If you use C++11, you can use std::to_string
#include <string>
std::string buf = std::to_string(i);
key1.append(buf);
key2.append(buf);
key3.append(buf);
or you could just use std:stringstream
#include <sstream>
std::stringstream ss;
ss << i;
key1.append(ss.str());
key2.append(ss.str());
key3.append(ss.str());