diff --git a/rclcpp/include/rclcpp/executor.hpp b/rclcpp/include/rclcpp/executor.hpp
index 555e31e..f21830b 100644
--- a/rclcpp/include/rclcpp/executor.hpp
+++ b/rclcpp/include/rclcpp/executor.hpp
@@ -45,7 +45,9 @@ public:
   {}
   virtual ~Executor() {}
 
-  void
+  virtual void spin() = 0;
+
+  virtual void
   add_node(rclcpp::node::Node::SharedPtr &node_ptr)
   {
     this->weak_nodes_.push_back(node_ptr);