OOP Inheritance Questions ...

monroe

I'm having issues with inheritance. If I have a memeber in a base class, and then I create a subclass that inherits form my baseclass, objects of the subclass have all the member variables and functions fo the base class. This much I get.

My confusion is wether or not it is good practice to access members (variables and funcitons) of the base class directly in the sub class? If I do acess base class members directly in my subclasses, then I could get into trouble if my base class changes. If I remove a member form my base class, then any subclass that accesses that memeber will be broken if I don't rememeber to update all of my subclasses, which seems to go against the rules and benefits of encapsulation?

Can someone clear this up for me?

ahundiak

Congratulations. You have just hit upon one of the major contradictions in oop programming - balancing encapsulation against inheritance. It's a widely discussed issue.

IMHO, if you own the classes then go ahead and access base member data in the derived classes. You will know what needs to be updated if changes are made.

However, if you are using someone else's classes then never access the member data directly. Only use public methods and then cross your fingers and hope that the method's functionality never changes.

BTW, PHP 5 is supposed to support interfaces which partually reduces the problems associated with using inherited methods.

fandelem

I believe the concept behind OOP is that you want to create functions in your base (or "super") class that access its variables. This way if you change the variable name, your getField() across 40 different applications will still function properly... and so it goes..

monroe

Originally posted by fandelem
I believe the concept behind OOP is that you want to create functions in your base (or "super") class that access its variables. This way if you change the variable name, your getField() across 40 different applications will still function properly... and so it goes..

Yes but what if the getField() in the base class changes as well?

for example, let's say the member var in the bass class is $myVar, and has associated accessor function: get_myVar(). So in my subclass, I use:
$something = $this->get_myVar();

Then later, the base class is changed and the $myVar memeber is removed all together (Along with it's accesor function get_myVar()). Now, if I instaitate my subclass, I'm going to start getting "Call to undefined function get_myVar()" errors. If I've used get_myVar extensively in the subclasses in my 40 different applications, I'm in big trouble and I have alot of work to do in order to make all my subclasses work again.

ahundiak

Here are a couple of articles on this issue. As the remarks at the end of the articles suggest, there is a wide range of opinions on the issue.
http://www.javaworld.com/javaworld/jw-09-2003/jw-0905-toolbox.html?