Re: Extended Return Statement with exception

["Randy Brukardt" <randy@rrsoftware.com>]

"Anh Vo" <anhvofrcaus@gmail.com> wrote in message
news:1132687889.831797.109510@o13g2000cwo.googlegroups.com...
> I have read the Extended Return Statement in Ada 2005 RM and Rationale.
> However, I am still not sure how it works. In other word, what will be
> returned when an exception occurs and handled the the exception handler
> within this extended return statement. Consider the following notional
> code, what exactly will be returned?
>
> type T is limited ...
> ...
>
> function Make (...) return T is
> begin
>    ...
>    return R : T do
>       ...
>       ...   -- Constraint_Error is raised here for example
>    exception
>       when Constraint_Error => null; -- handle Constraint_Error here.
>    end return;
> end Make;

(I changed your example a bit to be clear.)

The answer is that if the extended return completes normally, it's object is
returned. So, in this example, R is returned. You could complain that the
object isn't properly initialized, but of course if you put a handler here,
you ought to set up the object correctly (that is, if you handle the
exception, you really need to handle it, not just ignore it).

If you don't want to return R, you need to handle the exception outside of
the return statement:

function Make (...) return T is
begin
   ...
   begin
      return R : T do
      ...
      ...   -- Constraint_Error is raised here for example
      end return;
   exception
      when Constraint_Error =>
         return Some_Default_Value; -- handle Constraint_Error here.
   end;
end Make;

There are some interesting interactions between exceptions, finalization,
and return statements (imagine T has tasks or controlled components to make
your head hurt), but these shouldn't be a concern for anyone other than
implementers.

                                Randy.