Re: Ada95 OOP Questions

[Andre Spiegel <spiegel@berlin.informatik.uni-stuttgart.de>]

Spasmo writes:

> Well after looking at Barnes' Ada95 book and checking out the
> OOP sections I noticed that the Objects in Ada95 seem to
> consist purely of data, and no methods seem to be involved
> so that you have to pass these objects to functions in order
> to get stuff done.  Am I wrong in this (or did I somehow 
> miss something again?).

Ada Objects "consist" of both data and methods, just like in any other
OOP language.  Ada calls the methods "primitive subprograms", and they
are (roughly) defined as those subprograms

  "that are explicitly declared immediately within the same
   package_specification and that operate on the type." (RM95 3.2.3;6)

So, to translate your C++ example into equivalent Ada (not just
"simulating" it!), you can write


   package Persons is

      type Person is tagged private;

      procedure Walk (P : Person);
      procedure Talk (P : Person);
      procedure Eat  (P : Person);

   private

      type Person is tagged
         record
            Age  : Integer;  -- rather "Natural", I'd suggest 
                             -- (C++ doesn't have this)
            Name : String (1..256);
         end record;

   end Persons;


What should be pointed out is that C++ and Ada use slightly different
abstractions.  In C++, you have "classes" and "objects" (but sometimes
a class is referred to as an object, too); whereas in Ada you have 
"packages", "types", "classes of types", and "objects".  For a very
good discussion of this, see Tucker Taft's answer to Question 5.1 in
the Ada Programming FAQ (http://lglwww.epfl.ch/Ada/FAQ/programming.html).

Hope this helps,

Andre Spiegel
University of Stuttgart, Germany