A variable name (such as
number1) is any valid identifier that is not a keyword. An identifier is a series of characters consisting of letters, digits and underscores ( _ ) that does not begin with a digit. C++ is case sensitive—uppercase and lowercase letters are different, so
A1 are different identifiers.
Portability Tip 2.1
C++ allows identifiers of any length, but your C++ implementation may restrict identifier lengths. Use identifiers of 31 characters or fewer to ensure portability.
Good Programming Practice 2.4