#include <iostream>
#include <sstream>
#include <iomanip>

int main(int argc, char* argv[]) {
  std::stringstream aSs;
  aSs << std::left << std::setfill('0') << std::setw(12) << 123;
  
  std::string aStr = aSs.str();  // <--  get as std::string
  std::cout << aStr << '\n';
  
  return 0;
}