Interdependent classes

nightowl

What are the "best practices" for letting classes work together/talk to each other?

Let's say I have the class db which handles all database connections and queries. This one is of course used by a lot of other classes (class A, class 😎. How do I get started ?

Where do I define the parameters for class DB (username, dbname, ...)
How do I execute a query from one of the other classes?

class db {
     function init() {
           // set the parameters here ... 
     }
      function query($sql) {
          pg_query($con, $sql);
      }
}

class A {
     function get_data() {
        ... HOW to read the data? ... How to execute the query  

         via  class db ? 
     }
}


class B {
    function insert() {
         idem question ... should use teh same DB class

}

}

laserlight

Where do I define the parameters for class DB (username, dbname, ...)

It the db class, of course.
Though if you hardcode it in, then you may have to place the class file outside of the document root for security.
Otherwise, you could use a similiarly protected configuration file to supply the data at runtime.

How do I execute a query from one of the other classes?

In this case, one solution would be to instantiate a db object in an include file.
The files containing the declarations of classes A and B can then require_once() this include file, and make the db object global.

nightowl

If I define the connection parameters for my database class in the database class itself, every reuse of the class would need changements in the class itself. Using the same class from different applications would be impossible. Maybe this is not a huge problem but it sure is not what I was expecting to see as 'best practice'.

Another problem with your solution: creating an instance of the db class from an include file would work great but would require me to use $GLOBALS['db'] for example trough all of my A and B class - not exactly what I was hoping for ... I guess not creating an instance of it and using the db::query syntax would then be preferred, while loading the default connection settings hard-coded in the database class.

I hope somebody has a better solution ?

ahundiak

I'd suggest reading the examples for the PEAR DB class (pear.php.net) to get an idea of how to tie things together. The PEAR DB class is not necessarily the best abstraction class around but it's enough to get you started.

nightowl

I'm not actually searching for a database-abstraction layer but for an in-depth study of how to make classes (self written and existing ones) talk to each other. i'll have a look at the pear db class to see if i can learn something from it but I'm guessing you may have misunderstood my questioN?

laserlight

but for an in-depth study of how to make classes (self written and existing ones) talk to each other

Have you read up on object oriented programming?

nightowl

I'd be glad to read up on that. I've been browsing the web of course and I've found a couple of articles about OO Programming but most stop where it gets interesting. So far, no real information found about in-depth structures of OO programming.

If you could give me any pointers, that would be great !

laserlight

If I define the connection parameters for my database class in the database class itself, every reuse of the class would need changements in the class itself.

If you hardcode it, yes.
But you dont have to.
Consider a hypothetical database abstraction layer that merely caters for MySQL databases (so isnt much of an abstraction layer, actually), and does no error checking:

class MysqlDB {
	var $link;

function MysqlDB($host, $name, $pass, $database) {
	$this->link = mysql_connect($host, $name, $pass);
	mysql_select_db($database, $link);
}

function query($query_str) {
	return mysql_query($query_str, $this->link);
}
}

You can then instantiate different databases, for different scripts.
Actually, the only really useful thing about this class would be that it helps you manage using multiple databases simultaneously, but that's besides the point.

Another problem with your solution: creating an instance of the db class from an include file would work great but would require me to use $GLOBALS['db'] for example trough all of my A and B class

It doesnt.
You have to understand the nature of variabes in included files to know why.

I suspect that more canonical OOP would use inheritance, i.e. classes that need database handling would be derived from the database class.
I'm more comfortable with using a database object within such classes.

nightowl

I think you are not getting my point/question. That was: do you create an instance of the database class in every class that uses it ? Or do you use the class without creating an instance like db::query() ?

I don't like the idea of having to create 5 instances of my db class (or any other class) when I have 5 classes that rely on it. Not even to mention that, since I have 5 instances, I cannot configure it from one class and rely on it that the configuration will be correct when used from another class.

I may just have found the boundaries of the PHP object structure and I may need to use PHP 5 instead - but that was the question 🙂

goldbug

Sounds like design patterns are what you are searching for.

In this case, it sounds specifically like the Singleton pattern.

Some reading:
http://www.zend.com/zend/trick/tricks-app-patt-php.php
http://www.developer.com/design/article.php/10925_3345121_2
http://www.phppatterns.com/index.php/article/articleview/6/1/1/

Also a good dead-tree book on the subject (not specifically for PHP):
http://www.amazon.com/exec/obidos/tg/detail/-/0201633612/qid=1084910594/sr=8-1/ref=pd_ka_1/103-4977295-1859800?v=glance&s=books&n=507846

Another possibility to consider, is to use persistant database connections.

Sgarissta

You could also pass a reference to the DB object into the "constructor" of each class.
ex:

class DB {
....
}

class A {
    var $conn;
    var $var;

function A($conn,$var){
     $this->conn = $conn;
     $this->var = $var;
}

}
$db = &new DB();
$obj = &new A($db,'2');

nightowl

thx - that's what I was looking for I guess. I'll certainly give it a try.

nightowl

Wouldn't it be better to pass the var $db by reference to the instance of object A ? That way, any change of $db in the object would result in a change in the global instance too ?

laserlight

In this case, it sounds specifically like the Singleton pattern.

oh yeah, I couldnt look up my sources to answer "I hope somebody has a better solution ?" as I had forgotten the name :o

laserlight

Originally posted by nightowl
Wouldn't it be better to pass the var $db by reference to the instance of object A ? That way, any change of $db in the object would result in a change in the global instance too ?

That has already been done in the previous line.

Sgarissta

Originally posted by laserlight
That has already been done in the previous line.

Along with the oddities involving PHP and call-time pass by reference.