Identifiers In Language

• A valid identifier is a sequence of one or more letters, digits or underscore characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. 

• Only letters, digits and single underscore characters are valid. In addition, variable identifiers always have to begin with a letter. 

• They can also begin with an underline character (_), but in some cases these may be reserved for compiler specific keywords or external identifiers, as well as identifiers containing two successive underscore characters anywhere. In no case they can begin with a digit.

• Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword of the C++ language nor your compiler's specific ones, which are reserved keywords.

• The standard reserved keywords are: asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register,reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved words under some circumstances: and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq Your compiler may also include some additional specific reserved keywords.

• Very important: The C++ language is a "case sensitive" language. 

• That means that an identifier written in capital letters is not equivalent to another one with the same name but written in small letters. 

• Thus, for example, the RESULT variable is not the same as the result variable or the Result variable. These are three different variable identifiers.