hmmm section 18.3.2.4 of the standards says:
static constexpr int digits10; 11 Number of base 10 digits that can be represented without change.198 12 Meaningful for all specializations in which is_bounded != false.
static constexpr int max_digits10; 13 Number of base 10 digits required to ensure that values which differ are always differentiated. 14 Meaningful for all floating point types.
So I don't read this as number of digits to the right of the decimal point. I take it to be number of significant digits which is not the same to me. But maybe it is. In anycase it looks like I should be using max_digits10 instead of digits10+2 in anycase.
max_digits10 is a C++11 feature, but there's a config macro for it somewhere. However, you're confusing "how many digits do I need to fully represent this type", with "how do you want this type formatted". There are basically 3 formatting options: 1) std::ios_base::fixed is set. Then the precision is interpreted as the number of digits after the decimal point. 2) std::ios_base::scientific is set. The precision is interpreted as the number of significant digits to print. 3) Neither of the above are set (the default for a new stream). The formatter chooses either (1) or (2) based on conditions I don't recall, but basically large exponents go to (2). If you care about being able to re-read the value back in you have to choose (2), the other options are for "pretty printing" for humans to read. HTH, John.