Late static binding

Question

I've got a little problem. Here it is:

This is my singleton abstract class:

abstract class Singleton {

protected static $_instance = NULL;

/**
 * Prevent direct object creation
 */
final private function  __construct()
{
    $this->actionBeforeInstantiate();
}

/**
 * Prevent object cloning
 */
final private function  __clone() { }

/**
 * Returns new or existing Singleton instance
 * @return Singleton
 */
final public static function getInstance(){

    if(null !== static::$_instance){
        return static::$_instance;
    }
    static::$_instance = new static();
    return static::$_instance;
}

abstract protected function  actionBeforeInstantiate();

}

After that I create an abstract registry class:

abstract class BaseRegistry extends Singleton
{
    //...
}

Now it's time for session registry.

class BaseSessionRegistry extends BaseRegistry
{

//...

protected function actionBeforeInstantiate()
{
    session_start();
}

}

The final step:

class AppBaseSessionRegistryTwo extends BaseSessionRegistry { //... }

class AppBaseSessionRegistry extends BaseSessionRegistry { //... }

Testing

$registry = AppBaseSessionRegistry::getInstance();
$registry2 =AppBaseSessionRegistryTwo::getInstance();

echo get_class($registry) . '|' . get_class($registry2) . '<br>';

Output:

AppBaseSessionRegistry|AppBaseSessionRegistry

My expectations were:

AppBaseSessionRegistry|AppBaseSessionRegistryTwo

Why did I get such result? And how can I remake my code to obtain result that I expected?

UPDATE: i use this in my framework. And users will extend my BaseSessionRegistry class and will add their stuff. I want to solve this inside my framework classes

*(related)* [Who needs Singletons](http://stackoverflow.com/questions/4595964/who-needs-singletons/4596323#4596323) — Gordon, Sep 08 '12 at 11:27

xdazz · Accepted Answer · 2012-09-08T10:46:54.083

4

You need to do this:

class AppBaseSessionRegistryTwo extends BaseSessionRegistry {
    protected static $_instance = NULL;
    // ...
}

class AppBaseSessionRegistry extends BaseSessionRegistry { 
    protected static $_instance = NULL;
    //... 
}

If you don't declare static property separately, they will share the same static property of their parent.

Update: If you don't want the children declare the static property, then you could declare the static property as an array of the parent.

abstract class Singleton {

  protected static $_instances = array();

  // ...
  /**
   * Returns new or existing Singleton instance
   * @return Singleton
   */
  final public static function getInstance(){
    $class_name = get_called_class();
    if(isset(self::$_instances[$class_name])){
        return self::$_instances[$class_name];
    }
        return self::$_instances[$class_name] = new static();
    }

   abstract protected function  actionBeforeInstantiate();

}

edited Sep 08 '12 at 10:46

answered Sep 08 '12 at 10:27

xdazz

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Thanks, your solution work fine. But i use this in my frame work. And users will extend my BaseSessionRegistry class and will add their stuff. I want to solve this inside my framework classes. – warmspringwinds Sep 08 '12 at 10:36
Whats the point in having a singleton, if its replaced with another instance from another class? – JvdBerg Sep 08 '12 at 10:57
@JvdBerg: replaced? I created this class just to avoid copy pasting when creating new classes than i want to be singletons. – warmspringwinds Sep 08 '12 at 11:10

score 1 · Answer 2 · answered Sep 08 '12 at 10:30

Your first line:

$registry = AppBaseSessionRegistry::getInstance();

will fill the protected static $_instance with a AppBaseSessionRegistry instance. On your next line $registry2 =AppBaseSessionRegistryTwo::getInstance(); the protected static $_instance has already a value, and that instance will be returned.

Generally speaking, Singletons and statics are evil. Combining them is horrible. Get in the business of good OO programming and avoid Singletons and statics.

Maybe it's horrible) But it's also interesting to solve this problem. — warmspringwinds, Sep 08 '12 at 10:42

score 1 · Answer 3 · answered Sep 08 '12 at 10:44

 $registry = AppBaseSessionRegistry::getInstance();

The above statement set $_instance with the object of class AppBaseSessionRegistry
since in getInstance method we are checking if it has been set previously then return that value thus for your second statement

$registry2 =AppBaseSessionRegistryTwo::getInstance();

getInstance will return object of class AppBaseSessionRegistry i.e $registry1 and registry 2 referring to same object

Late static binding

3 Answers3