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.
- The Key - the syntactic symbol
- The syntactic symbols appropriate as keys in this association list are:
case-label,label,access-label,member-init-intro, andinher-intro. See Syntactic Symbols. Elements with any other value as a key get ignored.- The associate value - the “ACTION” list
- The action here is simply a list containing a combination of the symbols
beforeandafter. Unlike inc-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.