PHP: Problems using Singleton pattern and understanding __clone method

Question

I am trying to implement the singleton pattern in php like described here in Example #2: http://www.php.net/singleton

When I run the example code

$singleton = Example::singleton(); // prints "Creating new instance."
echo $singleton->increment(); // 0
echo $singleton->increment(); // 1

$singleton = Example::singleton(); // reuses existing instance now
echo $singleton->increment(); // 2
echo $singleton->increment(); // 3

it allways ends with Fatal Error 'Clone is not allowed.' directly after 'Creating new instance.'

I would expect that there is no reason for php to call the __clone-method. In another real-life project of mine I want to have a singleton PlayerManager that holds Player-objects in an array (loaded only once in __construct) and has functions like GetPlayers() or GetPlayersByID($id).

In my script I write something like

$pm = PlayerManager::GetInstance();
$p1 = $pm->GetPlayerByID(0);
echo $p1->SomeNumber; //100

$p1->SomeNumber = 200;
$p2 = $pm->GetPlayerByID(0);
echo $p2->SomeNumber; //100 and not 200, as I would expect

Can someome give me some hints how to implement the PlayerManager using the Singleton pattern correct? I'm not sure if it is only a problem with the singleton or also a problem with returning object references...

are you sure your code doesnt contain any of the last three lines from the example in the manual that explicitly state they will result in an error? — Gordon, Jan 30 '12 at 23:12

score 1 · Answer 1 · answered Jan 30 '12 at 23:02

1

I'm not quiet sure why you're getting that error (post your singleton class if you want help with that). Though I always preferred this version to the one you're using, it's a bit simpler: http://www.talkphp.com/advanced-php-programming/1304-how-use-singleton-design-pattern.html

So with the above, your code would look like:

class Counter
{
    $CurrentValue = 0;

    // Store the single instance of Database 
    private static $m_pInstance; 

    private function __construct() { } 

    public static function getInstance() 
    { 
        if (!self::$m_pInstance) 
        { 
            self::$m_pInstance = new Counter(); 
        } 

        return self::$m_pInstance; 
    }

    public function increment ($by)
    {
        $this->CurrentValue += $by;
        return $this->CurrentValue;
    }
    public function getValue ()
    {
        return $this->CurrentValue;
    }
}

And to use:

$counter = Counter::getInstance();
echo $counter->increment(); // 0
echo $counter->increment(); // 1

$counter = null;

$counter = Counter::getInstance(); // reuses existing instance now
echo $counter->increment(); // 2
echo $counter->increment(); // 3

Tell me how that works out for you.

answered Jan 30 '12 at 23:02

Jess

8,628
6
49
67

1

its also no longer a Singleton because it can be cloned and unserialized. The purpose of the Singleton pattern is to **make sure** there can be only one instance and provide a global access point to it. – Gordon Jan 30 '12 at 23:04
grossvogel Thanks for the code review, @Gordon A fair point, though to my understanding php never clones classes, and only does so when you directly tell it to. So unless you're trying to protect the code from yourself I see no issue with it, though I might have missed something? – Jess Jan 30 '12 at 23:07
well if that is the argument then why have a Singleton in the first place. After all, it's you who will call getInstance() in your code as well. With that argument you can simply do what you should do anyway: [Create Once Only](http://butunclebob.com/ArticleS.UncleBob.SingletonVsJustCreateOne) – Gordon Jan 30 '12 at 23:10
I've had many times where I wanted to share say a database class over multiple other classes. The singleton pattern is designed to share a single resource easily (say logs or a database connection) not a tie a coders hands. I'm not disagreeing with your point that isn't a true singleton pattern, I'm just pointing out that it would work fine for most (if not all) situations. – Jess Jan 30 '12 at 23:12
It's a creational pattern, so I have to disagree it's about sharing easily. Global Access is one only purpose of it. But if it was about global access only, you could simply use a static class. Or create a regular instance on top of your script and pull it in with the `global` keyword. It might sound pedantic, but if your version is good enough, its proof that you dont need a Singleton in the first place. I'd also argue [there is no UseCase for Singletons in PHP](http://stackoverflow.com/questions/4595964/who-needs-singletons/4596323#4596323) but that's a different story. – Gordon Jan 30 '12 at 23:29
I would argue the same thing, singletons are (by my definition) an easy way out of coding something well. Though that's why I like it I suppose, it's saved me days of re-factoring defunct code to add something. I use them because they give you the construct and others (like __get and __set) unlike static, and you don't need the global keyword in *every* function. They have their niche. – Jess Jan 30 '12 at 23:34
I wouldnt call shooting myself in the foot a niche, but hey, that's just me ;) – Gordon Jan 30 '12 at 23:45
Haha, psh, I won't be the one that does it, it's all the other devs that do that XD – Jess Jan 31 '12 at 00:18
Thx for the answers. I'm using PHP Version 5.2.4-pl2-gentoo, if it is important to know. I tried your code: $counter = Counter::getInstance(); echo $counter->increment(1).'
'; // 1 echo $counter->increment(1).'
'; // 2 echo $counter->increment(1).'
'; // 3 $counter = null; $counter = Counter::getInstance(); // reuses existing instance now echo $counter->increment(1).'
'; // 4 echo $counter->increment(1).'
'; // 5 echo $counter->increment(1).'
'; // 6 Result is: 1 2 3 1 2 3 It seems that i'm getting a completely new instance, no idea why. – Steven Grigoleit Feb 15 '12 at 08:15

PHP: Problems using Singleton pattern and understanding __clone method

1 Answers1