std::stof, std::stod, std::stold

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Defined in header <string>
float       stof ( const std::string& str, std::size_t* pos = nullptr );
(1) (since C++11)
float       stof ( const std::wstring& str, std::size_t* pos = nullptr );
(2) (since C++11)
double      stod ( const std::string& str, std::size_t* pos = nullptr );
(3) (since C++11)
double      stod ( const std::wstring& str, std::size_t* pos = nullptr );
(4) (since C++11)
long double stold( const std::string& str, std::size_t* pos = nullptr );
(5) (since C++11)
long double stold( const std::wstring& str, std::size_t* pos = nullptr );
(6) (since C++11)

Interprets a floating point value in a string str.

Let ptr be an internal (to the conversion functions) pointer of type char* (1,3,5) or wchar_t* (2,4,6), accordingly.

1) Calls std::strtof(str.c_str(), &ptr).
2) Calls std::wcstof(str.c_str(), &ptr).
3) Calls std::strtod(str.c_str(), &ptr).
4) Calls std::wcstod(str.c_str(), &ptr).
5) Calls std::strtold(str.c_str(), &ptr).
6) Calls std::wcstold(str.c_str(), &ptr).

Function discards any whitespace characters (as determined by std::isspace) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

  • decimal floating-point expression. It consists of the following parts:
  • (optional) plus or minus sign
  • nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
  • (optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 10)
  • hexadecimal floating-point expression. It consists of the following parts:
  • (optional) plus or minus sign
  • 0x or 0X
  • nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand)
  • (optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 2)
  • infinity expression. It consists of the following parts:
  • (optional) plus or minus sign
  • INF or INFINITY ignoring case
  • not-a-number expression. It consists of the following parts:
  • (optional) plus or minus sign
  • NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain digits, Latin letters, and underscores. The result is a quiet NaN floating-point value.
  • any other expression that may be accepted by the currently installed C locale

If pos is not a null pointer, then ptr will receive the address of the first unconverted character in str.c_str(), and the index of that character will be calculated and stored in *pos, giving the number of characters that were processed by the conversion.

Parameters

str - the string to convert
pos - address of an integer to store the number of characters processed

Return value

The string converted to the specified floating point type.

Exceptions

std::invalid_argument if no conversion could be performed.

std::out_of_range if the converted value would fall out of the range of the result type or if the underlying function (std::strtof, std::strtod or std::strtold) sets errno to ERANGE.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2009 C++11 std::out_of_range would not be thrown if the converted
value would fall out of the range of the result type
will throw
LWG 2403 C++11 stof called std::strtod or std::wcstod stof calls std::strtof or std::wcstof

See also

(C++11)(C++11)(C++11)
converts a string to a signed integer
(function)
(C++11)(C++11)
converts a string to an unsigned integer
(function)
converts a character sequence to an integer or floating-point value
(function)