PHP late static bindings (new static): How to ensure subclass constructor compatibility and handle divergent constructors?

Question

For some more complicated class hierarchy I was playin around with a minimal example for this problem a bit.

This class is given - the method "createOrUpdate()" may be modified:

class A {

    protected $a;
    protected $b;
    protected $c;

    function __construct($a,$b,$c) {
        $this->a = $a;
        $this->b = $b;
        $this->c = $c;
    }

    public static function createOrUpdate($a,$b,$c) {
        if(self::exists($b)) {
            someWhateverUpdate();
        } else {
            new static($a,$b,$c);
        }
    }
}

Now lets see what happens if we extend it:

class B extends A {
    function __construct($a,$b,$c) {
        parent::__construct($a,$b,$c);
    }
}
B::createOrUpdate("rock","this","now");

Works fine!

class C extends B {
    function __construct($a,$b) {
        parent::__construct($a,$b,"exactly");
    }
}
C::createOrUpdate("rock","this","now");

Works also fine however if somebody does createOrUpdate() parameter $c is lost silently!

class D extends A {
    protected $d;
    function __construct($a,$b,$c,$d) {
        $this->a = $a;
        $this->b = $b;
        $this->c = $c;
        $this->d = $d;
    }
}
D::createOrUpdate("rock","this","now");

Error: Throws ArgumentCountError

class E extends A {
        function __construct($b,$c) {
        $this->a = "Lorem";
        $this->b = $b;
        $this->c = $c;
    }
}

D::createOrUpdate("rock","this","now");

Error: Works but will behave completely unexpected.

Now my question is: Can I use some reflection within createOrUpdate() in order to check if the current subclass called is implementing the constructor correctly? How would you handle this if somebody else may implement further subclasses within the hierarchy?

nforced · Answer 1 · 2018-11-13T14:53:01.203

1

How about if you implement interface to your A class?

interface interfaceA {
    public function __construct($a, $b, $c);
}

class A implements interfaceA
{
 ...
}

This will force every extending class to either not have constructor or implement the one matching on class A or PHP Fatal error: Declaration of B::__construct($a, $b) must be compatible with interfaceA::__construct($a, $b, $c) will be thrown.

You can also add public static function createOrUpdate($a, $b, $c); to the interface to force all extending classes to implement such method.

edited Nov 13 '18 at 14:53

answered Nov 13 '18 at 14:35

nforced

361
3
14

Thanks I like the idea on putting an interface on top of the hierarchy. Making createOrUpdate abstract is also a good idea if code reuse can be neglected however in this case I want it. – Blackbam Nov 13 '18 at 14:45

score 1 · Answer 2 · answered Nov 13 '18 at 14:59

After experimenting a bit further also with the input of @nforced I found another good solution - make the constructor protected and final. Now subclasses are forced to implement a creation method (e.g. create()) which is using the desired constructor.

class A {

    protected $a;
    protected $b;
    protected $c;

    protected final function __construct($a,$b,$c) {
        $this->a = $a;
        $this->b = $b;
        $this->c = $c;
    }

    public static function createOrUpdate($a,$b,$c) {
        if(self::exists($b)) {
            updateMe();
        } else {
            new static($a,$b,$c);
        }
    }
}

You can also do `abstract class A` with `abstract public static function createOrUpdate($a,$b,$c);` to force subclasses to implement this method. — nforced, Nov 13 '18 at 15:10

score 0 · Answer 3 · answered May 21 '19 at 16:49

Have you tried to send an Object/Interface to the upsert() function? Like this:

class Parameters {
    private $a;
    private $b;
    private $c;
    ...
    setter&getter for attributes();
}
$paramObj = new Parameters();
$paramObj->setA('a');
...

D::upsert($paramObj);

class D extends A {
    public static function upsert(Parameters $param) {
        if ($param->getA()) {
            doSth();
        } else {
            $instance = new static();
            $instance->a = $param->getB();
        }
    }
}

This is only my little opinion and if I miss something, tell me, thanks.

PHP late static bindings (new static): How to ensure subclass constructor compatibility and handle divergent constructors?

3 Answers3