how to force implementation of an abstract class?

[Dormilich]

I've got 2 DB classes (see below) where one class extends my base DB class. I want the child classes (the User class is just an example) to have the static method getInstance() but I can't declare this function abstract (E_STRICT) and I don't see, how I can implement an interface.

does anyone have an idea how to solve that?

abstract class Base
{
    private static $PDO = NULL;
    private static $PS  = array();
 
    function __destruct()
    {
        self::$PDO = NULL;
    }
 
    protected static function load()
    {
        if (self::$PDO === NULL)
        {
            try {
                $dsn = 'mysql:host=' . DB_SERVER . ';dbname=' . DB_NAME_2;
                self::$PDO = new PDO($dsn, DB_USER, DB_PASS);
                self::$DBstatus = 'connected';
            }
            catch (PDOException $pdo)
            {
                ErrorLog::logException($pdo, __CLASS__, __METHOD__);
            }
        }
    }
 
    // some more general and useful methods
 
}

class User extends Base
{
    private static $instance = NULL;
 
    function __construct() {}
 
    function __clone() {}
 
# I want this function to be implemented
# by every child class of Base
# though not necessarily with the self::init() call
 
    public static function getInstance()
    {
        if (self::$instance === NULL)
        {
            self::$instance = new self;
            // this is where class specific code begins
            self::init();
        }
        return self::$instance;
    }
 
    private static function init()
    {
        parent::load();
        # prepare several statements
    }
 
}

[dlite922]

I'm kind of lost in what you're trying to accomplish. May I ask how you came to the conclusion you need this?

[Dormilich]

sure, I need to make sure the DB class I use is a Singleton. and I want the connector class as (kind of) parent.

[Atli]

You could simply have all child classes implement an interface, as well as expand the base class.

Then they would both inherit the requirements from the base class, and be required to implement everything from the interface.

For example:

<?php
// Interfaces for the base and the child classes.
interface iBase {
    public function connect();
}
interface iBaseChild {
    public static function getInstance();
}
 
// Base class
abstract class Base implements iBase {
    public function connect() {
        echo "Base connect" . PHP_EOL;
    }
}
 
// First Child class
class FirstChild extends Base implements iBaseChild {
    public static function getInstance() {
        return new FirstChild();
    }
}
 
// Second Child class
class SecondChild extends Base implements iBaseChild {
    // Will simply inherit the connect function from the parent
    // but fail because no getInstance function was defined.
}
?>

The second child class will fail, because it does not define your static getInstance method.

[Dormilich]

Thanks, that's clearing the matter (I didn't know that you can extend and implement at the same time).

do you know if you can force a private/protected static method to be implemented?

[bilibytes]

@Dormilich
Did you find out ?

[Dormilich]

kind of. you can't. simple reason is because they’re not public (interfaces only deal with public methods) and you can’t make them abstract because they’re static.