Convert subchannel_list code to C++.

5 files changed, 791 insertions, 707 deletions

diff --git a/src/core/ext/filters/client_channel/client_channel.cc b/src/core/ext/filters/client_channel/client_channel.cc
index 51f9ae000a..a10bfea8b1 100644
--- a/src/core/ext/filters/client_channel/client_channel.cc
+++ b/src/core/ext/filters/client_channel/client_channel.cc
@@ -924,7 +924,9 @@ typedef struct client_channel_call_data {
   // Note: We inline the cache for the first 3 send_message ops and use
   // dynamic allocation after that.  This number was essentially picked
   // at random; it could be changed in the future to tune performance.
-  grpc_core::InlinedVector<grpc_core::ByteStreamCache*, 3> send_messages;
+  grpc_core::ManualConstructor<
+      grpc_core::InlinedVector<grpc_core::ByteStreamCache*, 3>>
+      send_messages;
   // send_trailing_metadata
   bool seen_send_trailing_metadata;
   grpc_linked_mdelem* send_trailing_metadata_storage;
@@ -974,7 +976,7 @@ static void maybe_cache_send_ops_for_batch(call_data* calld,
             gpr_arena_alloc(calld->arena, sizeof(grpc_core::ByteStreamCache)));
     new (cache) grpc_core::ByteStreamCache(
         std::move(batch->payload->send_message.send_message));
-    calld->send_messages.push_back(cache);
+    calld->send_messages->push_back(cache);
   }
   // Save metadata batch for send_trailing_metadata ops.
   if (batch->send_trailing_metadata) {
@@ -1008,7 +1010,7 @@ static void free_cached_send_op_data_after_commit(
               "]",
               chand, calld, i);
     }
-    calld->send_messages[i]->Destroy();
+    (*calld->send_messages)[i]->Destroy();
   }
   if (retry_state->completed_send_trailing_metadata) {
     grpc_metadata_batch_destroy(&calld->send_trailing_metadata);
@@ -1032,7 +1034,7 @@ static void free_cached_send_op_data_for_completed_batch(
               "]",
               chand, calld, retry_state->completed_send_message_count - 1);
     }
-    calld->send_messages[retry_state->completed_send_message_count - 1]
+    (*calld->send_messages)[retry_state->completed_send_message_count - 1]
         ->Destroy();
   }
   if (batch_data->batch.send_trailing_metadata) {
@@ -1280,7 +1282,8 @@ static bool pending_batch_is_completed(
     return false;
   }
   if (pending->batch->send_message &&
-      retry_state->completed_send_message_count < calld->send_messages.size()) {
+      retry_state->completed_send_message_count <
+          calld->send_messages->size()) {
     return false;
   }
   if (pending->batch->send_trailing_metadata &&
@@ -1315,7 +1318,7 @@ static bool pending_batch_is_unstarted(
     return true;
   }
   if (pending->batch->send_message &&
-      retry_state->started_send_message_count < calld->send_messages.size()) {
+      retry_state->started_send_message_count < calld->send_messages->size()) {
     return true;
   }
   if (pending->batch->send_trailing_metadata &&
@@ -1817,7 +1820,7 @@ static void add_closures_for_replay_or_pending_send_ops(
   channel_data* chand = static_cast<channel_data*>(elem->channel_data);
   call_data* calld = static_cast<call_data*>(elem->call_data);
   bool have_pending_send_message_ops =
-      retry_state->started_send_message_count < calld->send_messages.size();
+      retry_state->started_send_message_count < calld->send_messages->size();
   bool have_pending_send_trailing_metadata_op =
       calld->seen_send_trailing_metadata &&
       !retry_state->started_send_trailing_metadata;
@@ -2133,7 +2136,7 @@ static void add_retriable_send_message_op(
             chand, calld, retry_state->started_send_message_count);
   }
   grpc_core::ByteStreamCache* cache =
-      calld->send_messages[retry_state->started_send_message_count];
+      (*calld->send_messages)[retry_state->started_send_message_count];
   ++retry_state->started_send_message_count;
   batch_data->send_message.Init(cache);
   batch_data->batch.send_message = true;
@@ -2254,7 +2257,7 @@ static subchannel_batch_data* maybe_create_subchannel_batch_for_replay(
   }
   // send_message.
   // Note that we can only have one send_message op in flight at a time.
-  if (retry_state->started_send_message_count < calld->send_messages.size() &&
+  if (retry_state->started_send_message_count < calld->send_messages->size() &&
       retry_state->started_send_message_count ==
           retry_state->completed_send_message_count &&
       !calld->pending_send_message) {
@@ -2274,7 +2277,7 @@ static subchannel_batch_data* maybe_create_subchannel_batch_for_replay(
   // to start, since we can't send down any more send_message ops after
   // send_trailing_metadata.
   if (calld->seen_send_trailing_metadata &&
-      retry_state->started_send_message_count == calld->send_messages.size() &&
+      retry_state->started_send_message_count == calld->send_messages->size() &&
       !retry_state->started_send_trailing_metadata &&
       !calld->pending_send_trailing_metadata) {
     if (grpc_client_channel_trace.enabled()) {
@@ -2325,7 +2328,7 @@ static void add_subchannel_batches_for_pending_batches(
     // send_message ops after send_trailing_metadata.
     if (batch->send_trailing_metadata &&
         (retry_state->started_send_message_count + batch->send_message <
-             calld->send_messages.size() ||
+             calld->send_messages->size() ||
          retry_state->started_send_trailing_metadata)) {
       continue;
     }
@@ -2976,6 +2979,7 @@ static grpc_error* cc_init_call_elem(grpc_call_element* elem,
                              calld->deadline);
   }
   calld->enable_retries = chand->enable_retries;
+  calld->send_messages.Init();
   return GRPC_ERROR_NONE;
 }
 
