#include<iostream>
using namespace std;
class complex
{
public:
    complex(){real=0;image=0;}
    complex(double r,double i):real(r),image(i){}
    complex(double r){real=r;image=0;}
    complex operator+(complex c1,complex c2);
    void display()
    {
            if(image<0)cout<<real<<image<<"i"<<endl;
            else cout<<real<<"+"<<image<<"i"<<endl;
    }
private:
    double real;
    double image;
};
complex complex:: operator+(complex c1,complex c2)
{
    return complex(c1.real+c2.real(),c1.image+c2.image);
}
int main()
{
    complex c1(1,2),c3;
    c3=c1+2.5;
    c3.display();
    return 0;
}

错误看不懂？

介个程序。。看得人眼花啊

class complex
{
public:
    friend complex operator+(complex c1,complex c2);
};
complex operator+(complex c1,complex c2)
{
    //return complex(c1.real+c2.real(),c1.image+c2.image);   c2.real多了个括号。
    return complex(c1.real+c2.real,c1.image+c2.image);
}