Re: Choice of variant record discriminant (design question)

["Jeffrey R. Carter" <spam@spam.com>]

Jacob Sparre Andersen wrote:

> I have declared a record type like this:
> 
>    type Preprocessed_Parameters (Measurement_Count : Natural := 0) is
>       record
>          case Measurement_Count is
>             when 0 =>
>                null;
>             when others =>
>                Mean_Value      : Scalar;
>                Scaled_Variance : Scalar;
>          end case;
>       end record;

Your record has 2 variants, so your discriminant should have 2 values.

> I can't do this, since the discriminant has to be static.  The obvious
> solution to the problem is to change the record declaration into:
> 
>    type Preprocessed_Parameters (Empty : Boolean := True) is
>       record
>          case Empty is
>             when True =>
>                null;
>             when False =>
>                Measurement_Count : Positive;
>                Mean_Value        : Scalar;
>                Scaled_Variance   : Scalar;
>          end case;
>       end record;
> 
> but I have a feeling there ought to be a more elegant solution.  Does
> somebody have a suggestion or two?

Now you have a discriminant with 2 values controlling a record with 2 variants. 
I find that more elegant than your 1st version.

-- 
Jeff Carter
"We burst our pimples at you."
Monty Python & the Holy Grail
16