The Effects of Buffering

The following example shows the effects of buffering. You might expect the program to print please wait, wait 5 seconds, and then proceed. It won’t necessarily work this way, however, because the output is buffered.

#include <iostream.h>
#include <time.h>

void main()
{
   time_t tm = time( NULL ) + 5;
   cout << "Please wait...";
   while ( time( NULL ) < tm )
      ;
   cout << "\nAll done" << endl;
}

To make the program work logically, the cout object must empty itself when the message is to appear. To flush an ostream object, send it the flush manipulator:

cout << "Please wait..." << flush;

This step flushes the buffer, ensuring the message prints before the wait. You can also use the endl manipulator, which flushes the buffer and outputs a carriage return–linefeed, or you can use the cin object. This object (with the cerr or clog objects) is usually tied to the cout object. Thus, any use of cin (or of the cerr or clog objects) flushes the cout object.