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From: ncohen@watson.ibm.com (Norman H. Cohen)
Newsgroups: comp.lang.ada
Subject: Re: Ada.strings.bounded problems?
Date: 18 Jan 1995 14:27:40 GMT
Organization: IBM T.J. Watson Research Center
Distribution: world
Message-ID: <3fj8gs$h9b@watnews1.watson.ibm.com>
References: <TARJEIJ.95Jan11183331@ulrik.uio.no> <D29L78.J9@nntpa.cb.att.com>
 <3f3rl7$kts@watnews1.watson.ibm.com>
 <Mats.Weber-1301952029290001@mlma11.matrix.ch>
 <3fduto$ta7@watnews1.watson.ibm.com>
 <Mats.Weber-1701951908250001@mlma11.matrix.ch>
Reply-To: ncohen@watson.ibm.com
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Date: 1995-01-18T14:27:40+00:00
List-Id: <comp.lang.ada>

In article <Mats.Weber-1701951908250001@mlma11.matrix.ch>,
Mats.Weber@matrix.ch (Mats Weber) writes: 

|> In article <3fduto$ta7@watnews1.watson.ibm.com>, ncohen@watson.ibm.com wrote: 
|>
...
|>
|> > Ada 95 allows predefined equality to be overridden, even for nonlimited
|> > types, so the declaration of a type Bounded_String with a discriminant
|> > specifying the maximum length should be accompanied by the declaration
|> > for the following function: 
|> >
|> >    function "=" (Left, Right: Bounded_String) return Boolean is ...
|>
|> This helps some, but still has severe problems. For instance if I write
|>
|>    type Pair is
|>       record
|>          First,
|>          Second : Bounded_String;
|>       end record;
|>
|> then "=" is broken for Pair. This situation is very bad because overriding
|> "=" on Bounded_String gives the programmer the feeling that the problem is
|> solved, but it reappears whenever a Bounded_String is used as a component
|> of a composite type.

Yes, but Ada 9X provides a straightforward solution for this too.  If a
private type is implemented as a tagged type with user-defined "=", then
equality tests for composites containing the private type invoke the
user-defined "=".  (See RM95 4.5.2(15) and 4.5.2(24).  In paragraph 24,
it is unclear whether a component belonging to a private type with a
tagged implementation is a "tagged component", but because of paragraph
15, it doesn't matter.)

Thus, what we have to write is: 

   package Interoperable_Bounded_Strings is
      type Bounded_String (Max_Length: Natural) is private;
      function "=" (Left, Right: Bounded_String) return Boolean;
      ...
   private
      type Bounded_String (Max_Length: Natural) is
         tagged record
            Current_Length : Natural;
            Text           : String (1 .. Max_Length);
         end record;
   end Interoperable_Bounded_Strings;

Because we have added the word "tagged" to the full-type declaration,
predefined "=" for type Pair will now invoke user-defined "=" for type
Bounded_String.

(This is not the usual use for tagged types, since, even though the full
type can be extended in the body or in a private child package, this is
unlikely to happen.  In this case, the type is declared tagged solely so
that the new composition rules for "=" will apply to it.  Tucker Taft
will correct me if I am wrong, but I believe that these are the rules he
would have liked to apply to ALL types if he had been starting with a
blank slate.  Upward compatibility with Ada 83 constrained him to "do the
right thing" only for tagged types, which of course do not arise in
Ada-83 programs.)

--
Norman H. Cohen    ncohen@watson.ibm.com