Hanging Colons
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   Using a mechanism similar to brace hanging (see *Note Hanging
Braces::), colons can also be made to hang using the variable
`c-hanging-colons-alist'.  The syntactic symbols appropriate for this
assocation list are: `case-label', `label', `access-label',
`member-init-intro', and `inher-intro'.  Note however that for
`c-hanging-colons-alist', ACTIONs as functions are not supported. See
also See Custom Brace and Colon Hanging for details.

   In C++, double-colons are used as a scope operator but because these
colons always appear right next to each other, newlines before and after
them are controlled by a different mechanism, called "clean-ups" in CC
Mode.  See Clean-ups for details.