Abstract Classes

sketchMedia · 11-26-2007, 07:30 PM

Hello all,

This is the second tutorial I have written for the wonderful community of TalkPHP :), the first one was about the singleton/factory pattern, within that article i briefly touched upon the idea of using interfaces, then Karl followed up with a great article explaining them further (Working with Interfaces).

In this short little tutorial I am going to be covering abstract classes within PHP5
(as i understand them, Ive never really been a big fan of OO theory, so if i get something wrong then please do correct me).

As you may recall interfaces do not and cannot define the operational behavior of an object, becasue interfaces are 100% abstracted, but rather they define how you (the developer) is to work with the class.

The way i visualize this is to think that i am entering into an 'agreement' between me and PHP, in other words I have defined a list of methods (rules) in the interface that I (or any other developer) must implement in any implemented class, so in any implemented classes i am agreeing to conform with the interface's rules. get me?

Abstract classes are a bit different, an abstract class is a partially abstracted class and can contain functioning methods and also abstracted methods that any class that inherits from it must implement.

confused? don't be, an abstract class merely exists to be extended (inherited) and contains methods and variables common to all derived sub-classes and therefore cannot be directly instantiated.

Now hopefully I've explained the differences, if not, this maybe useful to you, with an interface the creator is saying "Implemented class must implement X, Y, Z as defined in the Interface" where as an abstract he could say "extended class of abstract inherits X but must implement Y, Z as defined in abstract"

Right with the boring stuff out of the way, heres an example i will use from my last article, a DB class but in this case i want to have a common method for all of my objects that inherit the abstract, in this case its an error logging method (I haven't bothered making it a working example, but you should be able to alter to your needs):

PHP Code:


			

abstract class DB_Abstract
{
    //Noraml method, this get inherited by child objects (aslong as its public or protected)
    public function logError() { echo 'From: <b>' . __CLASS__ . '::' . __FUNCTION__ . '()</b><br />'; } 
    
    //Abstract methods, the inherited object must implement these much like an implemented class from an interface
    abstract public function connect();
    abstract public function escape_string();
    abstract public function insert();
    abstract public function select();
    abstract public function delete();
    abstract public function update();
    abstract public function close();
}

class DB_mysql extends DB_Abstract //we are inheriting DB_Abstract
{
    //We must implement all methods defined as abstract in the parent class
    public function connect(){ echo 'From: <b>' . __CLASS__ . '::' .__FUNCTION__ . '()</b><br />'; }
    public function escape_string() { echo 'From: <b>' . __CLASS__ . '::' . __FUNCTION__ . '()</b><br />'; }
    public function insert(){ echo 'From: <b>' . __CLASS__ . '::' . __FUNCTION__  . '()</b><br />'; }
    public function select(){ echo 'From: <b>' . __CLASS__ . '::' .__FUNCTION__ . '()</b><br />'; }
    public function delete(){ echo 'From: <b>' . __CLASS__ . '::' .  __FUNCTION__ . '()</b><br />'; }
    public function update(){ echo 'From: <b>' . __CLASS__ . '::' . __FUNCTION__ . '()</b><br />'; }
    public function close(){ echo 'From: <b>' . __CLASS__  . '::' . __FUNCTION__ . '()</b><br />'; }
}

Right, the first class has the keyword 'abstract' attached to the front of the class definition, which tells PHP that this class is abstract.

Im not going to explain most of the class because I have commented it and since this is an advanced tutorial you should be able to decipher it xD.

Anyway if we try to instantiate DB_Abstract,

PHP Code:


			
$mysql = new DB_Abstract();

PHP will produce :

HTML Code:

Fatal error: Cannot instantiate abstract class DB_Abstract

This is because you cant instantiate abstracts because they are like interfaces, abstracted types

So we must instantiate the derived object (should have been obvious but just incase anyone didnt see it)

PHP Code:


			
$mysql = new DB_mysql();

Now to illustrate this we can call:

PHP Code:


			
$mysql->connect();//from the DB_mysql class, that implements the abstract method in DB_Abstract
$mysql->logError();//from the method implemented in DB_Abstract

Because we have inherited, or in php 'extended' from the abstract, we have access to DB_Abstract::logError() from DB_mysql, but in the abstract we defined connect() as abstract so PHP will want an implementation of it within the inherited object, which in this case we do, but if i was to comment out connect() in DB_mysql, PHP will produce:

HTML Code:

 Fatal error: Class DB_mysql contains 1 abstract method and must therefore be declared abstract or implement the remaining methods.

So in short, abstracts are much like interfaces, but they allow you to have common methods inherited into derived objects, where as interfaces just enforce the API of the object.

