c++ - Does C++11 allow dollar signs in identifiers?

Question

Are dollar-signs allowed in identifiers in C++03? covers that dollar signs in identifiers are not allowed in C++03. GCC provides it as a C extension and properly gives a diagnostic in C++03 mode. However, in C++11, int $ = 0 will compile without warning.

This answer reasons that $ may be allowed because no diagnostic is required for implementation defined identifiers:

The answer here is "Maybe": According to §2.11, identifiers may consist of digits and identifier-nondigits, starting with one of the latter. identifier-nondigits are the usual a-z, A-Z and underscore, in addition since C++11 they include universal-character-names (e.g. uBEAF, UC0FFEE32), and other implementation-defined characters. So it is implementation defined if using $ in an identifier is allowed. VC10 and up supports that, maybe earlier versions, too. It even supports identifiers like こんばんわ.

But: I wouldn't use them. Make identifiers as readable and portable as possible. $ is implementation defined and thus not portable.

This language is present in the C++03 standard as well, so I don't find this to be a very convincing argument.

§2.10/2

In addition, some identifiers are reserved for use by C ++ implementations and standard libraries (17.6.4.3.2) and shall not be used otherwise; no diagnostic is required.

What change in the standard allows $ to be used as an identifier name?

Answer 1

This is implementation defined behavior, $ is not included in grammar for identifiers. The rules for identifier names in C++11 are:

1. It can not start with a number
2. Can be composed of letters, numbers, underscore, universal character names and implementation defined characters
3. Can not be a keyword

Implementation-defined characters are allowed and many compilers support as an extension, including gcc, clang, Visual Studio and as noted in a comment apparently DEC C++ compilers.

The grammar is covered in the draft C++ standard section 2.11 Indentifier, I added additional notes starting with <-:

identifier:
  identifier-nondigit            <- Can only start with a non-digit
  identifier identifier-nondigit <- Next two rules allows for subsequent 
  identifier digit               <-  characters to be those outlined in 2 above
identifier-nondigit:
  nondigit                       <- a-z, A-Z and _ 
  universal-character-name
  other implementation-defined characters
[...]

If we compile this code using clang with the -pedantic-errors flag it will not compile:

int $ = 0

and generates the following error:

error: '$' in identifier [-Werror,-Wdollar-in-identifier-extension]
int $ = 0;
    ^