Clear format flag.
  Clears the format flags corresponding to the bits set in parameter mask.

Parameters.

mask

Mask containing the bits to be cleared. It is a value of type fmtflags.

Return Value.
  none

Example.

// clearing flags
#include <iostream>
using namespace std;

int main () {
  cout.setf ( ios_base::hex, ios_base::basefield );  // set hex as the basefield
  cout.setf ( ios_base::showbase );                  // activate showbase
  cout << 100 << endl;
  cout.unsetf ( ios_base::showbase );                // deactivate showbase
  cout << 100 << endl;
  return 0;
}

See also.
  flags, setf
  fmtflags type, ios_base class