#include <iostream>
using namespace std;

string f(std::string&& s)
{
    std::string res(std::move(s));
    res += " plus extra";
    return res;
}

int main(void)
{
   std::string str = "A string";
   cout << f( std::move(str) ) << endl;
   std::cout << "'" << str << "'" << std::endl; // Unspecified behavior

   return 0;
}