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Subject: Re: [boost] [serialization] How are floating point values handled?
From: John Maddock (john_at_[hidden])
Date: 2013-08-08 12:22:04

> hmmm section 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

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.

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