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std::nan, std::nanf, std::nanl

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Defined in header <cmath>
float       nanf( const char* arg );
 (1) (since C++11)
double      nan ( const char* arg );
 (2) (since C++11)
long double nanl( const char* arg );
 (3) (since C++11)

Converts the character string arg into the corresponding quiet NaN value, as if by calling std::strtof, std::strtod, or std::strtold, respectively.

1) The call std::nanf("n-char-sequence"), where n-char-sequence is a sequence of digits, ASCII letters, and underscores, is equivalent to the call std::strtof("NAN(n-char-sequence)", (char**)nullptr);.
The call std::nanf("") is equivalent to the call std::strtof("NAN()", (char**)nullptr);.
The call std::nanf("string"), where string is neither an n-char-sequence nor an empty string, is equivalent to the call std::strtof("NAN", (char**)nullptr);.
2) Same as (1), but calls std::strtod instead of std::strtof.
3) Same as (1), but calls std::strtold instead of std::strtof.

Contents

[edit] Parameters

arg - narrow character string identifying the contents of a NaN

[edit] Return value

The quiet NaN value that corresponds to the identifying string arg or zero if the implementation does not support quiet NaNs.

If the implementation supports IEEE floating-point arithmetic (IEC 60559), it also supports quiet NaNs.

[edit] Error handling

This function is not subject to any of the error conditions specified in math_errhandling.

[edit] Example

Run this code
#include <cmath>
#include <cstdint>
#include <cstring>
#include <iostream>
 
int main()
{
    double f1 = std::nan("1");
    std::uint64_t f1n; std::memcpy(&f1n, &f1, sizeof f1);
    std::cout << "nan(\"1\") = " << f1 << " (" << std::hex << f1n << ")\n";
 
    double f2 = std::nan("2");
    std::uint64_t f2n; std::memcpy(&f2n, &f2, sizeof f2);
    std::cout << "nan(\"2\") = " << f2 << " (" << std::hex << f2n << ")\n";
}

Possible output: 

nan("1") = nan (7ff0000000000001)
nan("2") = nan (7ff0000000000002)

[edit] See also

(C++11)
 checks if the given number is NaN
(function) [edit]
(C++11)
 evaluates to a quiet NaN of type float
(macro constant) [edit]
[static]
 identifies floating-point types that can represent the special value "quiet not-a-number" (NaN)
(public static member constant of std::numeric_limits<T>) [edit]
[static]
 identifies floating-point types that can represent the special value "signaling not-a-number" (NaN)
(public static member constant of std::numeric_limits<T>) [edit]
[static]
 returns a quiet NaN value of the given floating-point type
(public static member function of std::numeric_limits<T>) [edit]
[static]
 returns a signaling NaN value of the given floating-point type
(public static member function of std::numeric_limits<T>) [edit]
C documentation for nanf, nan, nanl
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