message: Handle std::pair
diff: @@ -161,6 +161,13 @@ class DLL_PUBLIC Visitor {
  }
  }
  + template<typename T, typename U>
  + void visit(const std::pair<T, U>& p) {
  + this->visit(std::get<0>(p));
  + this->visit(std::get<1>(p));
  + }
  +
  +
  virtual void visit(size_t n);
  virtual void visit(const std::string& str);
  virtual void visit(const std::u16string& str);
  