Inexplicably, MS for VS 10 have chosen std::numeric_limits<float>::max_digits10 == 8 for VS 10 rather than 9, when the formula max_digits10 = 2 + std::numeric_limits<FPT>::digits * 3010/10000; and values have been known for 30 years. std::numeric_limits<T>:: max_digits10; is 'new for C++0X'. Briefly, max_digits10 is the number of decimal digits that you need to output to make sure that a one least significant bit (ULP) change produces a different decimal digit string. Any more digits are just noise with no useful information. Using max_digits10 rather than the formula above may be more efficient, and is certainly very much clearer. "Executive Summary" ================ Don't use max_digits10 for float or floating-point template types for MS VS - yet! You can safely (and very usefully) use cout.precision(std::numeric_limits<double>::max_digits10); when you want to be sure that all significant decimal digits are output. GCC provides the correct values of 9 for float (and 17 for double). The Fuller Story =========== This might affect anyone planning to use VS10 and this new (and very useful numeric_limits value) with built-in *type float, or a floating-point template type*, potentially, for example, with Boost Lexical_cast, Serialization. Round tripping would be 1 bit wrong in perhaps 1/3 of cases! If max_digits10 was used, Boost.Test might detect values that are different, but are reported to be the same, for example. As far as I know, none of these utilities yet try to use max_digits10. Mercifully, this fault only affects type T = float (not the most popular double or long double), and only in new code that uses the 'new for C++0X' numeric_limits<T>:: max_digits10. Other types like double and long double (and possibly user defined types that specialize numeric_limits) are, I believe, correct. So you can safely (and very usefully) use cout.precision(std::numeric_limits<double>::max_digits10); when you want to be sure that all significant decimal digits are output. (If you want this to be 'Boost portable' you will have to use the inscrutably named BOOST_NO_NUMERIC_LIMITS_LOWEST as a proxy for "C++0x numeric_limits before using new-for-C++0X max_digits10. Unless we add yet another macro for max_digits10? Groan.). The workaround is to continue to use the well-tried and test Kahan formula max_digits10 = 2 + std::numeric_limits<FPT>::digits * 3010/10000; Reported and confirmed as https://connect.microsoft.com/VisualStudio/feedback/details/668921/std-numer... ts10-value-of-8-is-wrong-and-should-be-9 (It might help if, you could confirm you can 'reproduce' this too). I trust this mistake will be corrected pronto - but I'm not holding my breath :-( I will post again if it is resolved. Paul --- Paul A. Bristow, Prizet Farmhouse, Kendal LA8 8AB UK +44 1539 561830 07714330204 pbristow@hetp.u-net.com