Re: types and subtypes

["Peter C. Chapin" <pchapin@sover.net>]

ada_student@yahoo.com wrote in news:1142273325.634632.41020
@j52g2000cwj.googlegroups.com:

> Why doesnt Ada subtyping also denote derivation as in the
> sense C++ base classes and derived classes do?
> 
> Why was Ada's subtyping defined to exclude derivation?

Consider this...

Suppose you had a C++ base class Vehicle from which you derived a class 
Car.

class Vehicle { };

class Car : public Vehicle { };

In OO terms, Car is a "subtype" of Vehicle. But this is consistent with 
Ada's Positive being a subtype of Integer. The set of Car values is a 
subset of the set of Vehicle values just as the set of Positive values 
is a subset of the set of Integer values. To say a Vehicle is a Car is 
to restrict it... no longer are you talking about just any Vehicle. It 
is the same with Ada subtypes as far as I can see.

Of course it is likely that class Car has more members than class 
Vehicle and thus has an extended implementation. However it defines a 
more restricted class of objects.

Peter