boost::spirit::karma::real_generator prints a number multiplied by 10

[Josef Zlomek <josef@…>]

When using the attached example, the number 0.09999999999999987 is printed as 0.999999999999999, i.e. multiplied by 10.

[Josef Zlomek <josef@…>]

Test that prints the number incorrectly

[code@…]

I found the source of the issue, its ​this line here. The call to log10 rounds up to 15 instead of 14.9999....

Fixing the issue is tricky, at least for me since I don't know many floating point tricks. The ideas I have:

• Divide by 10 in a loop to simulate what the int_inserter is doing below
• Have int_inserter write to a temporary stack buffer first (with output tracking), and then copy the stack buffer to the real one.
  • The stack buffer would be checked, and would error if it exceeded its bounds
• Remove the + 1 ​from this line.
  • Since the value no longer exceeds digits10, the log10 function no longer rounds.
  • Doesn't seem portable or reliable, but I could be mistaken.

Unfortunately I don't know of a way to reliably fix this without a performance impact. I can fix + add tests for this after some guidance.