| Re: Object-Oriented Programming Question [message #10672 is a reply to message #10535] |
Fri, 19 December 1997 00:00  |
Evilio del Rio
Messages: 17
Registered: December 1997
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Junior Member |
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On Wed, 17 Dec 1997, Peter Stoltz wrote:
> I define an object structure A that has as one of its data members
> another object B (A has a B). So far as I can tell, when one creates an
> instance of A, one cannot invoke the methods of class B through the
> syntax
>
> IDL> a=obj_new('A')
> IDL> a.b->some_method
>
> % Object instance data is not visible outside class methods
> % Execution halted
>
There are mainly two ways to put toghether two (or more) classes:
A) Composition: One component of a class is an Object.
A 'Leg' is a part of an 'Animal':
pro Animal__Define
tmp = {ANIMAL, ..., LeftFrontLeg : OBJ_NEW(),...}
...
end
...
function Animal::Init
...
self.LeftFrontLeg = OBJ_NEW('Leg')
...
end
B) Inheritance: One class is a special case of another one.
A 'Reptil' is a kind of 'Animal':
pro Reptil__Define
tmp = {Reptil, ..., INHERITS ANIMAL}
...
end
As a general rule, in OOP the implementation (the components and some
methods) of an object must be hiden from the user(*) of the Object. The user
must only know how to use of what kind of things an object does. This is the
"Need to Know" rule.
For example, to use a class representing Complex numbers you don't need to
know if its internal representation is cartesian (x,y) or polar (r,theta)
or any other convenient way. You just need to know that a complex is
'something' you can add, multiply, etc..., or calculate its module:
; Use (never changes)
oZ = OBJ_NEW('Complex')
...
r = oZ->Module()
; Polar Implementation (r, th) ; Cartesian Implementation (x,y)
function Complex::Module | function Complex::Module
return,self.R | return, SQRT(self.X^2 + self.Y^2)
end | end
Doing it this way, the code that uses the Complex class is stable against
implementation changes.
In your case, maybe what you need is to define Class A as a Subclass of
Class B, i.e.,
pro a__define
tmp = {A ,...(whatever)..., INHERITS B }
return
end
and then use it as
a->some_method (equivalent to a->b::some_method)
multiple inheritance is allowed (as far as no conflicting structure
member definitions are found)
pro a__define
tmp = {A ,...(whatever)..., INHERITS B, INHERITS C }
return
end
Hope this helps.
Cheers,
(*) Note: The user of an Object is the rutine(s) that uses this object but
is not a method of the Object class.
____________________________________________________________ ____
Evilio Jose del Rio Silvan Institut de Ciencies del Mar
E-mail: edelrio@icm.csic.es URL: http://www.bodega.org/
"Anywhere you choose,/ Anyway, you're gonna lose"- Mike Oldfield
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