Selects a base format flag
  dec sets the base format flag to represent integer values as decimal numbers.
  hex sets the base format flag to represent integer values as hexadecimal numbers.
  oct sets the base format flag to represent integer values as octal numbers.

Parameters.

str

Stream object where to apply. This is a manipulator parameter.

Return Value.
  A reference to the stream object.

Example.

// modify basefield
#include <iostream>
using namespace std;

int main () {
  int n;
  n=70;
  cout << dec << n << endl;
  cout << hex << n << endl;
  cout << oct << n << endl;
  return 0;
}

See also.
  flags, setf, unsetf
  ios_base class