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8.2 Hanging Colons

Using a mechanism similar to brace hanging (see Hanging Braces), colons can also be made to hang using the style variable c-hanging-colons-alist: when a colon is typed, CC Mode determines its syntactic context, looks this up in the alist c-changing-colons-alist and inserts up to two newlines accordingly. Here, however, If CC Mode fails to find an entry for a syntactic symbol in the alist, no newlines are inserted around the newly typed colon.

— User Option: c-hanging-colons-alist

The Key: the syntactic symbol
The syntactic symbols appropriate as keys in this association list are: case-label, label, access-label, member-init-intro, and inher-intro. See Syntactic Symbols. Elements with any other value as a key get ignored.
The associated value: the “ACTION” list
The action here is simply a list containing a combination of the symbols before and after. Unlike in c-hanging-braces-alist, functions as actions are not supported; there doesn't seem to be any need for them.

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.