inheritance vs composition 3 tier archiecture

[traineeirishprogrammer]

I am starting to develop applications in the form of 3 tier really (4 tier) architecture at the moment.

I am still learning and at the moment I am developing a simple guest book.
With the power of this tiering system however I hope to increase the functionality of it soon quickly and easily.

Anyway I am sort of getting confused with my class structures.

I dont know whether it is better to use inheritance (subclasses) or composition (putting objects inside classes.

Here is an example of the dilema.

inheritance

1. DB access layer
[PHP]class db
{
var $link
var $query
function db($host ........)

function ......
}[/PHP]

2.business layer


[PHP]class comments extends db{

}[/PHP]

-------------------------------------------------------
OR

composition

1. DB access layer
[PHP]class db
{
var $link
var $query
function db($host ........)

function ......
}[/PHP]
2.comments layer

[PHP]class comments
{
function comments
{
$db = new db;
}
function add_comments
{
.........
}

}[/PHP]__________________________________________________

which do you think is better?

Thanks

[Atli]

Hi.

I faced the same problem at one point. I started out by inheriting the Database class everywhere and that worked fine. Until I came across a situation where one of my classes needed to inherit another class.

At that point I stopped inheriting my Database class everywhere, which was always a mess to tell you the truth, and started to import it as a parameter in the constructor. Or, sometimes when I'm feeling lazy, I just create a global Database class I use everywhere.

So, the short answer is, don't inherit it. Create a instance of it inside your other classes.

[traineeirishprogrammer]

Yes I see. The more I think about it composition seems the best option. Maybe I could use inheritance with classes on the same tier but when jumping between tiers its a no no as you said. I think by its very nature this architecture dosent want caller to know the implementation of classes on a lower tier. Just use it. Thats why I think that a db class should be called with in the constructor of a higher (logic) layer.

Anyone with good experience of this type of thing could you please give your opinion thanks.