C++在编译的时候出的问题 看不懂 求解

Question

#include
using namespace std;
class Complex
{
public:
Complex();
Complex(double a, double b) { real = a, imag = b; }
friend Complex operator +(int &i, Complex &c);
friend Complex operator +(Complex &c1, Complex &c2);
void input();
void display();
private:
double real;
double imag;

};
Complex operator +(int &i, Complex &c)
{
return Complex(i + c.real, c.imag);
}
Complex operator +(Complex &c1, Complex &c2)
{
return Complex(c1.real + c2.real, c1.imag + c2.imag);
}
void Complex::input()
{
cout << "please input the complex ";
cin >> real >> imag;
}
void Complex::display()
{
cout << "(" << real << "," << imag << ")";
}
int main()
{
Complex c1, c2, c3, c4;
int i;
c1.input();
cout << "c1="; c1.display();
c2.input();
cout << "c2="; c2.display();
c3 = c1 + c2;
cout << "please input a number ";
cin >> i;
c4 = i + c1;
cout << "c1+c2="; c3.display();
cout << "c1+i="; c4.display();
return 0;
}

Answer 1

http://codepad.org/xAwxjyPw
通过在线编译。