NAME¶

std::atof - std::atof

Synopsis¶

Defined in header <cstdlib>
double atof( const char* str );

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 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) (since C++11)
* 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

Parameters¶

str - pointer to the null-terminated byte string to be interpreted

Return value¶

double value corresponding to the contents of str on success. If the converted value
falls out of range of the return type, the return value is undefined. If no
conversion can be performed, 0.0 is returned.

Example¶

// Run this code

#include <cstdlib>
#include <iostream>

int main()
{
std::cout << std::atof("0.0000000123") << '\n'
<< std::atof("0.012") << '\n'
<< std::atof("15e16") << '\n'
<< std::atof("-0x1afp-2") << '\n'
<< std::atof("inF") << '\n'
<< std::atof("Nan") << '\n'
<< std::atof("invalid") << '\n';
}

Output:¶

1.23e-08
0.012
1.5e+17
-107.75
inf
nan
0

See also¶

stof
stod
stold converts a string to a floating point value
(C++11) (function)
(C++11)
(C++11)
strtof converts a byte string to a floating-point value
strtod (function)
strtold
from_chars converts a character sequence to an integer or floating-point value
(C++17) (function)
atoi
atol converts a byte string to an integer value
atoll (function)
(C++11)
C documentation for
atof