[Bug libstdc++/14220] New: num_put::do_put() undesired float/double behavior

["sadfate at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

When a float or double is streamed to output in fixed-point format, rounding up
occurs well before the precision specified by the stream.

5.55555555e-17, when streamed to a stream with the scientific flag set and a
precision of 25 will print 5.55555555[#'s-up-to-20]e-17.  When streamed with the
fixed flag set, it will print out 0.0000000000000000560000000.  If the number is
especially small (e-20+), it will print out as all zeroes.

I looked at the code in 3.2.3 and 3.4 versions of g++, and saw that it was
because the max_digits is being set from the numeric_limits<double>::digits10
field.  The precision is then set to be less than or equal to the max_digits. 
So on my platform, where digits10 is 16 (+2), the precision will get set to 18,
max.  That works fine for scientific values, but I would argue that if I ask a
stream for 25 places of precision, and put a float/double into it that should be
represented by 0.0000000000000000555555555, that I would see that value, not a
rounded version.

My understanding is that the numeric_limits<double>::digits10 value for a
float/double amounts to the significant digits if it were in scientific
notation.  If that is the case, then the max_digits for fixed number would be
digits10 + the number of preceding zeroes, which is probably max_exponent10.

This was seen on Solaris 8 and Irix 6.5.22.

-- 
           Summary: num_put::do_put() undesired float/double behavior
           Product: gcc
           Version: 3.2.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sadfate at yahoo dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14220