A third category of tokens is identifiers. Identifiers are names of things that appear in
programs, such as variables, constants, and functions. All identifiers must obey C++’s rules for identifiers.
Identifier: A C++ identifier consists of letters, digits, and the underscore character (_ ) and must begin with a letter or underscore.
Some identifiers are predefined; others are defined by the user. In the C++ program in cout is a predefined identifier and num is a user-defined identifier. Two
predefined identifiers that you will encounter frequently are cout and cin. You have
already seen the effect of cout. Later in this chapter, you will learn how cin, which is
used to input data, works. Unlike reserved words, predefined identifiers can be redefined, but it would not be wise to do so.
Identifiers can be made of only letters, digits, and the underscore character; no other
symbols are permitted to form an identifier