One reason would be that you might not know the details of a derived class's
implementation details at design time.
For instance I could have an abstract class such as this:
Class MyTestClass
{
function doSomething()
{
doSomethingElse();
}
abstract function doSomethingElse() ;
}
As you can see I have an abstract function with no details. I know that
when the doSomething() function is called it must call the doSomethingElse
Function, but at the time that I define MyTestClass I don't know what that
is exactly.
Now say another developer needs to derive a class from MyTestClass. Because
the base class is abstract they must define an emplementation for the
doSomethingElse class such as:
Class MyDerivedClass extends MyTestClass
{
function doSomethingElse()
{
// here the developer must specify the specifics of what this method
does
}
}
Now in our business logic if we do the following:
$myClass = new MyDerivedClass();
$myClass->doSomething();
The code will actually execute the doSomethingElse() implementation that
has been defined in MyDerivedClass.
This can be extremely useful in developing frameworks that you want other
developers to extend on their own and also in business systems where you
might need to maintain a system by adding new classes/implementations over
time. Certain patterns also make heavy uses of interfaces and abstract
classes.
Perhaps a real-world example would make more sense. Does this help? If
not, let me know and I'll come up with a similar/real-world example.
Colin
On 10/23/05 4:40 AM, "Alan Lord" <lord_alan@xxxxxxxxxxx> wrote:
> Thanks Jasper,
>
> That makes sense.
>
> But what benefit is there is having it as an explicitly "abstract"
> class? Why can't it just be a "normal" class definition which you
> inherit from?
>
> Sorry to be so dense....
>
> Al
>
>> -----Original Message-----
>> From: Jasper Bryant-Greene [mailto:jasper@xxxxxxxxxxx]
>> Sent: 23 October 2005 09:19
>> To: Alan Lord
>> Cc: php-general@xxxxxxxxxxxxx
>> Subject: Re: Abstract Classes?
>>
>> On Sun, 2005-10-23 at 08:54 +0100, Alan Lord wrote:
>>> Hi All,
>>>
>>> I have started reading a couple of books about PHP 5 and
>> whilst most
>>> of it is comprehensible even to me :-), I fail to
>> understand WHY there
>>> is such a thing as an abstract class or method?
>>>
>>> I think I understand what it is: A class that can't itself be
>>> instantiated, only inherited from, or a method that
>> describes itself
>>> only in terms of what properties it accepts but no implementation
>>> detail.
>>>
>>> But could someone explain why I would want to use this? I'm
>> sure it is
>>> very useful but I can't quite see the benefit...
>>
>> Hi Alan
>>
>> Here's an example from an application framework I've been working on.
>>
>> It has classes to represent the different HTTP response statuses (like
>> 301 Moved Permanently, 304 Not Modified etc.) with required
>> and forbidden headers for each and different characteristics
>> (like no request-body allowed etc).
>>
>> I have an Abstract class called HTTP_Response, from which
>> HTTP_Response_Moved_Permanently, HTTP_Response_Not_Modified,
>> and many others all inherit.
>>
>> The reason HTTP_Response is abstract is because there's no
>> such thing as an HTTP_Response on its own. It has to be a
>> specific type of HTTP Response, that is a 301 Moved
>> Permanently or a 304 Not Modified.
>>
>> Does that help?
>>
>> --
>> Jasper Bryant-Greene
>> General Manager
>> Album Limited
>>
>> e: jasper@xxxxxxxxxxx
>> w: http://www.album.co.nz/
>> p: 0800 4 ALBUM (0800 425 286) or +64 21 232 3303
>> a: PO Box 579, Christchurch 8015, New Zealand
>>
>>
Thank you,
Colin Shreffler
Principal
303.349.9010 - cell
colin.shreffler@xxxxxxxxxxxxxxxxx
Warp 9 Software, LLC.
6791 Halifax Avenue
Castle Rock, CO 80104
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