In PHP, is it possible to return a class's constructor function?

Question

I've gotten myself into a situation in PHP where I need to access a class's constructor without knowing what the class is. I may be approaching this from the wrong design standpoint, so here is the situation - I am working in Drupal, writing a slight OO layer around their classless data-handling to allow some non-Drupal devs to join me on a project.

In Drupal, all content is considered a node. So, I made an abstract class - Node. Any content must have a specific content type. With OO, this is easy - I make a Person class that extends Node, or an Event class that extends Node, etc. Now comes the tricky part - part of my project allows these nodes to be "included" in other nodes. That is, if Node A includes Node B, then whenever A is displayed, the data from B is displayed. This means that whenever Node A is instantiated, it needs to instantiate Node B as well. BUT...Node is an abstract class. So, I can't instantiate a raw Node. I have to instantiate one of its implementing classes. So, as far as I can tell, I either need to write an abstract, static function that all extending classes must implement that returns the constructor...OR, I need to use reflection in some way to determine type, and somehow call the appropriate class constructor?

Without regard to Drupal, what is the most appropriate way to handle this issue from a PHP/OO programming standpoint?

Here is my code:

<?php

abstract class Node {

    public $title, $short_summary, $full_summary, $body, $uri, $machine_type_name, $included_content;

    public function __construct($node) {

        ##
        ## Set simple values
        $this->title = $node->title;
        $this->body = $node->body['und'][0]['safe_value'];

        ##
        ## Set clean uri if aliased
        if (drupal_lookup_path('alias', 'node/'.$node->nid)) {
            $this->uri = '/'.drupal_lookup_path('alias', 'node/'.$node->nid);
        } else {
            $this->uri = '/node/'.$node->nid;
        }

        ##
        ## Set short summary if exists, else short form of body text
        if(strlen($node->body['und'][0]['safe_summary'])) {
            $this->short_summary = $node->body['und'][0]['safe_summary'];
        } else {
            $this->short_summary = text_summary($node->body['und'][0]['safe_value'], NULL, 100);
        }

        ##
        ## Set full summary to concatenation of body
        $this->full_summary = text_summary($node->body['und'][0]['safe_value'], NULL, 600);

        ##
        ## Add included content if module is enabled
        if (module_exists('content_inclusion')) {
            // is this possible? Is there a better design pattern available?
            $this->included_content = Node::get_constructor(node_load($node->content_inclusion['und'][0]['value']));
        }

    }

    public static abstract function get_all_published();
    public static abstract function get_by_nid($nid);
    public static abstract function get_constructor();
}

?>

While I appreciate your help, I clearly have some understanding of the static keyword from my above code. Unless I'm missing something, I don't see how this addresses the problem in my question. — rybosome
– rybosome, Commented Dec 10, 2011 at 21:12
But have you met its identical twin static, used for late static binding? (docs) — salathe
– salathe, Commented Dec 10, 2011 at 21:16

Yes Barry · Accepted Answer · 2011-12-10 21:18:46Z

2

Do you mean this:?

class MySubClass extends Node
{
    public static function get_constructor()
    {
        return new self();
    }
}

// ...

$object = MySubClass::get_constructor();

Or if you have PHP 5.3 you can use late static binding with which you could use return new static(); or $class = get_called_class(); return new $class();. Etc.

Full example:

abstract class Node 
{
    public $test;

    public function __construct()
    {
        $this->test = 'Node';
    }

    public static function get_constructor()
    {
        return new static();
    }
}

class MySubClass extends Node
{
    public function __construct()
    {
        $this->test = __CLASS__ . PHP_EOL;
    }
}

// ...

$object = MySubClass::get_constructor();
echo $object->test; // echoes "MySubClass"

edited Dec 10, 2011 at 21:18

answered Dec 10, 2011 at 20:50

Yes Barry

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2 Comments

rybosome Over a year ago

Node is an abstract class, though, so it cannot be instantiated. I need an unknown class implementing Node to be instantiated.

Yes Barry Over a year ago

It should still work though. As long as you call it with your subclass name. (e.g. MySubClass::get_constructor();)

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