Identifiers in C++ can be composed of letters, digits, and the underscore character.
The language imposes no limit on name length.
Identifiers must begin with either a letter or an underscore. Identifiers are case-sensitive; upper- and lowercase letters are
distinct.
The language reserves a set of names. These names may not be used as identifiers.
The standard also reserves a set of names for use in the standard library.
The identifiers we define in our own programs may not contain two consecutive underscores, nor can an identifier begin with an underscore followed immediately by an uppercase letter. In addition, identifiers defined outside a function may not begin with an underscore.
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