Is there a difference between double dispatch and visitor pattern?

Question

Title says it all. If it helps, the languages in question are java and c++.

Answer 1

The visitor pattern is a means of adding a new operation to existing classes. Double dispatch is a means of dispatching function calls with respect to two (or, when generalised, more) polymorphic types, rather than a single polymorphic type, which is what languages like C++ and Java support directly.