C++

9.4    Identifiers

Many things have a name: variables, constants, functions, classes, macros, and many more. These names consist of letters, digits, and the underscore character _. By letters, we mean A to Z and a to z; you should not use umlauts and other international characters, although some compilers support them.

An identifier must not start with a digit—so you cannot use 9eye, for example.

Regarding the underscore, you should not start anything with two underscores __ and should avoid an underscore followed by an uppercase letter at the beginning, as these forms are reserved for the standard library internally: __number and _Text can cause problems, for example.

Here are a few valid identifiers—all of which are different, as C++ is case-sensitive: ...