PHP Singleton instantiation with String class name

Question

Im building a small MVC system and i want my controllers to be Singletons. I made the base controller "Controller" a singleton and every other controller extends from that. My Router processes the request from the URL and grabs the controller string name.

This is where im getting the private constructor error because i am trying to do this:

class IndexController extends Controller {
     //the "Index" part comes from the url
}

    class Controller {

        private $instance;

        /**
        *   Initializes a new Singleton Controller
        */
        private function __construct() {
        }

        /**
        *   Get the instance of the Controller
        */
        public static function getInstance(){
            if (null === self::$instance) {
                self::$instance = new self();
            }
            return self::$instance;
        }
   }

$className = "Controller";
$inst = new $className; //here is where i get the error
$inst = $className::getInstance() //also fails

I have done my research and i stumbled upon this (http://www.php.net/manual/en/reflectionclass.newinstancewithoutconstructor.php), however I am not sure if that will work or is the best method if it does.

Answer 1

$inst = new $className; //here is where i get the error This error is right, as the constructor is private.

$inst = $className::getInstance() //also fails This error is also right, as this syntax is not allowed.

If you want your conrollers to be singletons, you can, but you need to "twist" the rules for that.

b.t.w

1. Why on earth would you want to do this?
2. Why not just use an existing MVC (you do not need the entire FW).
3. replace the self with static as this will reference the actual controller you try to instantiate and not the Controller class, as static references only the class it is written in.

Twisting the rules: You will need to instantiate your controllers through a factory (it is a pattern). Usually the front controller is used for that.

Answer 2

A couple things really in regards to errors in using the "pattern"

class Controller {

    private static $instance;

    /**
    *   Initializes a new Singleton Controller
    */
    private function __construct() {
    }

    /**
    *   Get the instance of the Controller
    */
    public static function getInstance(){
        if (null === self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }

}

Note that the property is now static and the constructor is private as necessary for this anti-pattern.

Second your calling code should look like

$instance = Controller::getInstance();

You should never have to instantiate 'Controller' from the outside; the point of the pattern is to only allow access through Controller::getInstance()

---

That being said Singleton is little better than global state. Misko Hevery talks about it in this Google Clean Code Talk: Global State and Singleton. It is a really good video and explains far better than I can why Singletons are bad news. sourcemaking.com also has a good write-up on Singletons where they say that Singletons are unnecessary most of the time

Answer 3

class Controller {

        /**
         *   Let the __construct method be private to prevent new instance though new.
         */
        private function __construct() {}

        /**
         *   Get the instance of the Controller
         *   Here use the lazy loading. (need php >= 5.3)
         */
        public static function getInstance(){
            if (null === static::$instance) {
                static::$instance = new static();
            }
            return static::$instance;
        }
   }

class ControllerA extends Controller {
        //You need an static property to hold the instance.
        protected static $instance;
}

var_dump($a = ControllerA::getInstance());
var_dump($b = ControllerA::getInstance());
var_dump($a === $b);

If your Controller Name is a string, You can do like this:

$classname = "ControllerA";
call_user_func(array($classname, 'getInstance'));
call_user_func($classname .'::getInstance'); // As of 5.2.3

Answer 4

On this line:

$inst = $className::getInstance() //also fails

You are using the scope resolution operator. You should be using this '->' for access to object data members/methods.

$inst = $className->getInstance() //also fails

Otherwise, you need to use the scope resolution operator on the class rather than the object:

$inst = Controller::getInstance() //also fails

I do not know where you are getting the value off $className as a class. There is no class called IndexController.