@@ -3011,6 +3015,7 @@ static void cc_destroy_call_elem(grpc_call_element* elem,
           calld->pick.subchannel_call_context[i].value);
     }
   }
+  calld->send_messages.Destroy();
   GRPC_CLOSURE_SCHED(then_schedule_closure, GRPC_ERROR_NONE);
 }
 
diff --git a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
index 9090c34412..b593f93d7b 100644
--- a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
@@ -62,31 +62,57 @@ class PickFirst : public LoadBalancingPolicy {
  private:
   ~PickFirst();
 
+  class PickFirstSubchannelList;
+
+  class PickFirstSubchannelData
+      : public SubchannelData<PickFirstSubchannelList,
+                              PickFirstSubchannelData> {
+   public:
+    PickFirstSubchannelData(PickFirstSubchannelList* subchannel_list,
+                            const grpc_lb_user_data_vtable* user_data_vtable,
+                            const grpc_lb_address& address,
+                            grpc_subchannel* subchannel,
+                            grpc_combiner* combiner)
+        : SubchannelData(subchannel_list, user_data_vtable, address,
+                         subchannel, combiner) {}
+
+    void ProcessConnectivityChangeLocked(grpc_error* error) override;
+  };
+
+  class PickFirstSubchannelList
+      : public SubchannelList<PickFirstSubchannelList,
+                              PickFirstSubchannelData> {
+   public:
+    PickFirstSubchannelList(
+        PickFirst* policy, TraceFlag* tracer,
+        const grpc_lb_addresses* addresses, grpc_combiner* combiner,
+        grpc_client_channel_factory* client_channel_factory,
+        const grpc_channel_args& args)
+        : SubchannelList(policy, tracer, addresses, combiner,
+                         client_channel_factory, args) {}
+
+    void RefForConnectivityWatch(const char* reason);
+    void UnrefForConnectivityWatch(const char* reason);
+  };
+
   void ShutdownLocked() override;
 
   void StartPickingLocked();
   void DestroyUnselectedSubchannelsLocked();
 
-  static void OnConnectivityChangedLocked(void* arg, grpc_error* error);
-
-  void SubchannelListRefForConnectivityWatch(
-      grpc_lb_subchannel_list* subchannel_list, const char* reason);
-  void SubchannelListUnrefForConnectivityWatch(
-      grpc_lb_subchannel_list* subchannel_list, const char* reason);
-
-  /** all our subchannels */
-  grpc_lb_subchannel_list* subchannel_list_ = nullptr;
-  /** latest pending subchannel list */
-  grpc_lb_subchannel_list* latest_pending_subchannel_list_ = nullptr;
-  /** selected subchannel in \a subchannel_list */
-  grpc_lb_subchannel_data* selected_ = nullptr;
-  /** have we started picking? */
+  // All our subchannels.
+  RefCountedPtr<PickFirstSubchannelList> subchannel_list_;
+  // Latest pending subchannel list.
+  RefCountedPtr<PickFirstSubchannelList> latest_pending_subchannel_list_;
+  // Selected subchannel in \a subchannel_list_.
+  PickFirstSubchannelData* selected_ = nullptr;
+  // Have we started picking?
   bool started_picking_ = false;
-  /** are we shut down? */
+  // Are we shut down?
   bool shutdown_ = false;
-  /** list of picks that are waiting on connectivity */
+  // List of picks that are waiting on connectivity.
   PickState* pending_picks_ = nullptr;
-  /** our connectivity state tracker */
+  // Our connectivity state tracker.
   grpc_connectivity_state_tracker state_tracker_;
 };
 
@@ -138,13 +164,12 @@ void PickFirst::ShutdownLocked() {
   grpc_connectivity_state_set(&state_tracker_, GRPC_CHANNEL_SHUTDOWN,
                               GRPC_ERROR_REF(error), "shutdown");
   if (subchannel_list_ != nullptr) {
-    grpc_lb_subchannel_list_shutdown_and_unref(subchannel_list_, "pf_shutdown");
-    subchannel_list_ = nullptr;
+    subchannel_list_->ShutdownLocked("pf_shutdown");
+    subchannel_list_.reset();
   }
   if (latest_pending_subchannel_list_ != nullptr) {
-    grpc_lb_subchannel_list_shutdown_and_unref(latest_pending_subchannel_list_,
-                                               "pf_shutdown");
-    latest_pending_subchannel_list_ = nullptr;
+    latest_pending_subchannel_list_->ShutdownLocked("pf_shutdown");
+    latest_pending_subchannel_list_.reset();
   }
   TryReresolutionLocked(&grpc_lb_pick_first_trace, GRPC_ERROR_CANCELLED);
   GRPC_ERROR_UNREF(error);
@@ -192,14 +217,12 @@ void PickFirst::CancelMatchingPicksLocked(uint32_t initial_metadata_flags_mask,
 
