CODEWITHSUNDEEP
phpfunctionreferencestaticsingleton
SLC
SLC

Reputation: 2207

Static variable inside function can't hold reference to singleton

I've noticed a weird behavior with singletons in PHP there's no better way to explain this but with an example.

Let's say I have the following singleton class:

class Singleton
{
    protected function __construct()
    {
        // Deny direct instantion!
    }
    protected function __clone()
    {
        // Deny cloning!
    }
    public static function &Instance()
    {
        static $Instance;

        echo 'Class Echo'.PHP_EOL;
        var_dump($Instance);

        if (!isset($Instance)) {
            $Instance = new self;
        }

        return $Instance;
    }
}

And the following function:

function Test($Init = FALSE)
{
    static $Instance;

    if ($Init === TRUE && !isset($Instance)) {
        $Instance =& Singleton::Instance();
    }

    echo 'Function Echo'.PHP_EOL;
    var_dump($Instance);

    return $Instance;
}

And when I use the following:

Test(TRUE);
Test();
Singleton::Instance();

The output is:

Class Echo
NULL
Function Echo
object(Singleton)#1 (0) {
}
Function Echo
NULL
Class Echo
object(Singleton)#1 (0) {
}

As you can see the saved reference inside the function is lost after the execution even though the variable is static Also notice that the static variable inside the class method is working fine.

Is this supposed to be normal or I'm doing something wrong?

Upvotes: 4

Views: 1412

Answers (2)

Ja͢ck
Ja͢ck

Reputation: 173662

This behaviour is documented:

The Zend Engine 1, driving PHP 4, implements the static and global modifier for variables in terms of references. For example, a true global variable imported inside a function scope with the global statement actually creates a reference to the global variable. This can lead to unexpected behaviour.

This behaviour hasn't changed since ZE1 and the solution is simply to not assign a reference to a static variable, so:

$Instance = Singleton::Instance();

Update

I've simplified the issue below:

function test2($init = false, $value = null)
{
  static $test;

  if ($init) {
    $test =& $value;
  }

  var_dump($test);
}

$v = 1;

test2(true, $v);
test2();

Output:

int(1)
NULL

Upvotes: 6

Orangepill
Orangepill

Reputation: 24665

This is the normal pattern for a singleton in php. Note the $instance is a static property not a static variable within a function.

I am having trouble figuring out why but your above code can be fixed by doing one of the following.

1) Remove the reference assignment in the Test Function ($Instance =& Singleton::Instance(); becomes $Instance = Singleton::Instance();)

2) Remove the return by reference in the Instance Method (public static function &Instance() becomes public static function Instance())

3) Or both 1 & 2.

class Singleton{
    protected static $instance;

    protected function __construct(){

    }

    protected function __clone(){
        throw new Exception("Cannot clone a Singleton");
    }

    public static function getInstance(){
        if(!isset(static::$instance)){
            static::$instance = new static();
        }
        return static::$instance;
    }
}

Upvotes: 2

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