PHP OOP: Unique ID property among all instances of class

Question

My question is simple, but I can't seem to find any answer for it online. I will probably jump straight into the code:

class NewClas {
    public $id;

    public function __construct($id) {
        $this->id = $id;

        $this->checkVars();
    }

    public function checkVars() {
        if (empty($this->id)) {
            trigger_error('ID is a required parameter.');
        } elseif ($this->id WAS USED IN A PREVIOUS OBJECT) {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }

}

$object1 = new NewClass('id1');
$object2 = new NewClass('id2');
$object3 = new NewClass('id1'); // throws error, because id1 was already used

So - is it possible to check for uniqueness of a value of the property among all instances of the class? I am just getting started with OOP, so please go easy on me. :)

Also, I am aware of spl_object_hash but I would prefer work with IDs as readable strings, specified by a user.

Thanks in advance!

Alma Do · Accepted Answer · 2014-02-14T08:07:24.437

It is possible - if you'll store static registry of used id's. That's about:

class NewClass
{
    public $id;
    //here's your registry
    protected static $registry = array();

    public function __construct($id) 
    {
        $this->id = $id;

        $this->checkVars();
        //if not failed, add to registry:
        self::$registry[] = $id;
    }

    public function checkVars() 
    {
        if (empty($this->id)) 
        {
            trigger_error('ID is a required parameter.');
        }
        //checking if it's already used: 
        elseif (in_array($this->id, self::$registry)) 
        {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }

}

You can check this demo

YES Alma, that's exactly the thing I was looking for! Thanks for your time, OOP and Stack Exchange once again proved awesome. :) — Petr Cibulka, Feb 14 '14 at 08:12

score 1 · Answer 2 · answered Feb 14 '14 at 08:02

It won't throw any error. You are triggering the error using the trigger_error under the else block. That's the reason you are getting an error.

When you do this..

$object3 = new NewClass('id1');

The id1 is passed as the parameter to the constructor and it is set to the $id public variable. Now checkVars() is going to be called .. Here $this->id will not be empty, so it will go to the else block.

This is the right code actually..

<?php

class NewClass {
    public $id;

    public function __construct($id) {
        $this->id = $id;

        $this->checkVars();
    }

    public function checkVars() {
        if (empty($this->id)) {
            trigger_error('ID is a required parameter.');
        } else {
           // trigger_error('ID is already used.');
        }
    }

}

$object1 = new NewClass('id1');
$object2 = new NewClass('id2');
$object3 = new NewClass('id1');

Hello Shankar, I wasn't getting any error, I was just looking for a way to check for "uniqueness" of a property. My actual code is A LOT more difficult, here I was trying to come up with a simplified version. Thanks though! :) — Petr Cibulka, Feb 14 '14 at 08:11

score 0 · Answer 3 · answered Feb 14 '14 at 08:21

This is the right answer from above answer: But to respect SOLID OOP design principles I would recommend to make id private and use getters and setters to access it.

class NewClass
{
    private $id;
    //here's your registry
    public static $registry = array(); //since is static you can make it public

    public function __construct($id) 
    {
        $this->id = $id;

        $this->checkVars();
        //if not failed, add to registry:
        self::$registry[] = $id;
    }

    public function checkVars() 
    {
        if (empty($this->id)) 
        {
            trigger_error('ID is a required parameter.');
        }
        //checking if it's already used: 
        else if (in_array($this->id, self::$registry)) 
        {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }

To respect SOLID principles, you wouldn't use a static registry in the first place, since it's effectively global state. Also, Getters and Setter usually break object encapsulation and lead to consuming code making decisions that the object as the information expert should decide. — Gordon, Feb 14 '14 at 08:36

PHP OOP: Unique ID property among all instances of class

3 Answers3

This is the right code actually..