Polymorphism in PHP

[fil]

I'm implementing a project which relies heavily upon OO in PHP, and I've run into an issue that I don't quite know how to handle.

Imagine the following simple class heirarchy:

Shape
- Rectangle
- Square
- Ellipsis
- Circle

Everyone knows how to do the dead simple polymorphism like this:

$circ = new Circle();
$squ = new Square();

// Call virtual draw() on Shape
$circ->draw();
$squ->draw();

The issue I'm runing into:

I have a function which creates a generic shape:

function makeShape() { return new Shape(); }

How can I take the result of this shape and cast it (change it's type) to one of it's subclasses?

(i.e. In other languages this looks like: $myShape = (Circle) makeShape(); )

Thanks!

[ahundiak]

Which OOP language let's you change the class of an object with a simple cast?

Maybe you can post a few more details about what you are trying to do.

In a language like C++, you might create a circle class derived from a shape class with a circle attribute called radius.

If you have a function like
show_info(Shape *shape)
{
// This gives an error
cout << shape>radius;

// This works
cout << (Circle*)shape->radius;
}
You cannot get radius from a shape because the complier knows that shape has no radius. But you can use a cast to tell the complier that shape really is a circle. But, the cast itself is not changing the object and if the shape turned out to be a square then the code would fail at runtime.

Getting back to php, you don't need to do any casting. If shape has a radius attribute then it will work.

[fil]

OK, let's expand my example a bit:

Shape defines a draw() method on it. Each subclass overrides the draw() method to do some specific drawing (possbily with new member variables such as radius, possibly not).

Let's take an example where it doesn't use any extra info, but rather just has different behavior.

Here's some quick code:

$obj = new Shape();

// Draw the object normally
$obj->Draw();

// Draw the object as a 3D shape
$objTwo = (3DShape) $obj;
$obj->Draw();

You can't do this without casting because you have no context as to which version of Draw() you should be calling without casting the object.

( To expand a bit on the sceanrio. Here's some simplified code

// Creates an array generic shape based on xml
function createShapeObjectsFromXML($xml) {

}

$arrShapes = createShapeObjectsFromXML($xml);

// Now I want to manipulate and change those objects, some will be Circles, others squares, etc. I may set properties on those, I may not
(Code omitted)

for ($i = 0; $i < count($arrShapes); $i++) {
// Will call overriden Draw() function
$arrShapes[$i]->Draw();
}

Hoipe that makes things a little more clear)

[fil]

To change your example slightly:

If you have a function like
show_info(Shape *shape)
{
// This calls Shape:😃raw()
cout << shape->Draw()

// This calls Circle:😃raw()
cout << (Circle*)shape->Draw();
}

how would one do that in PHP?

[diego25]

"Object casting is not part of the implementation of PHP," according to http://www.php.net/manual/en/language.oop.php

With the advent of PHP 5, though, the OO support in PHP will be dramatically improved, such that Polymorphism may be possible.

Diego

[fil]

Thanks for the info Diego ... I'll try see if the serialize / unserialize strategy works for me.

[ahundiak]

What you are trying to do will work fine. Your createShapesFromXML will create either circle or squares based on xml data. Something like

switch($type)
{
case 'circle':
$shapes[] = new Circle($center,$radius);
break;
case 'square':
$shapes[] = new Square($lower_left,$upper_right);
break;
}
return shapes;

$shapes[0]->Draw() will now call the correct Draw() function.