Re: constant record components in Ada

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

Alexander V. Konstantinou wrote:
> 
> I am trying to obtain in Ada (95) the same functionality of the C++ struct
> declaration :
> 
>         struct map_pair {
>                 const Key_Type first;
>                 Value_Type second;
>         }
> 
> The idea is to provide a record in which the key value is constant, and the
> value type is mutable.  The Ada books I have looked at suggest using
> discriminants.  Unfortunately, discriminants have to be either discrete or
> access types.  In my case Key_Type is a generic parameter and restricting it
> to be a discrete type is too restrictive.  

Why is a discrete type too restrictive? Discrete types include integer
types and enumeration types; what other types are appropriate for keys?
Maybe fixed length strings?

>On the other hand, making the
> discriminant a pointer is in my view an ugly solution since it keeps that
> extra pointer around.
> 
> Any ideas on how to tackle this ?

You could define a limited type to hold the key; then users could not
change it.

> 
> Alexander V. Konstantinou
> --
> Alexander V. Konstantinou              http://www.cs.columbia.edu/~akonstan/
> akonstan@cs.columbia.edu               akonstan@acm.org

-- 
- Stephe