Re: Instantiating private types with discriminants?

[Rick H <rik_nntp@dsl.pipex.com>]

Ludovic Brenta <ludovic@ludovic-brenta.org> wrote:
> Here is a complete, working example, in case you're confused.  It
> summarises everything we've said before: package (possibly, like here,
> nested in the procedure), freezing rules, abstract procedure in the
> base type, no access type, and declaring the object at the last
> moment.
> 
> Now, can you answer this question: which subprograms in that example
> are primitive and which ones are not?
> 
Thanks, Ludovic.  I extented your example by parameterising Get
and creating two versions:

function Get (Item : Integer) return General_T'Class is
begin
   return Type_A' (Data => Item);
end Get;

function Get (Item : Float) return General_T'Class is
begin
   return Type_B' (Data => Item);
end Get;

and then Var_A can be declared as either of:
Var_A : General_T'Class := Get (10);
or
Var_A : General_T'Class := Get (10.0);


But all this has a side-effect that I haven't considered: the fact
that a variable declared as a class-wide type *requires* initialization
means that the compiler can determine the variable's type statically.
This means that the following will raise a constraint error:
declare
   Var_A : General_T'Class := Get (10);  -- Type_A
begin
   Put (Var_A);
   Ada.Text_IO.New_Line;
   Var_A := Get (100.0);      -- Type_B - constraint error!
   Put (Var_A);
end;