 void PickFirst::StartPickingLocked() {
   started_picking_ = true;
-  if (subchannel_list_ != nullptr && subchannel_list_->num_subchannels > 0) {
-    subchannel_list_->checking_subchannel = 0;
-    for (size_t i = 0; i < subchannel_list_->num_subchannels; ++i) {
-      if (subchannel_list_->subchannels[i].subchannel != nullptr) {
-        SubchannelListRefForConnectivityWatch(
-            subchannel_list_, "connectivity_watch+start_picking");
-        grpc_lb_subchannel_data_start_connectivity_watch(
-            &subchannel_list_->subchannels[i]);
+  if (subchannel_list_ != nullptr) {
+    for (size_t i = 0; i < subchannel_list_->num_subchannels(); ++i) {
+      if (subchannel_list_->subchannel(i)->subchannel() != nullptr) {
+        subchannel_list_->RefForConnectivityWatch(
+            "connectivity_watch+start_picking");
+        subchannel_list_->subchannel(i)->StartConnectivityWatchLocked();
         break;
       }
     }
@@ -215,7 +238,7 @@ void PickFirst::ExitIdleLocked() {
 bool PickFirst::PickLocked(PickState* pick) {
   // If we have a selected subchannel already, return synchronously.
   if (selected_ != nullptr) {
-    pick->connected_subchannel = selected_->connected_subchannel;
+    pick->connected_subchannel = selected_->connected_subchannel()->Ref();
     return true;
   }
   // No subchannel selected yet, so handle asynchronously.
@@ -228,11 +251,10 @@ bool PickFirst::PickLocked(PickState* pick) {
 }
 
 void PickFirst::DestroyUnselectedSubchannelsLocked() {
-  for (size_t i = 0; i < subchannel_list_->num_subchannels; ++i) {
-    grpc_lb_subchannel_data* sd = &subchannel_list_->subchannels[i];
+  for (size_t i = 0; i < subchannel_list_->num_subchannels(); ++i) {
+    PickFirstSubchannelData* sd = subchannel_list_->subchannel(i);
     if (selected_ != sd) {
-      grpc_lb_subchannel_data_unref_subchannel(sd,
-                                               "selected_different_subchannel");
+      sd->UnrefSubchannelLocked("selected_different_subchannel");
     }
   }
 }
@@ -249,7 +271,7 @@ void PickFirst::NotifyOnStateChangeLocked(grpc_connectivity_state* current,
 
 void PickFirst::PingOneLocked(grpc_closure* on_initiate, grpc_closure* on_ack) {
   if (selected_ != nullptr) {
-    selected_->connected_subchannel->Ping(on_initiate, on_ack);
+    selected_->connected_subchannel()->Ping(on_initiate, on_ack);
   } else {
     GRPC_CLOSURE_SCHED(on_initiate,
                        GRPC_ERROR_CREATE_FROM_STATIC_STRING("Not connected"));
@@ -258,24 +280,6 @@ void PickFirst::PingOneLocked(grpc_closure* on_initiate, grpc_closure* on_ack) {
   }
 }
 
