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 ...

---

Demo