Is there a primer on "pass by reference" somewhere?

bunner_bob

I suppose this could be considered a newbie question, but here it rests nonetheless...

I've just gotten started with OOP and classes, and this "pass by reference" thing has reared its head. I've never used it, I don't really understand it, and I want to know what the general rules and guidelines are for its use - when should I use it, when shouldn't I use it, what are the advantages?

I've got about 5000 lines of classes now, all "translated" from a version of this site I built in the "old" way. Not a single "pass by reference" in the batch, and everything works just fine, so at least in my case it's not a matter of having to do it that way or it won't work. But I'm sure there must be a reason for doing it.

Can anyone suggest a link that'll help me ease into the waters?

Bob

LordShryku

Best place I can think of would be the manual....

stfuji

Try

http://www.phpvolcano.com/articles/php5/obj-reference.php

or maybe

http://www.sitepoint.com/article/1264/7

There are tons more you might search on

pass by reference in google

-Geoffrey

AVANT

There are 2 main ways of passing variables into functions

pass by value
pass by reference

Pass by value into a function will allow the function to make a copy of the variable or value and use it in the function body.

Pass by reference into a function will allow the function to use that exact variable (utilize that memory).

Ever used a function before where you had to change the value of a variable within the function?
- In such cases, if you use pass by value, you will have to global the value in order to access it outside the function. Why? Because pass by value makes a copy of the variable and places it in seperate memory.
- But if you used pass by reference (add a & infront of the variable), you no longer need to global the variable because it's actually using that exact variable instead of working with a duplicate.

Hope that explains it for you.

yelvington

This will not make it any clearer but I can't help myself.

Nicklaus Wirth, the designer of the Pascal programming language, was asked: "How do you pronounce your name?" He replied: "That depends on whether you are calling me by reference or by value."

(Explanation of the pun: His name is pronounced "Veert," but most Americans would say, "Worth.")

bunner_bob

OK - let's see if I'm starting to get this:

Looking at classes specifically - when I say:
new MyClass();
right there I'm creating an instance of MyClass.

If I say:
$foo = new MyClass();
then I'm creating an instance and then immediately making a copy of it (into $foo).

But if I say:
$foo =& new MyClass();
then there is only one copy of the instance in existence. $foo is the real, accessible "pointer" to it - how I work with it, activate methods in it, etc.

Does that sound right?

What about when I want to return a value from the class - when I'd normally use:
$var = $foo->GetValue($bar);

should I say:
$var =& $foo->GetValue($bar);

or:
$var = $foo->GetValue(&$bar);

or both:
$var =& $foo->GetValue(&$bar);

and then there's the method inside the class:
function GetValue($input) {
// stuff
}

I think I understand that if I'm trying to pass the variable by reference, I also need the "&" in the function parameter - thus:
function GetValue(&$input) {
// stuff
}

is that right?

Final question: use of $this->referring to methods (or variables) inside a class. Do I want to pass by reference there?

For instance (this would be calling a method from inside its own class):
$this->var = $this->GetValue($input);

or:
$this->var =& $this->GetValue($input);

or:
$this->var = $this->GetValue(&$input);

what's the best approach in this case? Does it make any difference if I'm referencing into a class-wide attribute ($this->var) vs. just a temporary variable ($var)?

So far I have yet to run into a situation where I need a function or class to alter a variable outside the function/class, but I'm sure I've just been working around it until now, and once I really get the concept I'm going to be able to streamline things nicely.