Re: Ada decimal types

[Simon Wright <simon@pushface.org>]

"REH" <me@you.com> writes:

> "Keith Thompson" <kst-u@mib.org> wrote in message 
> news:lnmzdvttoc.fsf@nuthaus.mib.org...

>> And it's the small, not the delta, that needs to be 1.0e-6.  If you
>> specify a delta of 1.0e-6 for an ordinary fixed-point type without
>> specifying the small, you'll (probabaly) get a small of 2.0**-20.
>> (As you probably know.)
>
> It's not an binary fixed point, but a decimal fixed point.  I believe the 
> small has to be a power of ten, not two.  Anyways, that is a good point. 
> For a decimal type, do I need to specify the small?

On GNAT, Duration (which is a plain fixed-point type) is effectively
implemented as

   type Duration is delta 1.0e-9;
   for Duration'Small use 1.0e-9;
   for Duration'Size use 64;

which means that the lsb is 1 nanosecond. I'm pretty sure that a
compiler has to do what you tell it here (or reject the unit).

It may well be that a decimal fixed point type would have exactly the
same effect but I have never used one.

type My_Duration is digits 9 delta 1.0e-9;
for My_Duration'Size use 64;

One point we came unstuck over: depending on your machine, the
alignment of a 64-bit value may end up as 8 bytes, whereas the
alignment of your POSIX-like representation would be 4 bytes.

Had you considered just using Duration?