Explicit Constructor in C++

#include <iostream>
using std::cout;
using std::endl;

class complexNumbers {
  double real, img;
public:

  complexNumbers() : real(0), img(0) { }
  complexNumbers(const complexNumbers& c) { real = c.real; img = c.img; }
  complexNumbers( double r, double i = 0.0) { real = r; img = i; }
  friend void display(complexNumbers cx);
};

void display(complexNumbers cx){
  cout<<"Real Part: "<<cx.real<<" Imag Part: "<<cx.img<<endl;
}

int main() {
  complexNumbers one(1);
  display(one);
  display(300);
  return 0;
}