Re: Run-Time Type Assignment

[Jeffrey Carter <jrcarter@acm.org>]

"Robert C. Leif" wrote:
> 
> From: Bob Leif
> To: SK et al.
> How does your solution work, if the variants are:
> case Num_Parameters  is
>   when 1 =>
>     Parameter_1: Parameter_1_Type;
>   when 2 =>
>     Parameter_1: Parameter_1_Type;
>     Parameter_2: Parameter_2_Type;
>   when 3 =>
>     Parameter_1: Parameter_1_Type;
>     Parameter_2: Parameter_2_Type;
>     Parameter_3: Parameter_3_Type;
> end case;
> This is the equivalent of a tagged type.

One way is to make it a tagged type with a chain of types extended from
it. If you need/want to avoid tagged types, the composition equivalent
to type extension is to build up a bunch of record types, then combine
them in a variant record:

type Param_1_Data is record
   Parameter_1 : Parameter_1_Type;
end record;

-- Further types may be in separate packages if desired
type Param_2_Data is record
   Parent : Param_1_Data;
   Parameter_2 : Parameter_2_Type;
end record;

type Param_3_Data is record
   Parent : Param_2_Data;
   Parameter_3 : Parameter_3_type;
end record;

type Any_Param_Data (Num_Params : Whatever) is record
   case Num_Params is
   when 1 =>
      Parameter_1 : Param_1_Data;
   when 2 =>
      Parameter_2 : Param_2_Data;
   when 3 =>
      Parameter_3 : Param_3_Data;
   end case;
end record;

Run-time dispatching is also easily implemented using this approach, and
is left as an exercise for the reader.

-- 
Jeff Carter
"I wave my private parts at your aunties."
Monty Python & the Holy Grail