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From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
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Subject: Re: Instantiating private types with discriminants?
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Date: 2006-05-11T01:17:33+00:00
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Rick H wrote:
> 
> 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;

Constraint_Error is raised at run time, so this isn't static. In the 
most general case, a function returning the class-wide type may return 
any of the types in the class based on external events, and so be 
completely unknown statically (no matter how good a job of static 
analysis the compiler does). So there must be cases when the actual type 
of the variable is not known until run time. In practice, most compilers 
are not going to bother, so this will always result in dispatching.

One thing about class-wide variables is that they are constrained to the 
type of their initialization expression, and so can never be assigned 
another type in the class (hence Constraint_Error). This is why it's 
best to declare them as late as possible.

-- 
Jeff Carter
"My name is Jim, but most people call me ... Jim."
Blazing Saddles
39