Change associated clocks storage to unordered_set (ros2#2257)

Signed-off-by: Luca Della Vedova <lucadv@intrinsic.ai>

rclcpp/src/rclcpp/time_source.cpp

@@ -14,6 +14,7 @@
 
 #include <memory>
 #include <string>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -90,7 +91,7 @@ class ClocksState final
       }
     }
     std::lock_guard<std::mutex> guard(clock_list_lock_);
-    associated_clocks_.push_back(clock);
+    associated_clocks_.insert(clock);
     // Set the clock to zero unless there's a recently received message
     set_clock(last_time_msg_, ros_time_active_, clock);
   }
@@ -99,10 +100,8 @@ class ClocksState final
   void detachClock(rclcpp::Clock::SharedPtr clock)
   {
     std::lock_guard<std::mutex> guard(clock_list_lock_);
-    auto result = std::find(associated_clocks_.begin(), associated_clocks_.end(), clock);
-    if (result != associated_clocks_.end()) {
-      associated_clocks_.erase(result);
-    } else {
+    auto removed = associated_clocks_.erase(clock);
+    if (removed == 0) {
       RCLCPP_ERROR(logger_, "failed to remove clock");
     }
   }
@@ -179,8 +178,8 @@ class ClocksState final
 
   // A lock to protect iterating the associated_clocks_ field.
   std::mutex clock_list_lock_;
-  // A vector to store references to associated clocks.
-  std::vector<rclcpp::Clock::SharedPtr> associated_clocks_;
+  // An unordered_set to store references to associated clocks.
+  std::unordered_set<rclcpp::Clock::SharedPtr> associated_clocks_;
 
   // Local storage of validity of ROS time
   // This is needed when new clocks are added.