Extending A Class

[The Little Guy]

I have two classes:

 

- User

- Bank

 

Bank extends User, and what I want to do is get a public property from User, which is $id, in my file, I have something like this:

 

$user = new User();
$user->user_setup();
$bank = new Bank();
echo $bank->owner;

 

class User{
public $id, $first, $last, $email;
public function user_setup(){
	$this->id = $_SESSION['id'];
	$this->first = $_SESSION['first'];
	$this->last = $_SESSION['last'];
	$this->email = $_SESSION['email'];
}
}

 

class Bank extends User{
public $owner;
public function __construct(){
	$this->owner = parent::$id;
}
}

 

From what I have, I was expecting "$this->owner" from the Bank class, to equal my user id from the User class, which in this case should be 1. Any reason why it is not, am I doing it wrong?

[.josh]

properties from parent classes are used the same way as if it were declared in the child class. That is: $this->id is what you want to use.

[allworknoplay]

properties from parent classes are used the same way as if it were declared in the child class. That is: $this->id is what you want to use.

 

Agreed, and you should be able to do this too correct?

 

$bank->user_setup();

[.josh]

Well I suppose it depends on where you're trying to call it, don't you think? 

[cringe]

Little Guy,

 

You should consider making your class variables private or protected with public setters/getters. If class variables are public, a consumer can use them directly which breaks encapsulation and causes future problems if you decide to rework your class code.

 

 

[The Little Guy]

properties from parent classes are used the same way as if it were declared in the child class. That is: $this->id is what you want to use.

 

Then why is that not working?

 

I tried, but I get nothing...

 

In my construct I have this:

$this->owner = $this->$id;
echo $this->owner;

[Ken2k7]

You need to call the user_setup() method first

[The Little Guy]

You need to call the user_setup() method first

 

Like inside the Bank class?

 

And not like this:

$user = new User();
$user->user_setup();
$bank = new Bank();
echo $bank->owner;

 

?

[trq]

You need to call the user_setup() method first

 

Like inside the Bank class?

 

And not like this:

$user = new User();
$user->user_setup();
$bank = new Bank();
echo $bank->owner;

 

 

Yes. In your example $bank and $user are two completely different objects.

?

[The Little Guy]

If I did it like this would it work, where I call user_setup from the extended Bank class, instead of the User class?

 

$bank = new Bank();
$bank->user_setup();
echo $bank->owner;

[Daniel0]

Why would Bank extend User? Inheritance denotes an is-a relationship between the parent and child class.

[The Little Guy]

because "Bank" is the users bank account, it contains his/her banking information, such as:

Total cash

Loans

Deposits/Withdrawals

etc.

[Daniel0]

Yeah, but the Bank is not the User. You should be using composition instead.

[The Little Guy]

Without the user, you wouldn't have a bank account, because the bank account requires a user....

 

That is how I have been looking at this situation, I'm I looking at it the wrong way?

 

If you were to make this how would you go about it?

[Ken2k7]

The Little Guy, to put Daniel0's words into examples you can understand -

 

I have 2 classes - Vehicle and Car.

<?php
class Vehicle {
     private $plate_number;
     public function __construct ($plate) {
          $this->plate_number = $plate;
     }
     public function getPlateNumber () {
          return $this->plate_number;
     }
}

class Car extends Vehicle {
     private $num_doors;
     public function __construct ($num_doors, $plate_number) {
          parent::__construct($plate_number);
          $this->num_doors = $num_doors;
     }
}

 

I have Car extends Vehicle because a Car is a Vehicle; however a Vehicle is not always a Car. Just like a Bank is not a User. However, you can say Bank and User is a has-a relation instead of an is-a relation, just like a Vehicle and Car is a has-a relation, but Car and Vehicle is an is-a relation.

[The Little Guy]

OK, That makes sense, but would it be possible for you to explain to me why Bank and User are not a good relationship?

 

I'm not sure I understand that part (this is the first time I have use inheritance).

 

Thanks!

[Ken2k7]

Is a Bank a User or is a User a Bank? No. You can say a User is a Human. It's just logic.

 

Edit - to expand, think about what a Bank class does. Or better yet, think of the characteristics of a Bank. It has no relationship with a User. Banks need Users, but it isn't a User. It's like saying you need Aor, so let's have a Human extends Air.

[The Little Guy]

OK, so basically don't extend User with Bank.

 

So if you were to extend Bank, what types of things would be good extensions of Bank?

[Ken2k7]

... hmm... well... it depends. There are no set answer for that question. It just depends on the situation and what you want to code. I mean it's just pure logic. It's not really even programming-related. I guess you can say Bank extends Institution because a Bank is an Institution.

[Daniel0]

So if you were to extend Bank, what types of things would be good extensions of Bank?

 

Anything that logically is a particular type of bank.

[Ken2k7]

Yeah like BoA (Bank of America).

 

<?php

class BoA extends Bank {
     ...
}

 

[Daniel0]

Well, that depends on your implementation. For that to be true then you could say that an instance of BoA would be a particular branch of BoA. You couldn't say e.g. Daniel0 extends PHPFreaks_Member. It might not always be the case that there would be a logical choice for a subclass.

 

If you want a good example of how hierarchical inheritance you could take a look at the biological taxonomy for living organism. Overall you have life. Within life you have the different domains, and within the individual domains you have kingdoms, and so on...