The modulus of a complex number is a measure of the length of the vector representing the complex number. The modulus of a complex number a + bi is sqrt(a2 + b2), written |a + bi|. The norm of a complex number a + bi is (a2 + b2), so the modulus of a complex number is the square root of its norm.

// complex_abs.cpp
// compile with: /EHsc
#include <complex>
#include <iostream>
int main( )
{
using namespace std;
double pi = 3.14159265359;
// Complex numbers can be entered in polar form with
// modulus and argument parameter inputs but are
// stored in Cartesian form as real & imag coordinates
complex <double> c1 ( polar ( 5.0 ) ); // Default argument = 0
complex <double> c2 ( polar ( 5.0 , pi / 6 ) );
complex <double> c3 ( polar ( 5.0 , 13 * pi / 6 ) );
cout << "c1 = polar ( 5.0 ) = " << c1 << endl;
cout << "c2 = polar ( 5.0 , pi / 6 ) = " << c2 << endl;
cout << "c3 = polar ( 5.0 , 13 * pi / 6 ) = " << c3 << endl;
// The modulus and argument of a complex number can be recovered
// using abs & arg member functions
double absc1 = abs ( c1 );
double argc1 = arg ( c1 );
cout << "The modulus of c1 is recovered from c1 using: abs ( c1 ) = "
<< absc1 << endl;
cout << "Argument of c1 is recovered from c1 using:\n arg ( c1 ) = "
<< argc1 << " radians, which is " << argc1 * 180 / pi
<< " degrees." << endl;
double absc2 = abs ( c2 );
double argc2 = arg ( c2 );
cout << "The modulus of c2 is recovered from c2 using: abs ( c2 ) = "
<< absc2 << endl;
cout << "Argument of c2 is recovered from c2 using:\n arg ( c2 ) = "
<< argc2 << " radians, which is " << argc2 * 180 / pi
<< " degrees." << endl;
// Testing if the principal angles of c2 and c3 are the same
if ( (arg ( c2 ) <= ( arg ( c3 ) + .00000001) ) ||
(arg ( c2 ) >= ( arg ( c3 ) - .00000001) ) )
cout << "The complex numbers c2 & c3 have the "
<< "same principal arguments."<< endl;
else
cout << "The complex numbers c2 & c3 don't have the "
<< "same principal arguments." << endl;
}

Output

c1 = polar ( 5.0 ) = (5,0)
c2 = polar ( 5.0 , pi / 6 ) = (4.33013,2.5)
c3 = polar ( 5.0 , 13 * pi / 6 ) = (4.33013,2.5)
The modulus of c1 is recovered from c1 using: abs ( c1 ) = 5
Argument of c1 is recovered from c1 using:
arg ( c1 ) = 0 radians, which is 0 degrees.
The modulus of c2 is recovered from c2 using: abs ( c2 ) = 5
Argument of c2 is recovered from c2 using:
arg ( c2 ) = 0.523599 radians, which is 30 degrees.
The complex numbers c2 & c3 have the same principal arguments.