std::ostringstream::str

// ostringstream::rdbuf
#include <string>       // std::string
#include <iostream>     // std::cout
#include <sstream>      // std::ostringstream

int main () {
  std::ostringstream oss;
  oss << "One hundred and one: " << 101;
  std::string s = oss.str();
  std::cout << s << '\n';
  return 0;
}