CC Mode Manual
4.1.2 Hanging Colons
Using a mechanism similar to brace hanging (see section 4.1.1 Hanging Braces),
colons can also be made to hang using the style variable
c-hanging-colons-alist. The syntactic symbols appropriate for
this association list are:
Note however that for
c-hanging-colons-alist, ACTIONs as
functions are not supported. See also 7.5.2 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 section 4.1.5 Clean-ups, for details.