Re: others clause

[Mike Bishop <mbishop@ghgcorp.com>]

Chris Sparks wrote:
> 
> Why is the following not incorrect?
> 
> ---------------------------------------------------------------------
> procedure T is
> 
>   type Enum is (A, B, C, D, E, F);
> 
>   T1 : Enum    := Enum'First;
>   T2 : Integer := 0;
> 
> begin
>   case T1 is
>     when A      => null;
>     when B      => null;
>     when C      => null;
>     when D      => null;
>     when E      => null;
>     when F      => null;
>     when others => null;               --WHY ISN'T THIS ERRONEOUS?
>   end case;

According to the Ada 95 LRM, section 5.4, each value of T1 must be
covered either explicitly by a non-other discrete choice or by others.
That is the case in the above code. The LRM does not state that if all
values are covered by non-other choices, then others is not allowed. In
fact, using others is a good idea even when all values are explicitly
covered. If you add more values to the type but forget to modify the
case statement, you can still handle the new values in the others
choice.

>   case T2 is
>     when Integer'First .. -1 => null;
>     when 0                   => null;
>     when 1 .. Integer'Last   => null;
>     when others              => null;  --WHY ISN'T THIS ERRONEOUS?
>   end case;
> 
> end T;

Not only is others not erroneous here, it is required. If the type of
the case expression is root_integer or universal_integer, you must have
an others choice.

-- 
Mike Bishop         
mbishop@ghgcorp.com 
http://www.ghgcorp.com/mbishop/