Re: Controlled types and exception safety

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

Randy Brukardt wrote:
> 
> For an another explanation of this, see ARM 7.6(17.a-17.h).
> http://www.adaic.com/standards/95aarm/html/AA-A-5-3.html

Is it meaningful that you reference 7.6 and provide the URL of 5.3?

> The other issues are solveable, but this one is not.

This still doesn't seem like a problem to me. Suppose R is a discriminated 
record with default:

type R (D : D_Type := D_Type'First) is record
    ...
end record;

and we want to write Assign:

procedure Assign (To : in out R; From : in R);

1st thing is to check if To is constrained; if so, From must have matching 
constraints:

if To'Constrained and then To.D /= From.D then
    raise Constraint_Error;
end if;

If To is unconstrained, and we need to (be able to) change the constraints, we 
use an intermediate object:

Not_Constrained : declare
    Result : R (D => New_D);
begin -- Not_Constrained
    -- Assign to components of Result
    To := Result; -- or R'Bitwise_Copy (To => To, From => Result);
end Not_Constrained;

Maybe I'm still missing something. Anyway, I never thought user-defined 
assignment was a real need. Limited types and Assign procedures seem adequate to 
me. What Ada 83 really lacked was finalization. While I'm glad Ada 95 has it, I 
think I'd prefer

for R'Finalize use ...;

-- 
Jeff Carter
"English bed-wetting types."
Monty Python & the Holy Grail
15