C++ Primer, Fifth Edition

8.1.3. Managing the Output Buffer

Each output stream manages a buffer, which it uses to hold the data that the program reads and writes. For example, when the following code is executed

os << "please enter a value: ";

the literal string might be printed immediately, or the operating system might store the data in a buffer to be printed later. Using a buffer allows the operating system to combine several output operations from our program into a single system-level write. Because writing to a device can be time-consuming, letting the operating system combine several output operations into a single write can provide an important performance boost.

There are several conditions that cause the buffer to be flushed—that is, to be written—to the actual ...