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

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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 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 a decimal point character (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 (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.

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.

Contents

[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.

[edit] Example

#include <cstdlib>
#include <iostream>
 
int main()
{
    std::cout << std::strtod("0.0000000123", nullptr) << "\n";
 
    // the second arg of strto* is non-const. We must copy the string
    char src[] = "0.012  15e16 -0x1afp-2 inF Nan";
    char* c = src;
 
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
}

Output:

1.23e-08
0.012
1.5e+17
-107.75
inf
nan

[edit] See also

converts a byte string to a floating point value
(function) [edit]
C documentation for strtof, strtod, strtold