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3080. Floating point from_chars pattern specification breaks round-tripping

Section: 28.2.3 [charconv.from.chars] Status: C++20 Submitter: Greg Falcon Opened: 2018-03-12 Last modified: 2021-02-25

Priority: 0

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Discussion:

from_chars specifies that the '+' character is never matched, but to_chars specifies its output format in terms of printf(), which puts a '+' sign before positive exponents.

Since strtod() matches '+' signs, it is also desirable to accept '+' in exponents, so that code currently using strtod() can be migrated to from_chars() without a breaking semantic change.

[ 2018-03-27 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit 28.2.3 [charconv.from.chars] as indicated:

   from_chars_result from_chars(const char* first, const char* last, float& value,
                                chars_format fmt = chars_format::general);
   from_chars_result from_chars(const char* first, const char* last, double& value,
                                chars_format fmt = chars_format::general);
   from_chars_result from_chars(const char* first, const char* last, long double& value,
                                chars_format fmt = chars_format::general);
   

   -6- Requires: fmt has the value of one of the enumerators of chars_format.

   -7- Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that

   1. (7.1) — the only sign '+' that may only appear is '-'in the exponent part;

   2. (7.2) […]

   3. (7.3) […]

   4. (7.4) […]