What's class?

What's class?

[Rouault]

 What's class in Ada95?
 
 I should be intersted in hearing
 more about Class.

 Thanks.

 -- signature.all

Re: What's class?

[David Botton]

Take a look at OO programming in Ada on the Lovelace tutorial at:

http://adahome.com/Tutorials/Lovelace/lesson7.htm

and in particular section 7.3 on Polymorphism in Ada

http://adahome.com/Tutorials/Lovelace/s7s3.htm

other resource on learning Ada can be found at:

http://www.botton.com/ada/learn


David Botton


Rouault wrote:
> 
>  What's class in Ada95?
> 
>  I should be intersted in hearing
>  more about Class.
> 
>  Thanks.
> 
>  -- signature.all

Re: What's class?

[Matthew Heaney]

Rouault <rouault@yahoo.com> writes:

>  What's class in Ada95?
>  
>  I should be intersted in hearing
>  more about Class.


A "class" in Ada denotes a family of types.

For tagged types, the type T'Class denotes the type T and all its
descendents.

So if I have this hierarchy:

  type T is tagged private;

     type U is new T with private;

         type W is new U with private;

     type V is new T with private;


An object of "class-wide" type T'Class can be any of the "specific" types T,
U, V, or W.

An object of class-wide type U'Class can be any of the specific types U
or W.

This distinction is nice, because it allows you to see explicitly when
you have a specific type (T), or a type and any of its descents
(T'Class).  It makes it obvious that procedures like:

  procedure Print (Stack : in Stack_Type'Class);

are intended to work for Stack_Type and any type that derives from
Stack_Type.  

Print is not a primitive operation of type Stack_Type, and therefore
doesn't get inherited during derivations from Stack_Type.  There is one
and only one Print operation, and it works for any kind of stack (that
is, those that derive from stack, i.e. in Stack_Type'Class).

But if I see this:

  function Is_Full (Stack : in Bounded_Stack) return Boolean;

then I know that Is_Full is only intended to take an object of
(specific) type Bounded_Stack.  A stack that derives from Bounded_Stack
would indeed inherit operation Is_Full.

T'Class is a "class-wide" type, which denotes a family of "specific"
types: type T, and any type that derives from T.

Re: What's class?

[bill]

In article <m3k8x8fovj.fsf@mheaney.ni.net>, Matthew says...
>
 
>But if I see this:
>
>  function Is_Full (Stack : in Bounded_Stack) return Boolean;
>
>then I know that Is_Full is only intended to take an object of
>(specific) type Bounded_Stack.  A stack that derives from Bounded_Stack
>would indeed inherit operation Is_Full.
>
 
Since any type that extends a parent type, also inherits its operations,
then in this case, writing

         function is_full( stack: in bounded_stack'class) 
and
         function is_full( stack: in bounded_stack)


make no difference!

correct?

Bill.

Re: What's class?

[Jean-Pierre Rosen]

bill a �crit dans le message <7avvpf$pfl@drn.newsguy.com>...
>Since any type that extends a parent type, also inherits its
operations,
>then in this case, writing
>
>         function is_full( stack: in bounded_stack'class)
>and
>         function is_full( stack: in bounded_stack)
>
>
>make no difference!
>
>correct?
There is a huge difference between a class-wide operation and an
inherited operation.
An inheritede operation conceptually is part of the definition of the
type, and can be redefined by any descendant. In a sense, the original
operation serves as a "default value" for descendants that don't care
to redefine it.
OTOH,  a class wide operation operates conceptually on the full class;
it is not part of the type definition, it is not inherited, and in
cannot be redefined.

The nice thing with inherited operations is that you can redefine them
if you whish; the nice things with class-wide opertions is that you
are certain that nobody redefined the operation behind your back to
make it do something else.

Given your constraints, choose the appropriate one...
---------------------------------------------------------
           J-P. Rosen (Rosen.Adalog@wanadoo.fr)
Visit Adalog's web site at http://perso.wanadoo.fr/adalog

Re: What's class?

[Ed Falis]

On 23 Feb 1999 20:39:11 -0800, bill <bill@newsguy.com> wrote:

>  
> Since any type that extends a parent type, also inherits its operations,
> then in this case, writing
> 
>          function is_full( stack: in bounded_stack'class) 
> and
>          function is_full( stack: in bounded_stack)
> 
> 
> make no difference!
> 
> correct?
> 
> Bill.

