Identifiers in C++

A valid identifier is a sequence of one or more letters, digits or underline symbols ( _ ). The length of an identifier is not limited, although for some compilers only the 32 first characters of an identifier are significant.

Neither spaces nor marked letters can be part of an identifier. Only letters, digits and underline characters are valid. In addition, variable identifiers should always begin with a letter. They can also begin with an underline character ( _ ), but this is usually reserved for external links. 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 key word of the C++ language nor your compiler's specific ones since they could be confused with these. For example, the following expressions are always considered key words according to the ANSI-C++ standard and therefore they must not be used as identifiers:

asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, 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

Additionally, alternative representations for some operators do not have to 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 more specific reserved keywords. For example, many compilers which generate 16 bit code (like some compilers for DOS) also include far, huge and near as key words.

The C++ language is "case sensitive", 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 variable RESULT is not the same as the variable result nor the variable Result.