c-identifier-key is a variable defined in `cc-engine.el'.
Its value is

  • Automatically becomes buffer-local when set.

Regexp matching a fully qualified identifier, like "A::B::c" in
C++. It does not recognize the full range of syntactic whitespace
between the tokens; `c-forward-name' has to be used for that. It
should also not match identifiers containing parenthesis groupings,
e.g. identifiers with template arguments such as "A" in C++.