From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,fa1a697e52242fbf
X-Google-Attributes: gid103376,public
From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: What's class?
Date: 1999/02/24
Message-ID: <m3k8x8fovj.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 447745838
Sender: matt@mheaney.ni.net
References: <7av8du$rjj$1@news.kornet.nm.kr>
NNTP-Posting-Date: Tue, 23 Feb 1999 19:40:11 PDT
Newsgroups: comp.lang.ada
Date: 1999-02-24T00:00:00+00:00
List-Id: <comp.lang.ada>

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.