-void PickFirst::SubchannelListRefForConnectivityWatch(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason) {
-  // TODO(roth): We currently track this ref manually.  Once the new
-  // ClosureRef API is ready and the subchannel_list code has been
-  // converted to a C++ API, find a way to hold the RefCountedPtr<>
-  // somewhere (maybe in the subchannel_data object) instead of doing
-  // this manually.
-  auto self = Ref(DEBUG_LOCATION, reason);
-  self.release();
-  grpc_lb_subchannel_list_ref(subchannel_list, reason);
-}
-
-void PickFirst::SubchannelListUnrefForConnectivityWatch(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason) {
-  Unref(DEBUG_LOCATION, reason);
-  grpc_lb_subchannel_list_unref(subchannel_list, reason);
-}
-
 void PickFirst::UpdateLocked(const grpc_channel_args& args) {
   const grpc_arg* arg = grpc_channel_args_find(&args, GRPC_ARG_LB_ADDRESSES);
   if (arg == nullptr || arg->type != GRPC_ARG_POINTER) {
@@ -301,10 +305,10 @@ void PickFirst::UpdateLocked(const grpc_channel_args& args) {
             "Pick First %p received update with %" PRIuPTR " addresses", this,
             addresses->num_addresses);
   }
-  grpc_lb_subchannel_list* subchannel_list = grpc_lb_subchannel_list_create(
+  auto subchannel_list = MakeRefCounted<PickFirstSubchannelList>(
       this, &grpc_lb_pick_first_trace, addresses, combiner(),
-      client_channel_factory(), args, &PickFirst::OnConnectivityChangedLocked);
-  if (subchannel_list->num_subchannels == 0) {
+      client_channel_factory(), args);
+  if (subchannel_list->num_subchannels() == 0) {
     // Empty update or no valid subchannels. Unsubscribe from all current
     // subchannels and put the channel in TRANSIENT_FAILURE.
     grpc_connectivity_state_set(
@@ -312,10 +316,9 @@ void PickFirst::UpdateLocked(const grpc_channel_args& args) {
         GRPC_ERROR_CREATE_FROM_STATIC_STRING("Empty update"),
         "pf_update_empty");
     if (subchannel_list_ != nullptr) {
-      grpc_lb_subchannel_list_shutdown_and_unref(subchannel_list_,
-                                                 "sl_shutdown_empty_update");
+      subchannel_list_->ShutdownLocked("sl_shutdown_empty_update");
     }
-    subchannel_list_ = subchannel_list;  // Empty list.
+    subchannel_list_ = std::move(subchannel_list);  // Empty list.
     selected_ = nullptr;
     return;
   }
@@ -323,45 +326,48 @@ void PickFirst::UpdateLocked(const grpc_channel_args& args) {
     // We don't yet have a selected subchannel, so replace the current
     // subchannel list immediately.
     if (subchannel_list_ != nullptr) {
-      grpc_lb_subchannel_list_shutdown_and_unref(subchannel_list_,
-                                                 "pf_update_before_selected");
+      subchannel_list_->ShutdownLocked("pf_update_before_selected");
+    }
+    subchannel_list_ = std::move(subchannel_list);
+    // If we've started picking, start trying to connect to the first
+    // subchannel in the new list.
+    if (started_picking_) {
+      subchannel_list_->RefForConnectivityWatch("connectivity_watch+update");
+      subchannel_list_->subchannel(0)->StartConnectivityWatchLocked();
     }
-    subchannel_list_ = subchannel_list;
   } else {
     // We do have a selected subchannel.
     // Check if it's present in the new list.  If so, we're done.
-    for (size_t i = 0; i < subchannel_list->num_subchannels; ++i) {
-      grpc_lb_subchannel_data* sd = &subchannel_list->subchannels[i];
-      if (sd->subchannel == selected_->subchannel) {
+    for (size_t i = 0; i < subchannel_list->num_subchannels(); ++i) {
+      PickFirstSubchannelData* sd = subchannel_list->subchannel(i);
+      if (sd->subchannel() == selected_->subchannel()) {
         // The currently selected subchannel is in the update: we are done.
         if (grpc_lb_pick_first_trace.enabled()) {
           gpr_log(GPR_INFO,
                   "Pick First %p found already selected subchannel %p "
                   "at update index %" PRIuPTR " of %" PRIuPTR "; update done",
-                  this, selected_->subchannel, i,
-                  subchannel_list->num_subchannels);
+                  this, selected_->subchannel(), i,
+                  subchannel_list->num_subchannels());
         }
-        if (selected_->connected_subchannel != nullptr) {
-          sd->connected_subchannel = selected_->connected_subchannel;
+        if (selected_->connected_subchannel() != nullptr) {
+          sd->SetConnectedSubchannelFromLocked(selected_);
         }
         selected_ = sd;
         if (subchannel_list_ != nullptr) {
-          grpc_lb_subchannel_list_shutdown_and_unref(
-              subchannel_list_, "pf_update_includes_selected");
+          subchannel_list_->ShutdownLocked("pf_update_includes_selected");
         }
-        subchannel_list_ = subchannel_list;
+        subchannel_list_ = std::move(subchannel_list);
         DestroyUnselectedSubchannelsLocked();
-        SubchannelListRefForConnectivityWatch(
-            subchannel_list, "connectivity_watch+replace_selected");
-        grpc_lb_subchannel_data_start_connectivity_watch(sd);
+        subchannel_list_->RefForConnectivityWatch(
+            "connectivity_watch+replace_selected");
+        sd->StartConnectivityWatchLocked();
         // If there was a previously pending update (which may or may
         // not have contained the currently selected subchannel), drop
         // it, so that it doesn't override what we've done here.
         if (latest_pending_subchannel_list_ != nullptr) {
-          grpc_lb_subchannel_list_shutdown_and_unref(
-              latest_pending_subchannel_list_,
+          latest_pending_subchannel_list_->ShutdownLocked(
               "pf_update_includes_selected+outdated");
-          latest_pending_subchannel_list_ = nullptr;
+          latest_pending_subchannel_list_.reset();
         }
         return;
       }
@@ -375,74 +381,89 @@ void PickFirst::UpdateLocked(const grpc_channel_args& args) {
         gpr_log(GPR_DEBUG,
                 "Pick First %p Shutting down latest pending subchannel list "
                 "%p, about to be replaced by newer latest %p",
-                this, latest_pending_subchannel_list_, subchannel_list);
+                this, latest_pending_subchannel_list_.get(),
+                subchannel_list.get());
       }
-      grpc_lb_subchannel_list_shutdown_and_unref(
-          latest_pending_subchannel_list_, "sl_outdated_dont_smash");
+      latest_pending_subchannel_list_->ShutdownLocked("sl_outdated_dont_smash");
+    }
+    latest_pending_subchannel_list_ = std::move(subchannel_list);
+    // If we've started picking, start trying to connect to the first
+    // subchannel in the new list.
+    if (started_picking_) {
+      latest_pending_subchannel_list_->RefForConnectivityWatch(
+          "connectivity_watch+update");
+      latest_pending_subchannel_list_->subchannel(0)
+          ->StartConnectivityWatchLocked();
     }
-    latest_pending_subchannel_list_ = subchannel_list;
-  }
-  // If we've started picking, start trying to connect to the first
-  // subchannel in the new list.
-  if (started_picking_) {
-    SubchannelListRefForConnectivityWatch(subchannel_list,
-                                          "connectivity_watch+update");
-    grpc_lb_subchannel_data_start_connectivity_watch(
-        &subchannel_list->subchannels[0]);
   }
 }
 
