std::ios_base::pword

// pword example
#include <iostream>     // std::ios, std::cout, std::cerr, std::clog
const int name_index = std::ios::xalloc();
// stores pointer in extensible array:
void SetStreamName (std::ios& stream, const char* name) {
  stream.pword(name_index) = const_cast<char*>(name);
}
// custom manipulator that uses stored pointer:
std::ostream& StreamName (std::ostream& os) {
  const char* name = static_cast<const char*>(os.pword(name_index));
  if (name) os << name;
  else os << "(unknown)";
  return os;
}
int main()
{
  SetStreamName(std::cout, "standard output stream");
  SetStreamName(std::cerr, "standard error stream");
  std::cout << StreamName << '\n';
  std::cerr << StreamName << '\n';
  std::clog << StreamName << '\n';
  return 0;
}