Get/set the tied stream.
  The first syntax returns the tied stream. The tie stream is a stream syncronized with the sequence controlled by the stream buffer that is automatically flushed when more characters are needed.
  The second syntax ties the object to tiestr and returns a pointer to the previously tied ostream object.
  By default, the standard objects cin, cerr and clog are tied to cout.

Parameters.

tiestr

an output stream object to be tied to.

Return Value.
  A pointer to the stream object tied before the call, or if none, a null pointer.

Example.

// redefine tied object
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ostream *prevstr;
  ofstream filestr;
  filestr.open ("test.txt");

  *cin.tie() << "This is inserted into cout";
  prevstr = cin.tie (&filestr);
  *cin.tie() << "This is inserted into file";
  cin.tie (prevstr);

  filestr.close();

  return 0;
}

Basic template member declaration ( basic_ios<charT,traits> ):

basic_ostream<charT,traits> * tie () const; basic_ostream<charT,traits> * tie ( basic_ostream<charT,traits> tiestr );

See also.
  ios class