-void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
-  grpc_lb_subchannel_data* sd = static_cast<grpc_lb_subchannel_data*>(arg);
-  PickFirst* p = static_cast<PickFirst*>(sd->subchannel_list->policy);
+void PickFirst::PickFirstSubchannelList::RefForConnectivityWatch(
+    const char* reason) {
+  // TODO(roth): We currently track these refs manually.  Once the new
+  // ClosureRef API is ready, find a way to pass the RefCountedPtr<>
+  // along with the closures instead of doing this manually.
+  // Ref subchannel list.
+  Ref(DEBUG_LOCATION, reason).release();
+  // Ref LB policy.
+  PickFirst* p = static_cast<PickFirst*>(policy());
+  p->Ref(DEBUG_LOCATION, reason).release();
+}
+
+void PickFirst::PickFirstSubchannelList::UnrefForConnectivityWatch(
+    const char* reason) {
+  // Unref LB policy.
+  PickFirst* p = static_cast<PickFirst*>(policy());
+  p->Unref(DEBUG_LOCATION, reason);
+  // Unref subchannel list.
+  Unref(DEBUG_LOCATION, reason);
+}
+
+void PickFirst::PickFirstSubchannelData::ProcessConnectivityChangeLocked(
+    grpc_error* error) {
+  PickFirst* p = static_cast<PickFirst*>(subchannel_list()->policy());
   if (grpc_lb_pick_first_trace.enabled()) {
     gpr_log(GPR_DEBUG,
             "Pick First %p connectivity changed for subchannel %p (%" PRIuPTR
             " of %" PRIuPTR
             "), subchannel_list %p: state=%s p->shutdown_=%d "
             "sd->subchannel_list->shutting_down=%d error=%s",
-            p, sd->subchannel, sd->subchannel_list->checking_subchannel,
-            sd->subchannel_list->num_subchannels, sd->subchannel_list,
-            grpc_connectivity_state_name(sd->pending_connectivity_state_unsafe),
-            p->shutdown_, sd->subchannel_list->shutting_down,
+            p, subchannel(), Index(), subchannel_list()->num_subchannels(),
+            subchannel_list(),
+            grpc_connectivity_state_name(connectivity_state()),
+            p->shutdown_, subchannel_list()->shutting_down(),
             grpc_error_string(error));
   }
   // If the policy is shutting down, unref and return.
   if (p->shutdown_) {
-    grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-    grpc_lb_subchannel_data_unref_subchannel(sd, "pf_shutdown");
-    p->SubchannelListUnrefForConnectivityWatch(sd->subchannel_list,
-                                               "pf_shutdown");
+    StopConnectivityWatchLocked();
+    UnrefSubchannelLocked("pf_shutdown");
+    subchannel_list()->UnrefForConnectivityWatch("pf_shutdown");
     return;
   }
   // If the subchannel list is shutting down, stop watching.
-  if (sd->subchannel_list->shutting_down || error == GRPC_ERROR_CANCELLED) {
-    grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-    grpc_lb_subchannel_data_unref_subchannel(sd, "pf_sl_shutdown");
-    p->SubchannelListUnrefForConnectivityWatch(sd->subchannel_list,
-                                               "pf_sl_shutdown");
+  if (subchannel_list()->shutting_down() || error == GRPC_ERROR_CANCELLED) {
+    StopConnectivityWatchLocked();
+    UnrefSubchannelLocked("pf_sl_shutdown");
+    subchannel_list()->UnrefForConnectivityWatch("pf_sl_shutdown");
     return;
   }
   // If we're still here, the notification must be for a subchannel in
   // either the current or latest pending subchannel lists.
-  GPR_ASSERT(sd->subchannel_list == p->subchannel_list_ ||
-             sd->subchannel_list == p->latest_pending_subchannel_list_);
-  // Update state.
-  sd->curr_connectivity_state = sd->pending_connectivity_state_unsafe;
+  GPR_ASSERT(p->subchannel_list_ == subchannel_list() ||
+             p->latest_pending_subchannel_list_ == subchannel_list());
   // Handle updates for the currently selected subchannel.
-  if (p->selected_ == sd) {
+  if (p->selected_ == this) {
     // If the new state is anything other than READY and there is a
     // pending update, switch to the pending update.
-    if (sd->curr_connectivity_state != GRPC_CHANNEL_READY &&
+    if (connectivity_state() != GRPC_CHANNEL_READY &&
         p->latest_pending_subchannel_list_ != nullptr) {
       p->selected_ = nullptr;
-      grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-      p->SubchannelListUnrefForConnectivityWatch(
-          sd->subchannel_list, "selected_not_ready+switch_to_update");
-      grpc_lb_subchannel_list_shutdown_and_unref(
-          p->subchannel_list_, "selected_not_ready+switch_to_update");
-      p->subchannel_list_ = p->latest_pending_subchannel_list_;
-      p->latest_pending_subchannel_list_ = nullptr;
+      StopConnectivityWatchLocked();
+      subchannel_list()->UnrefForConnectivityWatch(
+          "selected_not_ready+switch_to_update");
+      subchannel_list()->ShutdownLocked("selected_not_ready+switch_to_update");
+      p->subchannel_list_ = std::move(p->latest_pending_subchannel_list_);
       grpc_connectivity_state_set(
           &p->state_tracker_, GRPC_CHANNEL_TRANSIENT_FAILURE,
           GRPC_ERROR_REF(error), "selected_not_ready+switch_to_update");
@@ -452,8 +473,8 @@ void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
       // re-resolution is introduced. But we need to investigate whether we
       // really want to take any action instead of waiting for the selected
       // subchannel reconnecting.
-      GPR_ASSERT(sd->curr_connectivity_state != GRPC_CHANNEL_SHUTDOWN);
-      if (sd->curr_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
+      GPR_ASSERT(connectivity_state() != GRPC_CHANNEL_SHUTDOWN);
+      if (connectivity_state() == GRPC_CHANNEL_TRANSIENT_FAILURE) {
         // If the selected channel goes bad, request a re-resolution.
         grpc_connectivity_state_set(&p->state_tracker_, GRPC_CHANNEL_IDLE,
                                     GRPC_ERROR_NONE,
@@ -462,17 +483,14 @@ void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
         p->TryReresolutionLocked(&grpc_lb_pick_first_trace, GRPC_ERROR_NONE);
         // In transient failure. Rely on re-resolution to recover.
         p->selected_ = nullptr;
-        grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-        p->SubchannelListUnrefForConnectivityWatch(sd->subchannel_list,
-                                                   "pf_selected_shutdown");
-        grpc_lb_subchannel_data_unref_subchannel(
-            sd, "pf_selected_shutdown");  // Unrefs connected subchannel
+        StopConnectivityWatchLocked();
+        subchannel_list()->UnrefForConnectivityWatch("pf_selected_shutdown");
+        UnrefSubchannelLocked("pf_selected_shutdown");
       } else {
-        grpc_connectivity_state_set(&p->state_tracker_,
-                                    sd->curr_connectivity_state,
+        grpc_connectivity_state_set(&p->state_tracker_, connectivity_state(),
                                     GRPC_ERROR_REF(error), "selected_changed");
         // Renew notification.
-        grpc_lb_subchannel_data_start_connectivity_watch(sd);
+        StartConnectivityWatchLocked();
       }
     }
     return;
@@ -486,26 +504,23 @@ void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
   //    for a subchannel in p->latest_pending_subchannel_list_.  The
   //    goal here is to find a subchannel from the update that we can
   //    select in place of the current one.
-  switch (sd->curr_connectivity_state) {
+  switch (connectivity_state()) {
     case GRPC_CHANNEL_READY: {
       // Case 2.  Promote p->latest_pending_subchannel_list_ to
       // p->subchannel_list_.
-      sd->connected_subchannel =
-          grpc_subchannel_get_connected_subchannel(sd->subchannel);
-      if (sd->subchannel_list == p->latest_pending_subchannel_list_) {
+      GetConnectedSubchannelFromSubchannelLocked();
+      if (p->latest_pending_subchannel_list_ == subchannel_list()) {
         GPR_ASSERT(p->subchannel_list_ != nullptr);
-        grpc_lb_subchannel_list_shutdown_and_unref(p->subchannel_list_,
-                                                   "finish_update");
-        p->subchannel_list_ = p->latest_pending_subchannel_list_;
-        p->latest_pending_subchannel_list_ = nullptr;
+        p->subchannel_list_->ShutdownLocked("finish_update");
+        p->subchannel_list_ = std::move(p->latest_pending_subchannel_list_);
       }
       // Cases 1 and 2.
       grpc_connectivity_state_set(&p->state_tracker_, GRPC_CHANNEL_READY,
                                   GRPC_ERROR_NONE, "connecting_ready");
-      p->selected_ = sd;
+      p->selected_ = this;
       if (grpc_lb_pick_first_trace.enabled()) {
         gpr_log(GPR_INFO, "Pick First %p selected subchannel %p", p,
-                sd->subchannel);
+                subchannel());
       }
       // Drop all other subchannels, since we are now connected.
       p->DestroyUnselectedSubchannelsLocked();
@@ -513,7 +528,8 @@ void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
       PickState* pick;
       while ((pick = p->pending_picks_)) {
         p->pending_picks_ = pick->next;
-        pick->connected_subchannel = p->selected_->connected_subchannel;
+        pick->connected_subchannel =
+            p->selected_->connected_subchannel()->Ref();
         if (grpc_lb_pick_first_trace.enabled()) {
           gpr_log(GPR_INFO,
                   "Servicing pending pick with selected subchannel %p",
@@ -522,40 +538,38 @@ void PickFirst::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
         GRPC_CLOSURE_SCHED(pick->on_complete, GRPC_ERROR_NONE);
       }
       // Renew notification.
-      grpc_lb_subchannel_data_start_connectivity_watch(sd);
+      StartConnectivityWatchLocked();
       break;
     }
     case GRPC_CHANNEL_TRANSIENT_FAILURE: {
-      grpc_lb_subchannel_data_stop_connectivity_watch(sd);
+      StopConnectivityWatchLocked();
+      PickFirstSubchannelData* sd = this;
       do {
-        sd->subchannel_list->checking_subchannel =
-            (sd->subchannel_list->checking_subchannel + 1) %
-            sd->subchannel_list->num_subchannels;
-        sd = &sd->subchannel_list
-                  ->subchannels[sd->subchannel_list->checking_subchannel];
-      } while (sd->subchannel == nullptr);
+        size_t next_index =
+            (sd->Index() + 1) % subchannel_list()->num_subchannels();
+        sd = subchannel_list()->subchannel(next_index);
+      } while (sd->subchannel() == nullptr);
       // Case 1: Only set state to TRANSIENT_FAILURE if we've tried
       // all subchannels.
-      if (sd->subchannel_list->checking_subchannel == 0 &&
-          sd->subchannel_list == p->subchannel_list_) {
+      if (sd->Index() == 0 && p->subchannel_list_ == subchannel_list()) {
         grpc_connectivity_state_set(
             &p->state_tracker_, GRPC_CHANNEL_TRANSIENT_FAILURE,
             GRPC_ERROR_REF(error), "connecting_transient_failure");
       }
       // Reuses the connectivity refs from the previous watch.
-      grpc_lb_subchannel_data_start_connectivity_watch(sd);
+      sd->StartConnectivityWatchLocked();
       break;
     }
     case GRPC_CHANNEL_CONNECTING:
     case GRPC_CHANNEL_IDLE: {
       // Only update connectivity state in case 1.
-      if (sd->subchannel_list == p->subchannel_list_) {
+      if (p->subchannel_list_ == subchannel_list()) {
         grpc_connectivity_state_set(&p->state_tracker_, GRPC_CHANNEL_CONNECTING,
                                     GRPC_ERROR_REF(error),
                                     "connecting_changed");
       }
       // Renew notification.
-      grpc_lb_subchannel_data_start_connectivity_watch(sd);
+      StartConnectivityWatchLocked();
       break;
     }
     case GRPC_CHANNEL_SHUTDOWN:
diff --git a/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.cc b/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.cc
index e534131c02..a9d9227ea5 100644
--- a/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.cc
@@ -73,23 +73,93 @@ class RoundRobin : public LoadBalancingPolicy {
  private:
   ~RoundRobin();
 
