- Lines beginning with a pound sign
(#) are directives for the preprocessor. They are not
regular code lines with expressions but indications for the
compiler's preprocessor. In this case the directive #include <iostream>
tells the preprocessor to include the iostream standard file. This
specific file (iostream) includes the declarations of the basic
standard input-output library in C++, and it is included because
its functionality is going to be used later in the program.