std::basic_streambuf::sputbackc

int_type sputbackc(char_type c) {
  if ( (!gptr()) || (gptr()==eback()) || (!traits_type::eq(c,gptr()[-1])) )
    return pbackfail (traits_type::to_int_type(c));
  gbump(-1);
  return traits_type::to_int_type(*gptr());
}

// sputbackc example
#include <iostream>     // std::cin, std::cout, std::streambuf, std::streamsize

int main () {
  char ch;
  std::streambuf * pbuf = std::cin.rdbuf();

  std::cout << "Please, enter some letters and then a number: ";
  do {
    ch = pbuf->sbumpc();

    if ( (ch>='0') && (ch <='9') )
    {
      pbuf->sputbackc (ch);
      long n;
      std::cin >> n;
      std::cout << "You entered number " << n << '\n';
      break;
    }
  } while ( ch != std::streambuf::traits_type::eof() );

  return 0;
}