3250. std::format: # (alternate form) for NaN and inf

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-08-05 Last modified: 2021-02-25

Priority: 0

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Discussion:

We have:

"For floating-point numbers, the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it."

So does that mean that infinity is formatted as "inf." and NaN as "nan."? (Or something like that? Where exactly do we add the decimal point in this case?) Or does this affect infinity but not NaN (because we can handwave that a NaN value is not a "floating-point number")?

I suspect that this should only cover finite floating-point numbers.

Victor Zverovich:

Right. So infinity and NaN should be still formatted as "inf" and "nan" without a decimal point.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   -6- […] For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for g and G conversions, trailing zeros are not removed from the result.