Re: Discriminant computation problem

[Martin Krischik <martin@krischik.com>]

Sandro Magi wrote:

> Jim Rogers <jimmaureenrogers@att.net> wrote in message
> news:<yjVld.906538$Gx4.504752@bgtnsc04-news.ops.worldnet.att.net>...
>> You might want to reconsider your design.
>> First of all, your chosen example is a very poor way to represent
>> a bit vector in Ada. This appears to be an Ada translation of a C
>> or C++ programming approach.
> 
> As I said, it was just the simplest example of what I was trying to
> do. It's not what I'm actually trying to do.
> 
>> My second point deals with your desire to perform a calculation
>> on a parameterized value. The more acceptable way to accomplish
>> such an end is the use of a generic package rather than a
>> record discriminant. The use of a generic package provides
>> encapsulation and information hiding for your operations that
>> prevent you from unnecessarily exposing internal data structures.
> 
> I was trying to avoid the extra step of instantiating a generic
> package. Furthermore, the types from one package would not be
> interoperable with the types from another package would they? ie.
> using my Bit_Vector example and assuming the package defines an Append
> operation:
> 
> procedure Test is
>   package Bit_Vector_1024 is new Bit_Vector(1024);
>   package Bit_Vector_2048 is new Bit_Vector(2048);
> 
>   b1024 : Bit_Vector_1024.Bit_Vector;
>   b2048 : Bit_Vector_2048.Bit_Vector;

Hint: When you have a package and a type with the same name you can't "use"
that package without risking name clashes. That's why most packages have
name for multiples like Unbounded_Strings.

> begin
>   ...
>   (initialize some data in each vector)
>   ...
>   --this would fail to type check wouldn't it?
>   Append(Data=>b1024, To=>b2048);
> end Test;
> 
> Sorry for asking something so obvious, but I can't check this myself
> at the moment.

Deppends on how the types and the parameters are defined. If both
Bit_Vector_1024.Bit_Vector and Bit_Vector_2048.Bit_Vector are subtypes with
a common ancestor then it well could work:

In Ada, with it's extensive type system, getting the types right is more
important then getting the procedures right.

With Regards

Martin

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