[Resolved] Problem: Accessing Variable from Another Class File

davidjam

Here is my problem in simplified code:

include "page-class.inc"

$thispage = new page;
// note: within the page constructor a $variable is set (i.e. $this->variable = "hello")

include "form-class.inc"

$thisform = new form;
// note: class form extends class page
// note: constructor assigns the $variable from the page object to $anothervariable (i.e. $this->anothervariable = $this->variable)

print $thisform->anothervariable;

// NO RESULT

Any thoughts...?

davidjam

Everytime I try to use global in the scope of the constructor I get a fatal error. I have also tried $GLOBALS

(Maybe simplifying the code made it too boring because I'm not getting any response. I will wait.)

Sgarissta

I'll assume you're using php4 in which case I don't believe extended classes call the parent constructor. Hence why your variable assignment is failing. I haven't messed with the inheritence bugs in PHP4 for a while, so maybe someone else can give you a work around.

Weedpacket

The PHP manual has a note to the effect that "it is your responsibility to propagate the calls to constructors upstream where appropriate". Which presumably just means

class form extends page
{
function form()
{ parent::page();
....
}
...
}

davidjam

I put the call to parent:page() in the extended class constructor and it worked. Thanks.

You know, that's a drag. Because I don't want to call the parent constructor, since it has all kinds of other stuff that I don't want repeated. I just want the $variable!

It's strange because I have another class that extends the base class which is in the same ".inc" file and it behaves properly: i.e. $this->base_variable works in the extended class. So it seems the problem is caused by having multiple ".inc" files.

Any thoughts on this?

davidjam

I figured it out! Here is the rule:

If an extended class that is in an "include" file OTHER THAN the base class requires a base class variable then you must use global on the existing object of the base class. The global command must exist in a function of the extended class. To illustrate:

include "base-class.inc"

$object = new base-class // object instantiated

include "extended-class.inc"

extended-class.inc:

class extended-class extends base-class {
var $variable
function extended-class () {
global $object; // bring the object in the local scope
$this->variable = $object->variable; // make the variable available to ALL of the extended-class (not just the constructor)
..}

I guess I just kept putting the global in the class braces. It goes in the function! I'm learning!

davidjam

Can one of you PHP gurus confirm this for me once and for all:

Do extended classes that exist in include files separate from the base class inherit the variables from the base class?

Sgarissta

Originally posted by davidjam
Can one of you PHP gurus confirm this for me once and for all:

Do extended classes that exist in include files separate from the base class inherit the variables from the base class?

They will inherit the variable, but not the value. The value is tied to a specific instance of the object. This behavior is the same regardless of what file the class definitions reside in. Example.

class Base {
   var $var;
}

class Child extends Base {
    var $var2;
}

$obj = new Base();
$obj->var="value";

$obj2 = new Child();
echo "Var=" . $obj2->var;

The above code will return "Var=" with no value specified. As the value of $var was never set by the Child object. The instance of Base, and the instance of Child are separate, and therefore so are the values of their variables.

davidjam

Thank you,

In fact the include file that had a base and a child class in it wasn't actually working like I imagined it was. $this in the child class was NOT getting the variable values from the base class.

So I will have to "string" the classes together by globalizing the instances of parent classes in the child class constructors. The only thing I don't like about this is that I have to use a consistent naming convention of my objects.

Is there a way of retreiving the object name that is instantiated so that the globalizing is not dependent on one particular name.

Sgarissta

I would say that the issues you're running into are more an issue of your design choices than limits in the language. Is the inheritence neccessary, does it provide some advantage over separate classes? Would a "repository" class holding the instances of your other classes solve the issue without the need for globals?

Personally I try to avoid pulling things from the global scope INTO my objects whenever possible. It defeats the point of encapsulation and makes the code harder to follow. So for a solution I'd look at rethinking your class structure, and I'll bet you can find a way around using globals for this.

davidjam

I like your advise. I will look into it.

However, at this point (and I may regret it later (i.e. "the point of encapsulation, etc")) the names of my objects and classes are really simple and logical: "page", "form", "sql" so globalizing these won't be hard to follow. At least that is what I think right now. What I want to avoid is duplicating variables in subclasses that have already been defined in higher classes.

But I will come up to the challenge and see if I can't break free from the globalizing problem...