message: Add missing member function
diff: @@ -87,6 +87,11 @@ public:
  return opts_.size();
  }
  + /// Returns the number of stored config options.
  + bool empty() const noexcept {
  + return opts_.empty();
  + }
  +
  /// Returns an iterator to the first ::config_option object.
  iterator begin() noexcept {
  return opts_.begin();
  