Answer 1

#include <iostream>
#include <sstream>
using namespace std;

int main() {
// str() overwrite
stringstream ss;
string s="hello my friend";
ss.str(s);
cout<<"1:"<<ss.str()<<endl;
s = "good morning my friend";
ss.str(s);
cout<<"2:"<<ss.str()<<endl;

// << append
stringstream ss2;
s="hello my friend";
ss2<<s;
cout<<"3:"<<ss2.str()<<endl;
s = "good morning my friend";
ss2<<s;
cout<<"4:"<<ss2.str()<<endl;
return 0;
}