In C++, only a few characters can be used as part of the identifier.

identifier:
    identifier-nondigit
    identifier identifier-nondigit
    identifier digit
identifier-nondigit:
    nondigit
    universal-character-name
    other implementation-defined characters
nondigit: one of
    a b c d e f g h i j k l m
    n o p q r s t u v w x y z
    A B C D E F G H I J K L M
    N O P Q R S T U V W X Y Z _
digit: one of
    0 1 2 3 4 5 6 7 8 9

However, there is one exception in VC. The "evil" extension allows "$" to be valid in identifier name.

It is used inside "sal.h" (Source Annotation Language). Like:

#define _$valid       Valid=SA_Yes

But I have no idea why they need "$" :P