[charconv.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.

[Note 1:

If the pattern allows for an optional sign, but the string has no digit characters following the sign, no characters match the pattern.

— end note]

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, after rounding according to round_to_nearest ([round.style]), and the member ec is value-initialized.

🔗

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

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.

Throws: Nothing.

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from_chars_result from_chars(const char* first, const char* last, floating-point-type& value,
                             chars_format fmt = chars_format::general);

Preconditions: 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

(6.1)
the sign '+' may only appear in the exponent part;
(6.2)
if fmt has chars_format::scientific set but not chars_format::fixed, the otherwise optional exponent part shall appear;
(6.3)
if fmt has chars_format::fixed set but not chars_format::scientific, the optional exponent part shall not appear; and
(6.4)
if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed.
[Example 1:
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/IEC 9899:2024, 7.24.2.6, 7.24.2.8