What is the nan() function in C++?

Overview

The nan() function is used to return a quiet NaN (Not-A-Number) value of type double.

The nan() function is defined in the <cmath> header file.

Syntax

double nan (const char* arg);

Parameter value

The nan() function takes the tagp as its parameter value. tagp represents an implementation-specific-string. When this string is empty (""), the nan() function returns a generic NaN value (the same that is returned by passing NaN to strtod).

Return value

The nan() function returns a quiet NaN value.

Example

#include <iostream>
#include <cmath>
#include <cstring>
using namespace std;
int main()
{
    // using the nan() function
    double var = nan("1");
    uint64_t dest;
    
    // copies variable var to dest
    // use <cstring> for memcpy()
    memcpy(&dest, &var, sizeof var);
    cout << "nan(\"1\") = " << var << " (" << hex << dest << ")\n";
    return 0;
}

Code explanation

  • Line 10: We implement the nan() function to the string value "1" and assign the output to a variable var.
  • Line 11: We create another variable dest.
  • Line 15: We copy the variable var to dest.
  • Line 16: We print our variables.

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