Print an even more meaningful message if WithinULP fails

[cschwan]

Thank you very much for implementing #1581; this feature request is a follow up to it.

The problem that I still have is best illustrated using an example:

../tests/regression_test_lusifer_psg_wz_scattering.cpp:65245: FAILED:
  CHECK_THAT( d.at(j), Catch::WithinULP(densities.at(channel).at(j), 8) )
with expansion:                                                          
  0.0 is within 8 ULPs of 0.0 ([0.00000000000000019, 0.00000000000000019])

which obviously is still not very meaningful, because my numbers are very small, and therefore I still don't see the relevant digits that differ.

Since it can happen in general, I think one should always print the numbers in scientific format instead (or check it the number is in the interval [0.1, 10), print it with the fixed format, else print it scientifically, but that's probably too complicated).

Note that if you decide to print floating point numbers in the scientific format, you should set the precision to max_digits - 1, where the -1 comes from the fact that the precision field changes its meaning to 'digits after the comma', see my question on stackoverflow, for example.

[cschwan]

To clarify: I suggest to show the numbers of the interval in scientific format.

As for the checked numbers themselves I am still a bit disappointed that they apparently can't be printed in the scientific format; would it be possible to globally change the behaviour of the printer (a desparate measure, but I'd be fine doing that in my own tests)?

[horenmar]

You could try modifying the precision settings for floating point stringification to change how the checked number is displayed.

As for the interval, I'll play with different options for a bit before deciding what to do there.