instance variables aka data members

[maxim]

im having trouble understanding whey we need to create instance variables (data members) in a class

 

consider this code.

 

<?php
/* Filename: DataMember.php
* Created on: May 15, 2007
* Author: maxim
* Email:  ocaumaxim@gmail.com
*/

$instance = new DataMember();
$instance->printString();

class DataMember {

private $str;

public function __construct() {
	$this->str = 'Hello World!'; 
}

public function printString() {
	print $this->str;
}
}
?>

 

now if you comment out or delete the data member declaration

<? private $str; ?>

 

the script will still work fine. it has something to do with the $this variable. could someone please explain

[Jenk]

PHP automatically creates a public instance variable, if you assign a value to that which does not exist.

[maxim]

so basically there is absolutely no point what so ever in declaring a instance variable public. and i realize why they are meant to be private its so you cant access it directly form the instance, correct ?

[utexas_pjm]

It's good practice to explicitly declare all of your member data, even though PHP will implicitly create new public member data for you if you assign a value.

 

Best,

 

Patrick

[Jenk]

so basically there is absolutely no point what so ever in declaring a instance variable public. and i realize why they are meant to be private its so you cant access it directly form the instance, correct ?

iirc it raises an E_NOTICE error when you don't declare the variables.

 

If you want to create a value object or similar, you can declare the __set() and __get() magic methods to store data cleanly.

<?php
class ValueObject
{
    private $_data = array();

    public function __get ($name)
    {
        if (!isset($this->_data[$name])) throw new IndexOutOfBoundsException($name . ' is not a valid member');

        return $this->_data[$name];
    }

    public function __set ($name, $val)
    {
        $this->_data[$name] = $val;
    }
}

$foo = new ValueObject;

$foo->bar = 'Hello Worls!';

try
{
    echo $foo->bar;
}
catch (IndexOutOfBoundsException $e)
{
    echo '$foo->bar has not been set yet!';
}
?>

[lee20]

It's good practice to explicitly declare all of your member data, even though PHP will implicitly create new public member data for you if you assign a value.

 

I agree that it's a good practice, and improves the readability of your class. Declaring your variables gives you a standard place to comment on what that variable is for.

 

It also prevents the E_NOTICE errors which some programmers choose to ignore but each error comes at a processing expense. And displaying all errors while you are developing/debugging is a useful tool and can reduce time wasted when you mispell a variable somewhere along the way.

 

Also, if you use PHP5's overloading capabilities, you would NOT declare variables that you wish to overload. So declaring or not declaring even your public variables can therefore determine the behavior of your class.