NAME¶

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

Synopsis¶

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 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

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

Parameters¶

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

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.

Example¶

// Run this code

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

int main()
{
const char* p = "111.11 -2.22 0X1.BC70A3D70A3D7P+6 -Inf 1.18973e+4932zzz";
char* end{};
std::cout << "Parsing \"" << p << "\":\n";
errno = 0;
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';
}

if (std::setlocale(LC_NUMERIC, "de_DE.utf8"))
{
std::cout << "With de_DE.utf8 locale:\n";
std::cout << " '123.45' -> " << std::strtod("123.45", 0) << '\n';
std::cout << " '123,45' -> " << std::strtod("123,45", 0) << '\n';
}
}

Possible output:¶

Parsing "111.11 -2.22 0X1.BC70A3D70A3D7P+6 -Inf 1.18973e+4932zzz":
'111.11' -> 111.11
' -2.22' -> -2.22
' 0X1.BC70A3D70A3D7P+6' -> 111.11
' -Inf' -> -inf
' 1.18973e+4932' -> range error, got inf
With de_DE.utf8 locale:
'123.45' -> 123
'123,45' -> 123.45

See also¶

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