$ symbol in c++ - c++

Question

I read the following code from an open source library. What confuses me is the usage of dollar sign. Can anyone please clarify the meaning of $ in the code. Your help is greatly appreciated!
__forceinline MutexActive( void ) : $lock(LOCK_IS_FREE) {}
void lock ( void );
__forceinline void unlock( void ) {
__memory_barrier(); // compiler must not schedule loads and stores around this point
$lock = LOCK_IS_FREE;
}
protected:
enum ${ LOCK_IS_FREE = 0, LOCK_IS_TAKEN = 1 };
Atomic $lock;

Answer 1

There is a gcc switch, -fdollars-in-identifiers which explicitly allows $ in idenfitiers.
Perhaps they enable it and use the $ as something that is highly unlikely to clash with normal names.
-fdollars-in-identifiers
Accept $ in identifiers. You can also explicitly prohibit use of $ with the option -fno-dollars-in-identifiers. (GNU C allows $ by
default on most target systems, but there are a few exceptions.)
Traditional C allowed the character $ to form part of identifiers.
However, ISO C and C++ forbid $ in identifiers.
See the gcc documentation. Hopefully the link stays good.

Answer 2

It is being used as part of an identifer.
[C++11: 2.11/1] defines an identifier as "an arbitrarily long sequence of letters and digits." It defines "letters and digits" in a grammar given immediately above, which names only numeric digits, lower- and upper-case roman letters, and the underscore character explicitly, but does also allow "other implementation-defined characters", of which this is presumably one.
In this scenario the $ has no special meaning other than as part of an identifier — in this case, the name of a variable. There is no special significance with it being at the start of the variable name.

Answer 3

Even if dollar sign are not valid identifiers according to the standard, it can be accepted. For example visual studio (I think ggc too but I'm not sure about that) seems to accept it.
Check this doc : http://msdn.microsoft.com/en-us/library/565w213d(v=vs.80).aspx
and this : Are dollar-signs allowed in identifiers in C++03?

Answer 4

The C++ standard says:
The basic source character set consists of 96 characters: the space
character, the control characters representing horizontal tab,
vertical tab, form feed, and new-line, plus the following 91 graphical
characters: 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 0 1 2 3 4 5 6 7 8 9
_ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ! = , \ " ’
There is no $ in the basic source character set described above; The $ character in your code is an extension to the basic source character set, which isn't required. Consider in Britain, where the pound symbol (£ or ₤) is used in place of the dollar symbol ($).