Re: Equality operator overloading in ADA 83

[mheaney@ni.net (Matthew Heaney)]

In article <335CAEFE.35DC@elca-matrix.ch>, Mats.Weber@elca-matrix.ch wrote:

>> It took me a while to realize that, yes, even for types (without access
>> objects) that have a physical length greater than the logical length, you
>> can make the predefined equality work.  This is true even for Ada 83, so
>> you may be able to apply this technique to your problem.
>
>Yes, but the performance penalty is pretty big and it's hard to find the
>bug if you forget the padding just once.

What are you saying?  That one should stick with the full-type-tagged
technique, and not use the pad-with-extra-characters technique?

The performance penalty is nothing compared to the fact that equality
doesn't work.  What's more important, program correctness or efficiency?

>
>GNAT 3.09 would fail, I just chacked: there is no padding of strings
>shorter than the max length. Making Max_Length a discriminant of a
>subcomponent of Bounded_String would solve the problem, probably better
>(at least cleaner) than padding with nulls.

But of course that solution would have a performance penalty too, right?

   type Bounded_String is private;

private

   subtype Length_Range is Natural range 0 .. Max_Length;

   type Bounded_String (Length : Length_Range := 0) is
      record
         Buffer : String (1 .. Length);
     end record;

end Bounded_Strings;

If I want to append characters to my string, that means I have to change
the value of the discriminant, and therefore must use aggregate assignment:

procedure Append (Str : String; To : in out Bounded_String) is
begin
   To := (Length => To.Length + Str'Length, Buffer => To.Buffer & Str);
end Append;

Without the (hidden) discriminant, I can just append the string to what's
already there:

procedure Append (Str : String; To : in out Bounded_String) is
begin
   To.Buffer (To.Length + 1 .. To.Length + Str'Length) := Str;
   To.Length := To.Length + Str'Length;
end Append;

N'est-ce pas?

Matt

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271