运算符重载friend与非friend的问题

#include <iostream>
using namespace std;
class complex{
    private:
        int real,imag;
    public:
        complex(int r=0,int i=0):real(r),imag(i){}
        friend complex operator +(complex c1);
        friend complex operator -(complex c1);
        void diplay();
};
void complex::diplay(){cout < <"real:" < <real < <"imag:" < <imag < <endl;}

complex operator +(complex c1){
    return complex(real+c1.real, imag+c1.imag);
}
complex operator -(complex c1){
    return complex(real-c1.real,imag-c1.imag);
}
int main(){
    complex c1(1,2),c2(2,3),c3;
    c1.diplay();
    c2.diplay();
    c3=c1+c2;
    c3.diplay();
    c3=c1-c2;
    c3.diplay();
}
//此处不能通过，没想通，面下面这个就可以运行，不知为什么？
#include <iostream>
using namespace std;
class complex{
    private:
        int real,imag;
    public:
        complex(int r,int i);
        void display();
        friend complex operator +(complex c1,complex c2);
        friend complex operator -(complex c1,complex c2);
};
complex::complex(int r,int i){
    real=r;
    imag=i;
}
complex complex::operator +(complex c1,complex c2){
    return complex(c1.real+c2.real,c1.imag+c2.imag);
}
complex complex::operator -(complex c1,complex c2){
    return complex(c1.real-c2.real,c1.imag-c2.imag);
}
void complex::display(){
    cout < <"real=" < <real < <"imag=" < <imag < <endl;
}
int main(){

}

因为如果是friend的话 参数必须显示有两个操作对象
而非friend的  因为其本身隐含了一个this参数 所以不要两个 

所以你第一个可行 第二个不行  把friend去掉就可以了

 

class complex{
    private:
        int real,imag;
    public:
        complex(int r=0,int i=0):real(r),imag(i){}
         complex operator +( complex c1);
          complex operator -( complex c1);
        void diplay();
};
void complex::diplay(){cout <<"real:" <<real <<"imag:" <<imag <<endl;}

complex complex::operator +(complex c1){
    return complex(real+c1.real, imag+c1.imag);
}
complex complex::operator -( complex c1){
    return complex(real-c1.real,imag-c1.imag);
}
int main(){
    complex c1(1,2),c2(2,3),c3;
    c1.diplay();
    c2.diplay();
    c3=c1+c2;
    c3.diplay();
    c3=c1-c2;
    c3.diplay();
}


输出如下
real:1imag:2
real:2imag:3
real:3imag:5
real:-1imag:-1
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