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Variants

std:: isnan

From cppreference.net
C++
Numerics library
Common mathematical functions
Nearest integer floating point operations
(C++11)
(C++11)
(C++11) (C++11) (C++11)
Floating point manipulation functions
(C++11) (C++11)
(C++11)
(C++11)
Classification and comparison
(C++11)
(C++11)
(C++11)
isnan
(C++11)
(C++11)
(C++11)
Types
(C++11)
(C++11)
(C++11)
Macro constants
Defined in header <cmath>
(1)
bool isnan ( float num ) ;

bool isnan ( double num ) ;

bool isnan ( long double num ) ;
(since C++11)
(until C++23)
constexpr bool isnan ( /*floating-point-type*/ num ) ;
(since C++23)
SIMD overload (since C++26)
Defined in header <simd>
template < /*math-floating-point*/ V >

constexpr typename /*deduced-simd-t*/ < V > :: mask_type

isnan ( const V & v_num ) ;
(S) (since C++26)
Defined in header <cmath>
template < class Integer >
bool isnan ( Integer num ) ;
(A) (since C++11)
(constexpr since C++23)
1) Determines if the given floating point number num is a not-a-number (NaN) value. The library provides overloads for all cv-unqualified floating-point types as the type of the parameter num . (since C++23)
S) The SIMD overload performs an element-wise std::isnan on v_num .
(See math-floating-point and deduced-simd-t for their definitions.)
(since C++26)
A) Additional overloads are provided for all integer types, which are treated as double .

Contents

Parameters

num - floating-point or integer value
v_num - a data-parallel object of std::basic_simd specialization where its element type is a floating-point type

Return value

1) true if num is a NaN, false otherwise.
S) A data-parallel mask object where the i th element equals true if v_num [ i ] is a NaN or false otherwise for all i in the range [ ​ 0 ​ , v_num. size ( ) ) .

Notes

There are many different NaN values with different sign bits and payloads, see std::nan and std::numeric_limits::quiet_NaN .

NaN values never compare equal to themselves or to other NaN values. Copying a NaN is not required, by IEEE-754, to preserve its bit representation (sign and payload ), though most implementation do.

Another way to test if a floating-point value is NaN is to compare it with itself: bool is_nan ( double x ) { return x ! = x ; } .

GCC and Clang support a -ffinite-math option (additionally implied by -ffast-math ), which allows the respective compiler to assume the nonexistence of special IEEE-754 floating point values such as NaN, infinity, or negative zero. In other words, std::isnan is assumed to always return false under this option.

The additional overloads are not required to be provided exactly as (A) . They only need to be sufficient to ensure that for their argument num of integer type, std :: isnan ( num ) has the same effect as std :: isnan ( static_cast < double > ( num ) ) .

Example

Run this code 
#include <cfloat>
#include <cmath>
#include <iostream>
int main()
{
    std::cout << std::boolalpha
              << "isnan(NaN) = " << std::isnan(NAN) << '\n'
              << "isnan(Inf) = " << std::isnan(INFINITY) << '\n'
              << "isnan(0.0) = " << std::isnan(0.0) << '\n'
              << "isnan(DBL_MIN/2.0) = " << std::isnan(DBL_MIN / 2.0) << '\n'
              << "isnan(0.0 / 0.0)   = " << std::isnan(0.0 / 0.0) << '\n'
              << "isnan(Inf - Inf)   = " << std::isnan(INFINITY - INFINITY) << '\n';
}

Output: 

isnan(NaN) = true
isnan(Inf) = false
isnan(0.0) = false
isnan(DBL_MIN/2.0) = false
isnan(0.0 / 0.0)   = true
isnan(Inf - Inf)   = true

See also

(C++11) (C++11) (C++11)
not-a-number (NaN)
(function)
(C++11)
categorizes the given floating-point value
(function)
(C++11)
checks if the given number has finite value
(function)
(C++11)
checks if the given number is infinite
(function)
(C++11)
checks if the given number is normal
(function)
(C++11)
checks if two floating-point values are unordered
(function)
C documentation for isnan