why nan is coming in c++ code example

Example 1: nan c++ example

#include <iostream>
#include <cmath>
using namespace std;

// main() section
int main()
{
    double nanValue;
    
    //generating generic NaN value
    //by passing an empty string
    nanValue = nan("");
    
    //printing the value 
    cout<<"nanValue: "<<nanValue<<endl;
    
    return 0;
}

Example 2: C++ is nan

doubel a = 0 ;
bool isnan_a = std::isnan(a) || std::isnan(-a);