+  class RoundRobinSubchannelList;
+
+  class RoundRobinSubchannelData
+      : public SubchannelData<RoundRobinSubchannelList,
+                              RoundRobinSubchannelData> {
+   public:
+    RoundRobinSubchannelData(RoundRobinSubchannelList* subchannel_list,
+                             const grpc_lb_user_data_vtable* user_data_vtable,
+                             const grpc_lb_address& address,
+                             grpc_subchannel* subchannel,
+                             grpc_combiner* combiner)
+        : SubchannelData(subchannel_list, user_data_vtable, address,
+                         subchannel, combiner),
+          user_data_vtable_(user_data_vtable),
+          user_data_(user_data_vtable_ != nullptr
+                     ? user_data_vtable_->copy(address.user_data)
+                     : nullptr) {}
+
+    void ProcessConnectivityChangeLocked(grpc_error* error) override;
+
+    void UnrefSubchannelLocked(const char* reason) override {
+      SubchannelData::UnrefSubchannelLocked(reason);
+      if (user_data_ != nullptr) {
+        GPR_ASSERT(user_data_vtable_ != nullptr);
+        user_data_vtable_->destroy(user_data_);
+        user_data_ = nullptr;
+      }
+    }
+
+    void* user_data() const { return user_data_; }
+
+    grpc_connectivity_state CheckConnectivityStateLocked() override {
+      prev_connectivity_state_ = SubchannelData::CheckConnectivityStateLocked();
+      return prev_connectivity_state_;
+    }
+
+   private:
+    const grpc_lb_user_data_vtable* user_data_vtable_;
+    void* user_data_ = nullptr;
+    grpc_connectivity_state prev_connectivity_state_ = GRPC_CHANNEL_IDLE;
+  };
+
+  class RoundRobinSubchannelList
+      : public SubchannelList<RoundRobinSubchannelList,
+                              RoundRobinSubchannelData> {
+   public:
+    RoundRobinSubchannelList(
+        RoundRobin* policy, TraceFlag* tracer,
+        const grpc_lb_addresses* addresses, grpc_combiner* combiner,
+        grpc_client_channel_factory* client_channel_factory,
+        const grpc_channel_args& args)
+        : SubchannelList(policy, tracer, addresses, combiner,
+                         client_channel_factory, args),
+          num_idle_(num_subchannels()) {}
+
+    void RefForConnectivityWatch(const char* reason);
+    void UnrefForConnectivityWatch(const char* reason);
+
+    void UpdateStateCountersLocked(grpc_connectivity_state old_state,
+                                   grpc_connectivity_state new_state);
+
+    size_t num_ready() const { return num_ready_; }
+    size_t num_transient_failure() const { return num_transient_failure_; }
+    size_t num_idle() const { return num_idle_; }
+
+   private:
+    size_t num_ready_ = 0;
+    size_t num_transient_failure_ = 0;
+    size_t num_idle_;
+  };
+
   void ShutdownLocked() override;
 
