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3243. std::format and negative zeroes

Section: 28.5.2 [format.string] Status: C++20 Submitter: Richard Smith Opened: 2019-07-31 Last modified: 2021-02-25

Priority: 2

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Discussion:

What are these: 

std::format("{}", -0.0);
std::format("{:+}", -0.0);
std::format("{:-}", -0.0);
std::format("{: }", -0.0);

with 

"{:{}} {}"

A negative zero is not a negative number, so I think the answer for an implementation that supports signed zeroes is "0", "-0", "0", " 0". Is that the intent? (Note that this doesn't satisfy to_chars' round-trip guarantee.)

Or should the behavior actually be that "+" means "insert a leading + if to_chars' output does not start with -" and " " actually means "insert a leading space if to_chars' output does not start with -" (that is, the same as "%+f" and "% f") — so that the result of all four calls is "-0"?

Victor Zverovich:

The latter. std::format is specified in terms of to_chars and the intent is to have similar round trip guarantee here, so the result should be "-0", "-0", "-0", "-0". This has also been extensively discussed in LEWG and the outcome of that discussion was to print '-' for negative zeros.

So I think it should be clarified that '-' and space apply to negative numbers and negative zero.

[2019-08-17 Priority set to 2 based on reflector discussion]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

  1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

    Table 59: Meaning of sign options [tab:format.sign]
    Option Meaning
    '+' Indicates that a sign should be used for both non-negative and negative numbers.
    '-' Indicates that a sign should be used only for negative numbers and negative zero (this is the default behavior).
    space Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

[2020-02-13, Prague]

Rebased and some wording finetuning by LWG.

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

Proposed resolution:

This wording is relative to N4849.

  1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

    Table 58: Meaning of sign options [tab:format.sign]
    Option Meaning
    '+' Indicates that a sign should be used for both non-negative and negative numbers.
    '-' Indicates that a sign should be used only for negative numbers and negative zero only (this is the default behavior).
    space Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.