From Jason Turner

[utility.from.chars]

C++17 → C++20 3.3 KB 71 lines
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- ### Primitive numeric input conversion <a id="utility.from.chars">[[utility.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`. Otherwise, the characters matching the pattern
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- are interpreted as a representation of a value of the type of `value`.
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- The member `ptr` of the return value points to the first character not
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- matching the pattern, or has the value `last` if all characters match.
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- If the parsed value is not in the range representable by the type of
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- `value`, `value` is unmodified and the member `ec` of the return value
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- is equal to `errc::result_out_of_range`. Otherwise, `value` is set to
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- the parsed value and the member `ec` is set such that the conversion to
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- `bool` yields `false`.
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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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- *Requires:* `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 a minus sign is the only sign that may
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- appear, 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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- *Requires:* `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 only sign that may appear is a minus sign;
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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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- ISO C 7.22.1.3, ISO C 7.22.1.4.
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-