I hope that made it clear, like i said, I've never been a guy that likes Object Oriented Theory, in fact it bores me to suicide sometimes so if there is something erroneous here then don't hesitate to shout at me :)

Karl · 11-27-2007, 04:14 PM

Nice article sketch, good follow up to our previous articles.

Wildhoney · 11-27-2007, 04:21 PM

As I understand it, all abstract functions also default to public, and although it is definitely good practice to put in the public, it's not a requirement to make the abstract class work well. The most perplexing part of abstracts, and interfaces while we're at it, were that they didn't really add anything to make my code work. After all, I could omit the abstract class, and my other classes that once inherited the abstract class, would still work perfectly fine.

However, and here is the really big however, since I've started to code with abiding by standards, you will realise that entering into an agreement with PHP is a big must, especially if it's not only you who's going to be editing the code. For instance, in programming there are million and one various ways to do the same task. There's a good chance that if you let someone loose on your code with a task in mind, they'd implement it in a only totally different way to which you have. Thus, when you enter into an agreement with PHP, everybody else who comes along must also enter into that agreement with PHP - there's no two ways about it! So if I am creating my child classes which hand out drinks to my PHP code (lucky PHP code!) then I can tell everybody who works on my code, to implement the giveDrink function, like so:

php Code:

abstract public function giveDrink($szDrink);

That way there are no arguments as to how something should be implemented, you've defined the rounds and everybody must abide by that, otherwise you get anarchy, and as I'm sure you're well aware, pure chaos isn't too far behind.

Essentially, you define the rules and everybody must stick to those rules. Great article, Sketch! I just felt it was necessary to cover the reasons why you would use it, I'm sure many are looking over this and thinking, pfft, I don't need that! Well, you will soon :) If you stick with your programming, I can assure you!

sketchMedia · 11-27-2007, 05:33 PM

Thats a good summary, yes i agree these features are pretty perplexing especially when your trying to learn them, because as you rightly say, they don't really do much apart from create rules and people may take the attitude 'Why the hell do i need that, i can get the same results with normal inheritance' i know i did, BUT when you actually start using them you see the power that they provide especially when there is more than one coder at work with the code.

I'm glad you like it.

bdm · 11-28-2007, 02:05 PM

Good article, keep em' coming. :)

Although, abstract classes and/or interfaces can't exactly force developers to ignore extending/implementing them.

Wildhoney · 11-28-2007, 02:39 PM

You're right there. A developer must implement and/or extend depending on whether it's an abstract or an interface. And that's possibly somewhat of a downfall. Although I can't seem to think of any other way around that, you'll just have to capitalise, underline and embolden the fact that ALL CHILD CLASSES MUST EXTEND THE LOVELY ABSTRACT.

dschreck · 11-28-2007, 06:39 PM

while we're on the topic of "forcing" someone to extend off of a class or what not.

There are ways to use PHP to restrict access to other parts of your core classes, by using the PHP function: is_subclass_of() ( you can grab a whole bunch of other class/object functions at http://www.php.net/manual/en/ref.classobj.php )

By using little tricks like this, you can enforce parent to child relationships. Also, consider the following, which are possible thanks to Abstract classes, as these are possible ways to also enforce parent to child relationships...

a. To Start, if you don't trust other developers to follow the rules, put all other developers are only allowed to edit files within a certain branch. This can be done with a version control client, or simply, giving them an FTP login that restricts them to a certain folder subset.

b. Keeping your core files out of their branches forces them to include them, and "interface" through them.

c. Keeping your methods private/protected, means you can restrict the developers ability to override your precautions already set in place.

d. Then, have 'sanity checks' to make sure access to the database and/or other core classes / modules is restricted to children / parent classes. (if you so choose)

But yes, extending or implementing, totally optional.
But you can gain a whole lot by extending or implementing abstract classes or interfaces.

A lot of PHP4 users don't really see this right off of the bat, thinking that this is one of those things for languages like Java or C++, classes with visibility restrictions etc etc, bleh. Those languages are very different than PHP, which is such a weakly typed language.

And I'll be honest, not everything you do needs an abstract class, or an interface. Experience will tell one day, "Hey, this would be a great place to plop down an interface." or "Damn, I should stick these methods in abstract classes."

I personally, love the flexibility you have with these features in PHP5.

So yeah, in my rambling I think I came to a point.

sketchMedia · 11-28-2007, 07:36 PM

thanks dschreck, some good points there, like the idea of forcing a parent - child relationship.
You cant force the developer to implement the interface, whereas if an abstract is used, if for example there is a method in the abstract that the devloper needs to inherit, then he will have to extend the abstract, therefore he will inherit the rules thats why i prefer abstracts to interfaces (not just because of that).

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Abstract classes are classes that contain one or more abstract methods (methods without implementation). nice articles.