   void StartPickingLocked();
   size_t GetNextReadySubchannelIndexLocked();
   void UpdateLastReadySubchannelIndexLocked(size_t last_ready_index);
-  void UpdateConnectivityStatusLocked(grpc_lb_subchannel_data* sd,
-                                      grpc_error* error);
-
-  static void OnConnectivityChangedLocked(void* arg, grpc_error* error);
-
-  void SubchannelListRefForConnectivityWatch(
-      grpc_lb_subchannel_list* subchannel_list, const char* reason);
-  void SubchannelListUnrefForConnectivityWatch(
-      grpc_lb_subchannel_list* subchannel_list, const char* reason);
+  void UpdateConnectivityStateLocked(grpc_connectivity_state state,
+                                     grpc_error* error);
 
   /** list of subchannels */
-  grpc_lb_subchannel_list* subchannel_list_ = nullptr;
+  RefCountedPtr<RoundRobinSubchannelList> subchannel_list_;
+  /** Latest version of the subchannel list.
+   * Subchannel connectivity callbacks will only promote updated subchannel
+   * lists if they equal \a latest_pending_subchannel_list. In other words,
+   * racing callbacks that reference outdated subchannel lists won't perform any
+   * update. */
+  RefCountedPtr<RoundRobinSubchannelList> latest_pending_subchannel_list_;
   /** have we started picking? */
   bool started_picking_ = false;
   /** are we shutting down? */
@@ -98,14 +168,8 @@ class RoundRobin : public LoadBalancingPolicy {
   PickState* pending_picks_ = nullptr;
   /** our connectivity state tracker */
   grpc_connectivity_state_tracker state_tracker_;
-  /** Index into subchannels for last pick. */
+  /** Index into subchannel_list_ for last pick. */
   size_t last_ready_subchannel_index_ = 0;
-  /** Latest version of the subchannel list.
-   * Subchannel connectivity callbacks will only promote updated subchannel
-   * lists if they equal \a latest_pending_subchannel_list. In other words,
-   * racing callbacks that reference outdated subchannel lists won't perform any
-   * update. */
-  grpc_lb_subchannel_list* latest_pending_subchannel_list_ = nullptr;
 };
 
