Miles,
I have come from a QBASIC to VFP to VC++ to PHP (VFP and PHP are my primary
languages), so I'll try to clear it up a bit.
In VFP, there is no constructor. I believe that the equivalent is the
init() method. The init() method, or the constructor (in PHP, C++, etc)
are useful for doing stuff that needs to be done when an object is
created. Likewise in true object oriented languages (PHP is not one of
them yet), there is a Destructor that is called when an object goes out of
scope or is deleted. In VFP, there is a Destroy() method to simulate this
behavior.
In VFP, there is the notion of Public, Private, and Hidden properties. PHP
does not yet support this behavior. In PHP, especially since it is such a
loosely typed language, you can add and remove properties on the
fly. Generally when programming in PHP, I like to identify private
properties and methods by placing a single underscore before their
name. This makes it clear for someone using the class not to ever
reference these entities.
If you want to see a neat example of object oriented programming in action
(not that I like M$ or anything), check out how the Microsoft Foundation
Classes (MFC) work. They effectively demonstrate public and private class
members, inheritance, constructors, destructors, parent objects, etc...
Hope it Helped...
-Jason Garber
IonZoft.com
At 09:10 AM 11/25/2001 -0400, Miles Thompson wrote:
>Chris,
>
>Not such a useless explanation at all, in fact a damned good one. Tight &
>concise, and you immediately related the functions to code examples, along
>with an easy-to-visualize object, the chair.
>
>But I have two questions:
>
>1. Why is a constructor class needed? My OO background is Visual FoxPro,
>and all it requires is a declaration of the class, it' methods and properties.
>
>2. Are the chairs properties exposed, or are they private? You use
>functions to both set and retrieve them, which implies that they are
>private. Even so, PHP's syntax is much cleaner than the tortured syntax
>used in VB.
>
>Miles Thompson
>
>At 03:28 AM 11/25/2001 -0800, you wrote:
>>On Sun, 25 Nov 2001, Rudi Ahlers wrote:
>> > Can anyone explain classes to me please? On many sites have I seen
>> classes,
>>
>>There are people earning PHD's while explaining classes. I know your
>>question originates from using PHP, and this is a PHP general mailing list
>>... but your question is just a tad too general for this list.
>>
>> > I'm not sure how these work, or how to use them. I'm still learning about
>>
>>Classes aren't unique to PHP. You need to learn a true object-oriented
>>programming language (C++, Java, ... etc.) to really learn classes.
>>Truly gifted individuals can learn object-oriented programming w/o too
>>much help, but they'd first have a firm grip on programming. I'd expect
>>the average, novice programmer to need a good amount of help learning &
>>understanding objecte-oriented programming ... like that attained from a
>>University, a good High School, or a lot of independent study and time
>>experimenting with code.
>>
>>That said ...
>>
>>You weren't completely missing the boat with your analogy of a class to a
>>variable, but in the same breath, that idea is totally missing the boat (I
>>can see from where you're coming, and to where you're headed). Classes
>>are an IDEA. They're not actually anything. They're the definition of
>>private and public data members and methods that should make up an object.
>>When a class is instantiated, an object is created with the defined data
>>members and methods available. You can then use the objects' methods to
>>set, get, alter, or otherwise interact with its data members, or to simply
>>perform a set of related operations. <Insert a couple semesters of theory
>>here.> That's my feeble attempt to explain classes. It's abstract, I
>>know, and possibly not a help at all. But it's because of the paragraph
>>above this one.
>>
>>Let's look at some petty code:
>>
>>class chair{
>> // DATA
>> var num_legs;
>> var color;
>> var num_arms;
>>
>> // METHODS
>> function chair( $legs = 3, $arms = 0 ){ //CONSTRUCTOR
>> $this->num_legs = $legs;
>> $this->num_arms = $arms;
>> }
>>
>> function setLegs( $legs ){
>> $this->num_legs = $legs;
>> return;
>> }
>> function getLegs( ){
>> return $this->num_legs;
>> }
>>
>> // ... *clip* ...
>>}
>>
>>
>>Above is the [incomplete] definition of a chair class. As you can see,
>>the class is useless. But you can instantiate the class, and create a
>>useable object ... you can now create a chair.
>>
>>$myChair = new chair( 4, 2 );
>>
>>Now I have a chair [object] with 4 legs and 2 arms, called $myChair.
>>Let's have the chair tell us how many legs it has ...
>>
>>$numLegsOnMyChair = $myChair->getLegs();
>>print( "My Chair has $numLegsOnMyChair legs." );
>>
>>Lets change how many legs the chair has ...
>>
>>$myChair->setLegs( 7 ); // very odd, seven-legged chair
>>
>>We should have a chair with 7 legs now, instead of 4. Prove it ...
>>
>>$numLegsOnMyChair = $myChair->getLegs();
>>print( "My Chair has $numLegsOnMyChair legs." );
>>
>>(As you alluded to previously, the object $myChair is seemingly a variable
>>that has access to scripts ... but I hope you see now that $myChair is an
>>object with methods and data members, not a variable.)
>>
>>That's very simple, and not too useful. Which brings me to the next
>>point. Knowing object-oriented programming is to know when and when not
>>to use it (especially with PHP). I don't need a class definition and an
>>object for a one-time use 7-legged chair. But I may need a class
>>definition if I were making many complete graph data structures, each with
>>a number of nodes equal to a unique number in the Fibonacci sequence. I
>>wouldn't want to re-code the basic logic of a complete graph over and over
>>for each graph. I could just instantiate graph objects, each with
>>different numbers of nodes, much like I did with the chair, above.
>></ramble>
>>
>> g.luck,
>> ~Chris /"\
>> \ / September 11, 2001
>> X We Are All New Yorkers
>> / \ rm -rf /bin/laden
>>
>>
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>
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