On Fri, 4 Aug 2006 12:16:03 +0100, "Paul A Bristow" <pbristow@hetp.u-net.com> wrote:
And in rely to Gennaro's question,
| stream.precision(2 + std::numeric_limits<Source>::digits * 301/1000);
| Does that work for non IEEE 754 floating points too? | I guess not. And in that case we have to check.
It should be as good as you can get - certainly MUCH better than just using the default of 6 decimal digits. This is because numeric_limits<>::digits is the precision of the significand in bits, and the formula divides by log2 ~= 3 to convert binary to decimal and adds 2 to make sure there are enough to ensure a 1 least significant bit change shows up.
Paul, I understand the formula :-) And I'm aware of your N2005 and N1171. My question was just if it (the formula) requires IEEE754 or not. Of course max_digits10 is different in that respect, as it doesn't need to be calculated from digits10. We are calculating instead, and I'm not sure the formula is valid *in general*, probably because I haven't thought enough about it. As to the docs, this is what I would add. Feedback welcome. lexical_cast<> offers the following guarantees: - in the absence of overflow/underflow, * if a "decimal string" with at most numeric_limits<T>::digits10 significant digits is converted to a floating type T and back to a string the result will equal the original string value. * if a floating point value having type F is converted to a string allowing at least max_digits10 [or our formula here] significant decimal digits and back to F the result will be the original number. -- [ Gennaro Prota, C++ developer for hire ]