C++ reference: ostream::flush

iostream hierarchy
ostream:: flush

cplusplus.com

ostream& flush ( );

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 slower 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.

none

Return Value.
*this

Example.

// Flushing files
#include <fstream>
using namespace std;

int main () {

  ofstream outfile ("test.txt");

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

  outfile.close();

  return 0;
}

When this example is executed the content of the file test.txt is updated 100 times.

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

basic_ostream& flush ( );