| Re: Abstract Objects and Methods [message #12111] |
Tue, 23 June 1998 00:00 |
Phillip & Suzanne
Messages: 31
Registered: June 1998
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Member |
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J.D. Smith wrote:
>
> Phillip David wrote:
>>
>> Does anyone know of any way to make sure a method has been overriden by a
>> child class? The only thought I have is to make the class return a
>> {structure/object/status} that contains a status code, with one specifying
>> that the abstract routine was invoked rather than the concrete subclass of the
>> abstract routine. Any better ideas?
>>
>> Phillip
>
> How about just:
>
> pro Abstract::aMethod, arg1, arg2
> message, 'This is an abstract method and must be overridden in class
> '+obj_class(self)
> end
>
> with the understanding that aMethod not be chained to in the overridding
> method of a subclass? Or am I missing something?
J.D. -- This is perfect. Thanks for the tip. BTW, this also works for making
the Init function abstract.
--- sample ---
pro abstract::method
message, 'Method is abstract in class abstract'
end
function abstract::init
message, 'Class abstract is abstract and cannot be instantiated'
end
pro abstract__define
struct = {ABSTRACT, NULL:0b}
end
--- end of sample ---
By the way, I also learned another important lesson about objects. Be sure
you define both an init function (even if it only returns the value '1' (i.e.,
success)) and a cleanup routine (even with an empty body). When running IDL
5.1 under Win/NT, a class with an implicit Init and Cleanup takes about 4
seconds for each operation. When the operations are defined explicitly, even
with empty bodies, the operations are almost instantaneous.
Phillip
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| Re: Abstract Objects and Methods [message #12113 is a reply to message #12111] |
Tue, 23 June 1998 00:00  |
J.D. Smith
Messages: 214
Registered: August 1996
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Senior Member |
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Phillip David wrote:
>
> While looking into using IDL's OO-based technologies, I came up with another
> interesting question. Is there any way in IDL to create an abstract method?
> An abstract method is a method common to all objects of a particular class
> whose implementation is not defined in the parent class. For example, if I
> had a class "Shape", and wanted it to have a method "Area", I would like to
> declare the method Area as abstract because no particular method makes sense.
> For a circle, area = pi * r^2; for a square, area = side^2; for a rectangle,
> area=l*w; .... An abstract method requires subclasses to define the method,
> but doesn't specify the implementation (other than perhaps the calling sequence).
>
> Any class that contains an abstract method must also be abstract. That's an
> easier problem to solve. Here's a basic abstract class:
>
> ----- Sample code -----
>
> function abstract::init
> ok = Widget_Message(/Error, $
> [ 'Class ABSTRACT cannot be instantiated.', $
> 'It is an abstract class.', $
> 'You must create a subclass of it.'])
> return, 0
> end
>
> pro abstract__define
> struct = {ABSTRACT, NULL:0b}
> end
>
> ----- End sample code -----
>
> When you create a concrete subclass, the init method doesn't call its parent's
> init method (or else it fails), but it returns success when it succeeds.
>
> Does anyone know of any way to make sure a method has been overriden by a
> child class? The only thought I have is to make the class return a
> {structure/object/status} that contains a status code, with one specifying
> that the abstract routine was invoked rather than the concrete subclass of the
> abstract routine. Any better ideas?
>
> Phillip
How about just:
pro Abstract::aMethod, arg1, arg2
message, 'This is an abstract method and must be overridden in class
'+obj_class(self)
end
with the understanding that aMethod not be chained to in the overridding
method of a subclass? Or am I missing something?
JD
--
J.D. Smith |*| WORK: (607) 255-5842
Cornell University Dept. of Astronomy |*| (607) 255-4083
206 Space Sciences Bldg. |*| FAX: (607) 255-5875
Ithaca, NY 14853 |*|
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