#include Preprocessing Directive
#include <iostream> // allows program to output data to the screen
is a preprocessing directive, which is a message to the C++ preprocessor (introduced in Section 1.4). Lines that begin with # are processed by the preprocessor before the program is compiled. This line notifies the preprocessor to include in the program the contents of the input/output stream header <iostream>. This header is a file containing information used by the compiler when compiling any program that outputs data to the screen or inputs data from the keyboard using C++’s stream input/output. The program in Fig. 2.1 outputs data to the screen, as we’ll soon see. We discuss headers in more detail in Chapter 6 and explain the contents ...