转换构造函数与类型构造函数与运算符重载函数

#include 
using namespace std;
class complex
{
public:
    complex(){real=0;image=0;}
    complex(double r){real=r;image =0;}   ///转换构造函数，只有一个形参。将double 转换成complex类型。
    complex(double r,double i){real=r;image=i;}
    friend complex operator+(complex &c1,complex &c2);
    void display();
     operator double (){return real;}  ///类型转换函数，将complex类型转换成double类型。
private:
    double real,image;
};

complex operator+(complex &c1,complex &c2)
{
    complex c;
    c.real=c1.real+c2.real;
    c.image=c1.image+c2.image;
    return c;
}
void complex::display()
{
    cout<<"("<