Re: Need exception despite Machine_Overflows is false

[Matthew Heaney <matthew_heaney@acm.org>]

Stephen Leake <Stephen.Leake@gsfc.nasa.gov> writes:

> Pascal MALAISE <malaise@magic.fr> writes:
> 
> > Using gnat 3.10p on Linux,
> > I make several computations on long_float (s), then use a
> > text_io.float_io
> > to display the results => NaN*****
> > 
> > There is an overflow, Gnat uses the libc, long_float'Machine_Overflows
> > is false...
> > fair enough.
> > 
> > What can I do to get a constraint_error raised when there is an
> > overflow?
> > Don't tell me that I have to check, via an interface to C finite(), the
> > result of
> > each operation!
> 
> You could try adding a range constraint to your basic floating point type:
> 
> type Real is digits 9 range -1.0e307 .. +1.0e307;
> 
> As long as your limits are tighter than the machine limits, the
> compiler will insert explicit checks. Of course, this will slow things
> down, but I assume you could remove the limits after you find the bug.
> 
> This is one reason not to use the predefined Float type!

I think this statement is misleading.

The problem here has nothing to do with the "predefined Float type."
The "problem" is that an unconstrained type was used, when a constrained
type was desired.  You would have the same problems if you did:

  type My_Float is digits 6;

This declares an _unconstrained_ floating point subtype, meaning that no
constraint checks are performed.

Perhaps a better statement would be:

  This is one reason not to use unconstrained floating point subtypes!