The difference is if you plan to allow a change to the representation or implementation of bounded_stack in its descendants, and expect to implement the "stack full" 
operation in terms of one or more alternate representations in the is_full function.

The classwide operation could in many cases be implemented in terms of other dispatching operations specific to the descendants.  Or, if you intend the descendants 
to provide their own implementations of the function in all cases, use the specific argument and declare the function as abstract.

- Ed

Re: What's class?

[dennison]

In article <1103_919865565@DZOG-CHEN>,
  falis@ma.aonix.com (Ed Falis) wrote:
> On 23 Feb 1999 20:39:11 -0800, bill <bill@newsguy.com> wrote:
>
> >
> > Since any type that extends a parent type, also inherits its operations,
> > then in this case, writing
> >
> >          function is_full( stack: in bounded_stack'class)
> > and
> >          function is_full( stack: in bounded_stack)
> >
> >
> > make no difference!
> >
> > correct?
>
> The difference is if you plan to allow a change to the representation or
> implementation of bounded_stack in its descendants, and expect to implement
> the "stack full" operation in terms of one or more alternate representations
> in the is_full function.

There's also the minor difference that a user of a type extended from
bounded_stack will have to with bounded_stack's package as well as the new
type's package to get at the first definition. The second definition they get
straight from the new type's package.

Also, the first function can be written in any package at all (assuming you
don't need special visibility to write its body, while the second has to be
written in bounded_stack's package spec to be inherited.

T.E.D.

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Re: What's class?

[Matthew Heaney]

bill <bill@newsguy.com> writes:

> Since any type that extends a parent type, also inherits its operations,
> then in this case, writing
> 
>          function is_full( stack: in bounded_stack'class) 
> and
>          function is_full( stack: in bounded_stack)
> 
> 
> make no difference!
> 
> correct?

Not quite.  That would only be true if the derived type didn't override
the Is_Full operation.

The purpose in making it primitive (instead of class-wide) is to allow
the derived type to override the operation.  A class-wide operation is
not override-able.

There's also a name-space issue.  Primitive operations of the derived
type are implicitly declared at the point of derivation.  However, a
class-wide operation isn't inherited, and so does not exist where the
(derived) type is (unless of course the derivation is in the same
package as the class-wide operation).

In your example above, you wouldn't get an Is_Full operation at the same
place as the (derived) type, and you'd have to go back to the original
package in which the Is_Full operation is declared, in order to use it.

In the pattern community, class-wide operations are called "template
methods."  You can read all about the finer points of template method
patterns in Ada95 by reading the Template Method post in the June 98
patterns archive at the ACM.

<http://www.acm.org/archives/patterns.html>



  

Re: What's class?

[Gyeongmoon Ryu]

..........

I don't see things the same way... for the following reasons:

A class is an implementation of an Types ( (such as tagged, record,
subprogram...)  It specifies a data structure and the permissible
operational methods that apply to each of its objects) )

In the Ada, object types are implemented as PACKAGE.

-- IF package = C++(class)  --  FALSE or exception
-- IF package = OOP(class) -- TRUE ?  I don no.. :(

-- Gyeongmoon Ryu.
-- Please forgive me for my poor English.
-- Email @abstract.

Re: What's class?

[Matthew Heaney]

"Gyeongmoon Ryu" <rkm579@mail.hitel.net> writes:

> ..........
> 
> I don't see things the same way... for the following reasons:
> 
> A class is an implementation of an Types ( (such as tagged, record,
> subprogram...)  It specifies a data structure and the permissible
> operational methods that apply to each of its objects) )
> 
> In the Ada, object types are implemented as PACKAGE.
> 
> -- IF package = C++(class)  --  FALSE or exception
> -- IF package = OOP(class) -- TRUE ?  I don no.. :(


This is wrong.  The term "class" in Ada has a very specific meaning,
different from its use in other languages.  It was the Ada-meaning of
the term that the original poster was interested in.

It is also untrue that, in Ada, a type is implemented as a package.  In
Ada, a type is implemented as a ... type!