对左值和右值的理解

#include <bits/stdc++.h>
using namespace std;
class Num{
    int real,imag;
public:
    Num(int real,int imag):real(real),imag(imag){cout<<"calling constructing"<<endl;}
    Num(const Num&n){
        cout<<"calling copy"<<endl;
        real=n.real;
        imag=n.imag;
    }
    Num operator +(const Num&n){
        cout<<"calling +"<<endl;
        return {real+n.real,imag+n.imag};
    }
    Num operator =(const Num&n){
        cout<<"calling ="<<endl;
        real=n.real;
        imag=n.imag;
    }
};

int main(){
    Num N1(5,6);
    Num N2(6,9);
    Num n3=N1+N2;



    return 0;
}

看结果

结果出乎意料，本来以为会有calling copy出现但是没有，然后发现
如果改为

就会有

因为N1+N2不是左值（引用），所以会导致跟预期不一样