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# std::strtof, std::strtod, std::strtold

From cppreference.com
< cpp‎ | string‎ | byte

Defined in header `<cstdlib>` |  |
float strtof( const char* str, char** str_end ); |  | (since C++11)
double strtod( const char* str, char** str_end ); |  |
long double strtold( const char* str, char** str_end ); |  | (since C++11)

Interprets a floating point value in a byte string pointed to by `str`.

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)

- binary 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)

- 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 alphanumeric characters. The result is a quiet NaN floating-point value.

- any other expression that may be accepted by the currently installed C locale

The functions sets the pointer pointed to by `str_end` to point to the character past the last character interpreted. If `str_end` is NULL, it is ignored.

### [edit] Parameters

str | - | pointer to the null-terminated byte string to be interpreted
str_end | - | pointer to a pointer to character.

### [edit] Return value

Floating point value corresponding to the contents of `str` on success. If the converted value falls out of range of corresponding return type, range error occurs and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, ​0​ is returned and *str_end is set to str.

### [edit] Example

Run this code

    #include <iostream>
    #include <string>
    #include <cerrno>
    #include <cstdlib>

    int main()
    {
        const char* p = "111.11 -2.22 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz";
        char* end;
        std::cout << "Parsing \"" << p << "\":\n";
        for (double f = std::strtod(p, &end); p != end; f = std::strtod(p, &end))
        {
            std::cout << "'" << std::string(p, end-p) << "' -> ";
            p = end;
            if (errno == ERANGE){
                std::cout << "range error, got ";
                errno = 0;
            }
            std::cout << f << '\n';
        }
    }

Output:

    Parsing "111.11 -2.22 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz":
    '111.11' -> 111.11
    ' -2.22' -> -2.22
    ' 0X1.BC70A3D70A3D7P+6' -> 111.11
    '  1.18973e+4932' -> range error, got inf

### [edit] See also

atof | converts a byte string to a floating point value (function) [edit]
wcstof wcstod wcstold | converts a wide string to a floating point value (function) [edit]
from_chars (C++17) | converts a character sequence to an integer or floating-point value (function) [edit]
C documentation for strtof, strtod, strtold

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