Question

If I have:

abstract class AbstractSingleton
{
    protected static $instance;

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

    protected function __construct() { }
}

Can a user who extends the class redefine the visibility of __construct?

class Singleton extends AbstractSingleton
{
    public function __construct() { } // That would be a problem
}

And second question, what happens for subclasses if __construct is defined as private in AbstractSingleton? Is there not constructor at all? Is a new default constructor provided to the class, and if yes, with what visibility?

Answer 1

Well, for starters, you don’t need to return a reference (&) from your method. Since PHP 5, objects are passed and returned by reference by default.

To your actual question, for a child to access a member of a parent, its access modifier needs to be at least protected. That counts for constructors as well. So, in your case, a subclass would be able to do:

abstract class AbstractSingleton
{
    protected __ construct() {}
}

class Singleton extends AbstractSingleton
{
    public __construct()
    {
        parent::__construct();

        // more code
    }
}

The way to get around that is to use the final keyword. See: PHP documentation.

For your second question, if you make AbstractSingleton’s constructor private and try to invoke one on its child, then a default, public constructor will be used instead, just like always.

EDIT: With all that said, Singletons are hardly ever a good idea, especially in PHP. Chances are, you can solve your problem with merely taking advantage of PHP objects automatically being passed/returned by reference. If not, Dependency Injection is a far more OOP friendly alternative.