Building a Singleton Trait with PHP 5.4

Question

We recently had a discussion if it was possible to build a trait Singleton PHP Traits and we played around with it a possible Implementation but ran into issues with building one.

This is an academic exercise. I know that Singletons have very little - if not to say no - use in PHP and that one should 'just create one' but just for exploring the possibilities of traits:

<?php trait Singleton {     protected static $instance;     final public static function getInstance()     {         return isset(static::$instance)             ? static::$instance             : static::$instance = new static;     }     final private function __construct() {         static::init();     }     protected function init() {}     final private function __wakeup() {}     final private function __clone() {}     }  class A  {     use Singleton;     public function __construct() {         echo "Doesn't work out!";     } }  $a = new A(); // Works fine 

reproduce: http://codepad.viper-7.com/NmP0nZ

The question is: It is possible to create a Singleton Trait in PHP?

Answer 1

Quick solution we've found (thanks chat!):

If a trait and a class both define the same method, the one of class if used

So the Singleton trait only works if the class that uses it doesn't define a __construct()

Trait:

<?php trait Singleton {     protected static $instance;     final public static function getInstance()     {         return isset(static::$instance)             ? static::$instance             : static::$instance = new static;     }     final private function __construct() {         $this->init();     }     protected function init() {}     final private function __wakeup() {}     final private function __clone() {}     } 

Example for a consuming class:

<?php     class A  {     use Singleton;      protected function init() {         $this->foo = 1;         echo "Hi!\n";     } }  var_dump(A::getInstance());  new A(); 

The var_dump now produces the expected output:

Hi! object(A)#1 (1) {   ["foo"]=>   int(1) } 

and the new fails:

Fatal error: Call to private A::__construct() from invalid context in ... 

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Demo