are:

Is it correct my logic/pseudo-code?
To transale this pseudo-code to PHP, is it good idea to create a recursive method or interactive method?
There is another way to check a circular reference? If so, please tell me!

HTML

<html>
<body>
<head>
<style>
body{ font:12px Verdana, Geneva, sans-serif; font-weight:bold}
td{ font:11px Verdana, Geneva, sans-serif;}
</style>
</head>
<?php
abstract class AbstractInstrument{
	
	private $name;
	private $category;
	private $instruments = array();
	
	public function add(AbstractInstrument $instrument){
		array_push($this->instruments, $instrument);	
	}
	
	public function remove(AbstractInstrument $instrument){
		array_pop($this->instruments, $instrument);
	}
	
	public function hasChildren(){
		return (bool) (count($this->instruments) > 0);
	}
	
	public function getChild($i){
		return $instruments[$i];
	}
	
	public function getDescription(){
		echo "- one ".$this->getName();
		if($this->hasChildren()){
			echo " which includes:<br>";
			foreach($this->instruments as $instrument){
				echo "<table cellspacing=\"5\" border=\"0\"><tr><td>   </td><td>-";
				$instrument->getDescription();
				echo "</td></tr></table>";
			}
		}
	}
	
	public function setName($name){
		$this->name=$name;
	}
	
	public function getName(){
		return $this->name;
	}
	
	public function setCategory($category){
		$this->category=$category;
	}
	
	public function getCategory(){
		return $this->category;
	}
}

class Guitar extends AbstractInstrument{
	function __construct($name){
		parent::setName($name);
		parent::setCategory("guitars");
	}
}

class DrumSet extends AbstractInstrument{
	function __construct($name){
		parent::setName($name);
		parent::setCategory("drums");
	}
}

class SnareDrum extends AbstractInstrument{
	function __construct($name){
		parent::setName($name);
		parent::setCategory("snare drums");
	}
}

class BaseDrum extends AbstractInstrument{
	function __construct($name){
		parent::setName($name);
		parent::setCategory("base drums");
	}
}

class Cymbal extends AbstractInstrument{
	function __construct($name){
		parent::setName($name);
		parent::setCategory("cymbals");
	}
}

$drums = new DrumSet("tama maple set");
$drums->add(new SnareDrum("snare drum"));
$drums->add(new BaseDrum("large bass drum"));

$cymbals = new Cymbal("zildjian cymbal set");
$cymbals->add(new Cymbal("small crash"));
$cymbals->add(new Cymbal("large high hat"));

$drums->add($cymbals);
//$cymbals->add($drums); //Uncomment this and then it will show an error!!

$guitar = new Guitar("gibson les paul");
echo "List of instruments: <p>";
$drums->getDescription();
$guitar->getDescription();
?>
</body>
</html>

Posted 13-Feb-13 4:59am

User 8438611

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Comments

Sergey Alexandrovich Kryukov 13-Feb-13 13:41pm

Before we continue, can you explain what's wrong in circular references? They are pretty much a commonplace, can be found anywhere. What's wrong?
—SA

[no name] 13-Feb-13 16:46pm

I think a circular reference is a mistake very serious. Please copy code, uncomment "//$cymbals->add($drums)" and test! This will origin a error. I want prevent this mistake.

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_Maxxx_ · Answer 1 · 2013-02-13T13:18:00

Solution 1

Your $drums is actually a drum set, not a collection of drums, isn't it?

So should you not be adding each cymbal to that collection rather than adding cymbal to a collection of cymbals then adding that collection to the drum set?

So I would have a Drum Set to which I add each drum and each cymbal(or add a collection of drums and a collection of cymbal)

If Cymbals needs a reference to the Drum Kit then all good - the collection of Cymbals' parent would be the drum kit.

Posted 13-Feb-13 13:18pm

_Maxxx_

Comments

[no name] 14-Feb-13 5:38am

My $drums is actually a drum set, not a collection of drums.

No. I should not be adding drums to the cymbals that already belongs to drums! This origin a circular reference.

A drum set is a composite of (1 or more)drum(s) and (1 or more)cymbal(s).

My questions are:
1)Is it correct my logic/pseudo-code?
2)To transale this pseudo-code to PHP, is it good idea to create a recursive method or interactive method?
3)There is another way to check a circular reference? If so, please tell me!

_Maxxx_ 14-Feb-13 6:02am

You say you shouldn't be adding drums to the cymbals - then surely the answer is "then don't"
The way you are setting up our drum$ is that it is a collection that contain individual drums plus a collection of cymbals. M

[no name] 14-Feb-13 6:09am

OK.

But can you answer my questions?

_Maxxx_ · Answer 2 · 2013-02-14T00:18:00

It is not possible to answer your question sensibly;

Your pseudo code will always return true given the info you have given.

if you can add a drum kit to a cymbal, could you also add a cymbal to a cymbal, or a drum to a cymbal? Your Cymbal object seems to act both as an individual and a collection of cymbals (there is no Cymbalset class) so I cannot deduce what your intentions may be.

If you talk abstractly of collections that can hold collections and you do not want to add collection a to collection b if some object already in collection b contains collection a, then you would need to traverse the entire tree every time you want to add to any collection; If I had to do it I would do it recursively - but as I said earlier, i wouldn't do it like this in the first place.