PHP: Detect instances of same class

Question

I have a PHP Class which requires a unique value in its constructor. If multiple instances of the same class are passed the same value the results are horrific.

How would I go about detecting other instances of a Class so I can check and prevent this from happening before constructing any new ones?

Possible duplicate of stackoverflow.com/questions/475569/… — kba
– kba, Commented Jan 6, 2012 at 14:55

Jani Hartikainen · Accepted Answer · 2012-01-06 14:52:34Z

4

A simple solution would be to keep a static array of the values inside the class. Then, when a new instance is created, check the static array's contents in the constructor.

Something like..

class Foo {
    private static $usedValues = array();

    public function __construct($val) {
        if(in_array($val, self::$usedValues)) {
            throw new Exception('blah');
        }

        self::$usedValues[] = $val;
    }
}

answered Jan 6, 2012 at 14:52

Jani Hartikainen

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2 Comments

kba Over a year ago

+1 I was just about to write the same thing. Here's another implementation example.

George Reith Over a year ago

So simple yet so clever, thanks! I knew static existed for a reason :D

Rudolph Gottesheim · Accepted Answer · 2012-01-06 15:04:45Z

2

I think the multiton pattern is right for you.

class Foo {
    static protected $_instances = array();
    static public function getInstance($id) {
        if(!self::exists($id)) {
            self::$_instances[$id] = new Foo($id);
        }
        return self::$_instances[$id];
    }
    static public function exists($id) {
        return isset(self::$_instances[$id]);
    }
    protected function __construct($id) {
    }
}

edited Jan 6, 2012 at 15:04

answered Jan 6, 2012 at 14:54

Rudolph Gottesheim

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7 Comments

kba Over a year ago

@MarkBaker, @Rudolph, I don't think this is actually a correct answer. First of all, shouldn't $_instances be static? Second, it seems that OP wants to know if an instance already exists with that ID - not just retrieve it if it exists.

Rudolph Gottesheim Over a year ago

Thank, I just spotted that error myself and corrected it. I'm guessing that what I wrote is what he really needs, but a static public exists($id) method is written pretty easily.

George Reith Over a year ago

Thanks, this pattern is interesting me. But I am confused because the outcome seems to be new Foo($id) but if this is called in the constructor then doesn't this recreate an instance of itself and attempt to check it again? Seems kind of wasteful to run the script twice and construct two instances to perform this check, unless I am mistaken?

Mark Baker Over a year ago

@GEorge, no it doesn't create two instances, because the getInstance() method is called statically.... static calls (and that includes the exists() method as well) don't create a new instance unless explicitly via "new"

Rudolph Gottesheim Over a year ago

It isn't called in the constructor. The custructor is deliberately left empty and set to protected/private so it can only be called from within the class - getInstance().

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