From Jason Turner

[charconv.from.chars]

C++17 → C++20 3.5 KB 77 lines
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+ ### Primitive numeric input conversion <a id="charconv.from.chars">[[charconv.from.chars]]</a>
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+
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+ All functions named `from_chars` analyze the string \[`first`, `last`)
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+ for a pattern, where \[`first`, `last`) is required to be a valid range.
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+ If no characters match the pattern, `value` is unmodified, the member
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+ `ptr` of the return value is `first` and the member `ec` is equal to
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+ `errc::invalid_argument`.
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+
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+ [*Note 1*: If the pattern allows for an optional sign, but the string
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+ has no digit characters following the sign, no characters match the
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+ pattern. — *end note*]
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+
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+ Otherwise, the characters matching the pattern are interpreted as a
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+ representation of a value of the type of `value`. The member `ptr` of
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+ the return value points to the first character not matching the pattern,
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+ or has the value `last` if all characters match. If the parsed value is
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+ not in the range representable by the type of `value`, `value` is
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+ unmodified and the member `ec` of the return value is equal to
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+ `errc::result_out_of_range`. Otherwise, `value` is set to the parsed
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+ value, after rounding according to `round_to_nearest` [[round.style]],
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+ and the member `ec` is value-initialized.
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+
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+ ``` cpp
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+ from_chars_result from_chars(const char* first, const char* last,
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+ see below& value, int base = 10);
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+ ```
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+
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+ *Preconditions:* `base` has a value between 2 and 36 (inclusive).
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+
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+ *Effects:* The pattern is the expected form of the subject sequence in
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+ the `"C"` locale for the given nonzero base, as described for `strtol`,
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+ except that no `"0x"` or `"0X"` prefix shall appear if the value of
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+ `base` is 16, and except that `’-’` is the only sign that may appear,
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+ and only if `value` has a signed type.
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+
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+ *Throws:* Nothing.
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+
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+ *Remarks:* The implementation shall provide overloads for all signed and
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+ unsigned integer types and `char` as the referenced type of the
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+ parameter `value`.
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+
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+ ``` cpp
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+ from_chars_result from_chars(const char* first, const char* last, float& value,
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+ chars_format fmt = chars_format::general);
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+ from_chars_result from_chars(const char* first, const char* last, double& value,
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+ chars_format fmt = chars_format::general);
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+ from_chars_result from_chars(const char* first, const char* last, long double& value,
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+ chars_format fmt = chars_format::general);
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+ ```
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+
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+ *Preconditions:* `fmt` has the value of one of the enumerators of
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+ `chars_format`.
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+
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+ *Effects:* The pattern is the expected form of the subject sequence in
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+ the `"C"` locale, as described for `strtod`, except that
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+
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+ - the sign `’+’` may only appear in the exponent part;
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+ - if `fmt` has `chars_format::scientific` set but not
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+ `chars_format::fixed`, the otherwise optional exponent part shall
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+ appear;
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+ - if `fmt` has `chars_format::fixed` set but not
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+ `chars_format::scientific`, the optional exponent part shall not
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+ appear; and
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+ - if `fmt` is `chars_format::hex`, the prefix `"0x"` or `"0X"` is
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+ assumed. \[*Example 1*: The string `0x123` is parsed to have the value
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+ `0` with remaining characters `x123`. — *end example*]
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+
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+ In any case, the resulting `value` is one of at most two floating-point
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+ values closest to the value of the string matching the pattern.
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+
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+ *Throws:* Nothing.
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+
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+ See also: ISO C 7.22.1.3, 7.22.1.4
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+