c++类的问题

#include <iostream>
using namespace std;
class Complex                                     //定义Complex类
{public:
Complex(){real=0;imag=0;}            //定义构造函数
Complex(double r,double i){real=r;imag=i;}     //构造函数重载
Complex complex_add(Complex &c2);              //声明复数相加函数
void display();                                //声明输出函数
 private:
double real;                                   //实部
double imag;                                   //虚部
};

Complex Complex::complex_add(Complex &c2)
{Complex c;
c.real=real+c2.real;
c.imag=imag+c2.imag;
return c;}   

void Complex::display()                            //定义输出函数
{cout<<"("<<real<<","<<imag<<"i)"<<endl;}

int main()
{Complex c1(3,4),c2(5,-10),c3;                     //定义3个复数对象
c3=(c2);                            //调用复数相加函数
cout<<"c1="; c1.display();                        //输出c1的值
cout<<"c2="; c2.display();                        //输出c2的值
cout<<"c1+c2="; c3.display();                     //输出c3的值
return 0;
}