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4430. from_chars should not parse "0b" base prefixes

Section: 28.2.3 [charconv.from.chars] Status: New Submitter: Jan Schultke Opened: 2025-10-20 Last modified: 2025-10-25

Priority: Not Prioritized

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Discussion:

C23 added support for the "0b" and "0B" base prefix to strtol, and since the wording of from_chars for integers is based on strol, this inadvertently added support for parsing "0b" prefixes to from_chars.

Proposed resolution:

This wording is relative to N5014.

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

   constexpr from_chars_result from_chars(const char* first, const char* last,
                                          integer-type& value, int base = 10);
   

   -2- Preconditions: base has a value between 2 and 36 (inclusive).

   -3- Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0b" or "0B" prefix shall appear if the value of base is 2, no "0x" or "0X" prefix shall appear if the value of base is 16, and except that '-' is the only sign that may appear, and only if value has a signed type.

   -4- Throws: Nothing.