Variables - a portion of  memory to store a determined value.

Identifiers - are used as names for variables and other items in a C++ program.

Valid Identifiers - is a sequence of one or more letters, digits or underscores characters. Neither spaces nor punctuation marks or symbols can be an identifiers.

You have to consider when inventing your own identifiers is that they cannot match any of the reserved keywords of the C++ language nor your compilers 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

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