Flushing the Output Buffer

Consider what happens as a program uses cout to send bytes on to the standard output. Because the ostream class buffers output handled by the cout object, output isn’t sent to its destination immediately. Instead, it accumulates in the buffer until the buffer is full. Then the program flushes the buffer, sending the contents on and clearing the buffer for new data. Typically, a buffer is 512 bytes or an integral multiple thereof. Buffering is a great time-saver when the standard output is connected to a file on a hard disk. After all, you don’t want a program to access the hard disk 512 times to send 512 bytes. It’s much more effective to collect 512 bytes in a buffer and write them to a hard disk in a single disk operation.

For screen output, however, filling the buffer first is less critical. Indeed, it would be inconvenient if you had to reword the message “Press any key to continue” so that it consumed the prerequisite 512 bytes to fill a buffer. Fortunately, in the case of screen output, the program doesn’t necessarily wait until the buffer is full. Sending a newline character to the buffer, for example, normally flushes the buffer. Also as mentioned before, most C++ implementations flush the buffer when input is pending. That is, suppose you have the following code:

cout << "Enter a number: ";
float num;
cin >> num;

The fact that the program expects input causes it to display the cout message (that is, flush the "Enter a number: " message) immediately, even though the output string lacks a newline character. Without this feature, the program would wait for input without prompting the user with the cout message.

If your implementation doesn’t flush output when you want it to, you can force flushing by using one of two manipulators. The flush manipulator flushes the buffer, and the endl manipulator flushes the buffer and inserts a newline character. You use these manipulators the way you would use a variable name:

cout << "Hello, good-looking! " << flush;
cout << "Wait just a moment, please." << endl;

Manipulators are, in fact, functions. For example, you can flush the cout buffer by calling the flush() function directly:

flush(cout);

However, the ostream class overloads the << insertion operator in such a way that the following expression gets replaced with the flush(cout) function call:

cout << flush

Thus, you can use the more convenient insertion notation to flush with success.