class inheritance and set variables

Marino

I am trying to understand how to work with classes. and I've got the following problem/question.

Best explained at the hand of the following example:
These are the classes:

class a {

var $x;
var $y;

function a($x,$y) {
$this->x = $x;
$this->y = $y;
}
}

class b extends a {
var $z;

function b () {
$z = $this->$x . $this->$y;
print $this->z;
}
}

I set $x and $y the following way:

$i = new a("Hel", "lo");

Now I want to print the string "Hello"

$j = new b()

What I would like is this to work, but ofcourse: how does class be know the values set in class a without refering to $i ?
I could solve the problem by rewriting the class b and also provide the strings "Hel" and "lo" when constructing the new class b, but that's is something i want to avoid because when my classes get bigger they will have lots of constructor values, and when my class-tree grows..
You can see where I am going with this.

So help me please...

goldbug

$i is an instance of class 'a'
$j is an instance of class 'b'

They are two separate instances. $j cannot automatically know what is going on in $i (unless it's a reference, of course).

Just remember, when you use the 'new' keyword, you are creating an instance of a class, not using a static class.

For example.
$a = new classA();
$b = new classA();
$c = new classA();

$a, $b, and $c refer to different objects that just happen to have the same underlying structure/interface, but they cannot automatically share data.

... also, while I'm at it.... in your class b constructor, you should use $this->z instead of $z 🙂

Edit:
P.S. It's probably not a good idea to cram all that logic into your constructor. The way you have them written now, they are no different than a common function. Consider breaking up the constructor logic into other methods to set that stuff/print to screen. That way your subclasses can use the methods, and you don't cram too much stuff into your constructors.

Marino

Ah yes. You are absolutely right (also about the $this->z, it was an example, not actual tested code :p )

So would you please tell me how to rewrite this example code so that i have only to initiate class a once and let class b use the values set ($i) ?

PS. to your ps, I know. My actual classes would be far to big to post here 😉

Marino

hmm. damn. Ignore this message. Can't seem to delete this (accidental) post. Moderator!? Sorry.
Still would like your help on this topic tough 😉

reyjrar

You'd need to pass the Object you're extending into the constructor of class b. This isn't really that great of a problem to solve with OO though. Here's how you can get it to work:

class b extends a {
 var $z;
 function b ($a) {
   $this->x = $a->x;
   $this->y = $a->y;
   $this->z = $this->x . $this->y;
 }
}

I don't see a way around this, of course its difficult to do in any language that supports OO a lot better than PHP, so its no surprise that its difficult in PHP 🙂.

There's not enough detail in your post to figure out why this functionality would be necessary. It sounds like a design decision, so instead I propose an alternate solution that will probly do what you want:

class a {
   var $x;
   var $y;

/* constructor and all the other stuff */

function z() {
   return $this->x . $this->y;
}
}

now, in your main code, when you need to get the value of z in your example, you call z() on a. This gives you the ability to always have the most recent version of the value which would be hell any other way.

$a = new a("Hel", "lo");
echo $a->z();  # "Hello"
$a->y = 'l';
echo $a->z(); # "Hell";
$a->y = " low";
$a->x = "How";
echo $a->z(); # "How low"

Design. Design. Design.

Marino

hmm. doesn't really help me. Could you try rewriting this code?

class basic {
var debug;

function a($debug) {
$this->debug = $debug;
}

function slashn() {
if ($this->debug) print "\n";
}
}

class html extends basic {
var debug;

function basic($debug) {
$this->debug = $debug;
}

function nextline() {
print "<br />"; $this->slashn();
}
}

$debug = false

$Cbasic = new basic ($debug);
$Chtml = new html($debug);

And so there are a lot of other classes which extend either basic or html.

Help me not to set the $debug every time I make a class!

swr

In that case, I would use a global variable.

Usually it's better to avoid global variables, but if something really is global (like a debug setting) then a global variable is the right tool for the job.

Marino

Not too femiliar with global variables. Where in the code should i put the text global $debug then?

swr

Like this:

function slashn() {
    global $debug;
    if ($debug) print "\n";
}

Or this:

function slashn() {
    if ($GLOBALS['debug']) print "\n";
}

To set $debug, just do this:
$debug = 1;
in global context (not inside a function or class).

More info here:
http://www.php.net/manual/en/language.variables.scope.php

Marino

it is like this:

 
class basic { 
var debug; 

function a($debug) { 
$this->debug = $debug; 
} 

function slashn() { 
if ($this->debug) print "\n"; 
} 
} 

class html { 
var basic;

function basic($basic) { 
$this->basic = $basic; 
} 

function nextline() { 
print "<br />"; $this->basic->slashn(); 
} 
} 

$debug = false 

$Cbasic = new basic ($debug); 
$Chtml = new html($Cbasic);

I'm amazed that all you guys couldn't tell me this sooner!

Well I am off building my classes 😃