Flush buffer.
  If the stream is a buffered stream, the buffer is synchronized with the physical media associated with it.
  Buffered streams are usually used when a physical device that is significantly slowlier than memory access (like files) is associated with a stream. This allows to perform the input and output operations on a memory buffer instead of directly on the device, thus accelerating considerably the operations. A call to this member function forces the stream's buffer to syncronize its data with the content of the physical device associated with it.

Parameters.

os

Stream in which the insertion has to be performed.
This is a manipulator parameter that is automatically assumed if used with insertion operator (operator<<).

Return Value.
  A reference to the stream object (parameter os).

Example.

// Flushing files (flush manipulator)
#include <fstream>
using namespace std;

int main () {

  ofstream outfile ("test.txt");

  for (int n=0; n<100; n++)
    outfile << n << flush;

  outfile.close();

  return 0;
}

Basic template declaration:

template <class charT, class traits>   basic_ostream<charT,traits>& flush ( basic_ostream<charT,traits>& os );

See also.
endl, flush function   ostream class