c++运算符重载字符串想加

#include 
using namespace std;
class C{
    private:
        string s;
    public:
        C(){}
        C(string a){
            s=a;
        }
        C operator+(C &b){
            C c;
            c.s=s+b.s;
            return c;
        }
        void show(){
            cout<