Class vars - public or private?

Hi:

I don't often delve into OO but it's becoming more common in the PHP projects I develop.

I just wanted to know simply - what are public and private variables within a class? Can you give me some basic examples?

Also while we're on the subject - what are the differences between and circumstances of use of constructor and other types of class-methods?

Thanks a lot :)

Public variables are accessible from outside the class, which is frowned upon in the world of object oriented design. Private variables are only accessible to instances of the same class.

The constructor is a special method that is automatically called when you create an instance of a class. It is used for initialising the instance. True OO languages also supply a corresponding method called a destructor, which is called automatically when an instance is destroyed.

Quote:

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Originally Posted by AutisticCuckoo

Public variables are accessible from outside the class, which is frowned upon in the world of object oriented design. Private variables are only accessible to instances of the same class.

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Why is it frowned upon and are you able to give me a simple example of an instance of a class whose variables are accessible from outside of it?

Quote:

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Originally Posted by AutisticCuckoo

The constructor is a special method that is automatically called when you create an instance of a class. It is used for initialising the instance. True OO languages also supply a corresponding method called a destructor, which is called automatically when an instance is destroyed.

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How exactly would you construct it and what does doing so actually do? Again an example or two would be very helpful :)

Thanks a lot.

Public data members break one of the key OO concepts, namely 'encapsulation'. Encapsulation means that an 'instance' (i.e. an object) of a class controls its internal data, and only allows access to it through member functions. You can have a getFoo() function that returns some data (by value, or as a copy), and a setFoo() function that allows others to modify data in a controlled way. For instance, if the data is a numeric value that must not be negative, the setFoo() member function could reject any value below zero.

Example class:

PHP Code:

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class Foo
{
  public  $a;
  private $b;
}

var $myFoo = new Foo;

$myFoo->a = 13; // Allowed.
$myFoo->b = 69; // Not allowed! $myFoo->b is private. 


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The constructor is called when you create an instance of a class. Its name is the same as the class name. Example:

PHP Code:

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class Bar
{
  public $x;

  function Bar()
  {
    $this->x = 1;
  }
}

var $myBar = new Bar;
echo $myBar->x; // Writes '1'. 


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Note that you don't explicitly call $myBar->Bar(). It's called automatically when you do 'new Bar'.

Thank you - that was all very helpful! :)

What I do is to keep a class property private, and all methods private as well, unless for certain a method has to be accessed outside the class.

If you extend a class from a parent class you may note you are unable to use private so you use protected instead, as I see it anyways :)

Also note that if you are using the PHP5 public/private/protected keywords, you also have at your disposal the __construct() function. You can use this as your constructor instead of a function that uses the same name as the class.

Quote:

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Originally Posted by AutisticCuckoo

For instance, if the data is a numeric value that must not be negative, the setFoo() member function could reject any value below zero.

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In PHP5 you get another option.

Say you have a read/write property of a class, and you don't want to bother with any checking of what the property is set to. Instead of using a get/set pair, you can just use a public variable. But what if you want to add some sort of responce to the variable being set later?

Well, you can make the variable protected, and then use PHP's __get($val) and __set($key, $val) magic functions to handle the request. This way, for simple public variables, you have a way to "upgrade" to a get/set system, but let yourself code more quickly using publics when you don't need to do anything complicated.

hth,
Douglas

Why getter and setter methods are evil
More on getters and setters

I quite often use protected myself....

I know some people might look down at that as it's encorages the use of inheritance. However I quite often seem to end up generating an interface, then making a abstract base class to implment basic functionallity, and then entend once from there. I do normally try to avoid over entending where possible, however there are some cases where extending a bit is a better solution. Personally, I don't have too much issue with extending, provided the child class is not completely replacing what is defined in the base class. Likewise, there is no point in extending if the child class isn't using methods in the base class as this is also bloat.

I often avoid using public where possible. I find in most cases getters and setters are much better as they allow you to impose restrictions, and do validation etc, which i like to do as it is defensive coding (and thus saves a fair bit of debugging in the long term)