error: ‘WaitSet’ is not a member of ‘rclcpp’

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Hi,

when I try to use wait_set in my code copied from this link:

#include <rclcpp/rclcpp.hpp>
rclcpp::WaitSet wait_set;
wait_set.add_subscription(subscription);
auto ret = wait_set.wait(std::chrono::seconds(10));
if (ret.kind() == rclcpp::WaitResultKind::Ready) {
   std_msgs::msg::String msg;
   rclcpp::MessageInfo info;
   auto ret_take = subscription->take(msg, info);
   if (ret_take) {
        RCLCPP_INFO(this->get_logger(), "got message: %s", msg.data.c_str());
    } else {
        RCLCPP_ERROR(this->get_logger(), "no message received");
   }
} else {
RCLCPP_ERROR(this->get_logger(), "couldn't wait for message");

}

I face a problem : error: ‘WaitSet’ is not a member of ‘rclcpp’ rclcpp::WaitSet wait_set;`

How can I solve this problem? thanks in advance.