Promote

class Promote {
 public:
  Promote(const std::initializer_list<const char *> &syms);
  std::vector<const char *> Syms() const; // Return the symbol name involved in type promotion.
  Promote(const Promote &other) = delete;
  Promote &operator=(const Promote &other) = delete;
  Promote(Promote &&other) noexcept;
  Promote &operator=(Promote &&other) noexcept;
 private:
  std::shared_ptr<void> data_;
};