23 General utilities library [utilities]

23.2 Utility components [utility]

23.2.9 Primitive numeric input conversion [utility.from.chars]

All functions named from_­chars analyze the string [first, last) for a pattern, where [first, last) is required to be a valid range.
If no characters match the pattern, value is unmodified, the member ptr of the return value is first and the member ec is equal to errc​::​invalid_­argument.
Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value.
The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match.
If the parsed value is not in the range representable by the type of value, value is unmodified and the member ec of the return value is equal to errc​::​result_­out_­of_­range.
Otherwise, value is set to the parsed value and the member ec is set such that the conversion to bool yields false.
from_chars_result from_chars(const char* first, const char* last, see below& value, int base = 10);
Requires: base has a value between 2 and 36 (inclusive).
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 "0x" or "0X" prefix shall appear if the value of base is 16, and except that a minus sign is the only sign that may appear, and only if value has a signed type.
Throws: Nothing.
Remarks: The implementation shall provide overloads for all signed and unsigned integer types and char as the referenced type of the parameter value.
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);
Requires: fmt has the value of one of the enumerators of chars_­format.
Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that
  • the only sign that may appear is a minus sign;
  • if fmt has chars_­format​::​scientific set but not chars_­format​::​fixed, the otherwise optional exponent part shall appear;
  • if fmt has chars_­format​::​fixed set but not chars_­format​::​scientific, the optional exponent part shall not appear; and
  • if fmt is chars_­format​::​hex, the prefix "0x" or "0X" is assumed.
    [Example
    :
    The string 0x123 is parsed to have the value 0 with remaining characters x123.
    end example
    ]
In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.
Throws: Nothing.
See also: ISO C 7.
22.
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3, 7.
22.
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4