Re: Ada subtypes and base types

[Jeffrey A Law <law at redhat dot com>]

On Tue, 2006-02-21 at 14:34 -0500, Robert Dewar wrote:
> Jeffrey A Law wrote:
> 
> > So, if we have an object with the range based on its type of
> > [0, 0x7fffffff] and we add 1 to that object, the resulting range
> > should be [1, 0x7fffffff].   ie, 0x80000000 is not a valid value
> > for the type.  Right?
> 
> The actual rule in Ada works like this:
> 
>      type X is range 0 .. 16#7fff_ffff#;
> 
>      Y : X;
> 
>      Y := (Y + 1) - 1;
> 
> If Y is X'Last, then the addition of 1 must either raise an
> overflow exception or "work". Works means give its proper
> mathematical value.
> 
> So this assignment can either raise CE, or leave Y unchanged.
> Either is OK.
So in the case above, the set of permissible values is
[1, 0x7fffffff] after the addition, right?   It's also valid
to raise a CE.

Jeff