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

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Numerics library
Common mathematical functions
Nearest integer floating point operations
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Floating point manipulation functions
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Classification and comparison
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Macro constants
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

Parameters

arg - narrow character string identifying the contents of a NaN

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.

Error handling

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

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)

See also

(C++11)
checks if the given number is NaN
(function)
(C++11)
evaluates to a quiet NaN of type float
(macro constant)
[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> )
[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> )
[static]
returns a quiet NaN value of the given floating-point type
(public static member function of std::numeric_limits<T> )
[static]
returns a signaling NaN value of the given floating-point type
(public static member function of std::numeric_limits<T> )
C documentation for nanf , nan , nanl