Re: Zoo question

[kst@thomsoft.com (Keith Thompson)]

In <3210A142.2781E494@escmail.orl.mmc.com> Ted Dennison <dennison@escmail.orl.mmc.com> writes:
> Ken Garlington wrote:
> > 
> > Here's a little brain teaser we received recently. Assuming no typos or
> > other obvious syntax errors, will the Fill procedure work in Ada 83?
> 
> OK. I'll bite. It looks like it would probably perform as advertised, with
> no optimization set on the compiler. The following constraint check code -
> 
> > 
> >       Get_Next_Animal: begin -- look carefully at this code!
> >         Next_Animal := Animal_ID'Succ(Next_Animal);
> >       exception
> >         when others => Next_Animal := Animal_ID'First;
> >       end Get_Next_Animal;
> 
> isn't guaranteed to work (assuming "work" means raise an execption within
> this block when Animal_ID'Succ(Next_Animal) > Animal_ID'last).

I believe it *is* guaranteed (by the language) to work.  In both Ada 83
and Ada 95, the Animal_ID'Succ attribute is a function that takes a
value of type Animal_ID'Base and returns a value of type Animal_ID'Base.
Since Animal_ID'Base will almost certainly have bounds greater than
1..5, Animal_ID'Succ(5) will return 6, without raising an exception.

However, attempting to assign the resulting value to Next_Animal *must*
raise Constraint_Error, since Next_Animal is of subtype Animal_ID and
is constrained to the range 1..5.

I can imagine a compiler implementer mistakenly thinking that the Succ
attribute returns a value of the prefix subtype, and optimizing out
the check.  If this is what you're seeing, it's clearly a compiler bug.

Incidentally, this worked as expected (i.e., properly raised
Constraint_Error) on every Ada 83 and Ada 95 compiler I tried.

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com <*>
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