Extending a singleton class

Question

i used to create an instance of a singleton class like this:

$Singleton = SingletonClassName::GetInstance();

and for non singleton class:

$NonSingleton = new NonSingletonClassName;

i think we should not differentiate how we create an instance of a class whether this is a singleton or not. if i look in perception of other class, i don't care whether the class we need a singleton class or not. so, i still not comfortable with how php treat a singleton class. i think and i always want to write:

$Singleton = new SingletonClassName;

just another non singleton class, is there a solution to this problem ?

Answer 1

It better be the other way around - provide a factory-method for non-singletons, and get instances of them using:

$NonSingleton = NonSingletonClassName::createInstance();

This is a recommended best practice for Java (in Effective Java), but it applies to most object-oriented languages.

Answer 2

I wouldn't recommend it as it would make your code a lot harder to understand (People think new means an entirely new object). But then I wouldn't recoemmed the use of Singletons.

The basic idea of this code is that there is a wrapper around a singleton. All functions and variables accessed through that wrapper actually effect the singleton. It isn't perfect as the code below doesn't implement a lot of magic methods and SPL interfaces but they can be added in if required

Code

/**
 * Superclass for a wrapper around a singleton implementation
 *
 * This class provides all the required functionality and avoids having to copy and
 * paste code for multiple singletons.
 */
class SingletonWrapper{
    private $_instance;
    /**
     * Ensures only derived classes can be constructed
     *
     * @param string $c The name of the singleton implementation class
     */
    protected function __construct($c){
        $this->_instance = &call_user_func(array($c, 'getInstance'));
    }
    public function __call($name, $args){
        call_user_func_array(array($this->_instance, $name), $args);
    }
    public function __get($name){
        return $this->_instance->{$name};
    }
    public function __set($name, $value){
        $this->_instance->{$name} = $value;
    }
}

/**
 * A test singleton implementation. This shouldn't be constructed and getInstance shouldn't
 * be used except by the MySingleton wrapper class.
 */
class MySingletonImpl{
    private static $instance = null;
    public function &getInstance(){
        if ( self::$instance === null ){
            self::$instance = new self();
        }
        return self::$instance;
    }

    //test functions
    public $foo = 1;
    public function bar(){
        static $var = 1;
        echo $var++;
    }
}

/**
 * A wrapper around the MySingletonImpl class
 */
class MySingleton extends SingletonWrapper{
    public function __construct(){
        parent::__construct('MySingletonImpl');
    }
}

Examples

$s1 = new MySingleton();
echo $s1->foo; //1
$s1->foo = 2;

$s2 = new MySingleton();
echo $s2->foo; //2

$s1->bar(); //1
$s2->bar(); //2