 RoundRobin::RoundRobin(const Args& args) : LoadBalancingPolicy(args) {
@@ -115,7 +179,7 @@ RoundRobin::RoundRobin(const Args& args) : LoadBalancingPolicy(args) {
   UpdateLocked(*args.args);
   if (grpc_lb_round_robin_trace.enabled()) {
     gpr_log(GPR_DEBUG, "[RR %p] Created with %" PRIuPTR " subchannels", this,
-            subchannel_list_->num_subchannels);
+            subchannel_list_->num_subchannels());
   }
   grpc_subchannel_index_ref();
 }
@@ -144,30 +208,30 @@ size_t RoundRobin::GetNextReadySubchannelIndexLocked() {
             "[RR %p] getting next ready subchannel (out of %" PRIuPTR
             "), "
             "last_ready_subchannel_index=%" PRIuPTR,
-            this, subchannel_list_->num_subchannels,
+            this, subchannel_list_->num_subchannels(),
             last_ready_subchannel_index_);
   }
-  for (size_t i = 0; i < subchannel_list_->num_subchannels; ++i) {
+  for (size_t i = 0; i < subchannel_list_->num_subchannels(); ++i) {
     const size_t index = (i + last_ready_subchannel_index_ + 1) %
-                         subchannel_list_->num_subchannels;
+                         subchannel_list_->num_subchannels();
     if (grpc_lb_round_robin_trace.enabled()) {
       gpr_log(
           GPR_DEBUG,
           "[RR %p] checking subchannel %p, subchannel_list %p, index %" PRIuPTR
           ": state=%s",
-          this, subchannel_list_->subchannels[index].subchannel,
-          subchannel_list_, index,
+          this, subchannel_list_->subchannel(index)->subchannel(),
+          subchannel_list_.get(), index,
           grpc_connectivity_state_name(
-              subchannel_list_->subchannels[index].curr_connectivity_state));
+              subchannel_list_->subchannel(index)->connectivity_state()));
     }
-    if (subchannel_list_->subchannels[index].curr_connectivity_state ==
+    if (subchannel_list_->subchannel(index)->connectivity_state() ==
         GRPC_CHANNEL_READY) {
       if (grpc_lb_round_robin_trace.enabled()) {
         gpr_log(GPR_DEBUG,
                 "[RR %p] found next ready subchannel (%p) at index %" PRIuPTR
                 " of subchannel_list %p",
-                this, subchannel_list_->subchannels[index].subchannel, index,
-                subchannel_list_);
+                this, subchannel_list_->subchannel(index)->subchannel(),
+                index, subchannel_list_.get());
       }
       return index;
     }
@@ -175,21 +239,21 @@ size_t RoundRobin::GetNextReadySubchannelIndexLocked() {
   if (grpc_lb_round_robin_trace.enabled()) {
     gpr_log(GPR_DEBUG, "[RR %p] no subchannels in ready state", this);
   }
-  return subchannel_list_->num_subchannels;
+  return subchannel_list_->num_subchannels();
 }
 
 // Sets last_ready_subchannel_index_ to last_ready_index.
 void RoundRobin::UpdateLastReadySubchannelIndexLocked(size_t last_ready_index) {
-  GPR_ASSERT(last_ready_index < subchannel_list_->num_subchannels);
+  GPR_ASSERT(last_ready_index < subchannel_list_->num_subchannels());
   last_ready_subchannel_index_ = last_ready_index;
   if (grpc_lb_round_robin_trace.enabled()) {
     gpr_log(GPR_DEBUG,
             "[RR %p] setting last_ready_subchannel_index=%" PRIuPTR
             " (SC %p, CSC %p)",
             this, last_ready_index,
-            subchannel_list_->subchannels[last_ready_index].subchannel,
-            subchannel_list_->subchannels[last_ready_index]
-                .connected_subchannel.get());
+            subchannel_list_->subchannel(last_ready_index)->subchannel(),
+            subchannel_list_->subchannel(last_ready_index)
+                ->connected_subchannel());
   }
 }
 
@@ -219,14 +283,12 @@ void RoundRobin::ShutdownLocked() {
   grpc_connectivity_state_set(&state_tracker_, GRPC_CHANNEL_SHUTDOWN,
                               GRPC_ERROR_REF(error), "rr_shutdown");
   if (subchannel_list_ != nullptr) {
-    grpc_lb_subchannel_list_shutdown_and_unref(subchannel_list_,
-                                               "sl_shutdown_rr_shutdown");
-    subchannel_list_ = nullptr;
+    subchannel_list_->ShutdownLocked("rr_shutdown");
+    subchannel_list_.reset();
   }
   if (latest_pending_subchannel_list_ != nullptr) {
-    grpc_lb_subchannel_list_shutdown_and_unref(
-        latest_pending_subchannel_list_, "sl_shutdown_pending_rr_shutdown");
-    latest_pending_subchannel_list_ = nullptr;
+    latest_pending_subchannel_list_->ShutdownLocked("rr_shutdown");
+    latest_pending_subchannel_list_.reset();
   }
   TryReresolutionLocked(&grpc_lb_round_robin_trace, GRPC_ERROR_CANCELLED);
   GRPC_ERROR_UNREF(error);
@@ -273,32 +335,12 @@ void RoundRobin::CancelMatchingPicksLocked(uint32_t initial_metadata_flags_mask,
   GRPC_ERROR_UNREF(error);
 }
 
-void RoundRobin::SubchannelListRefForConnectivityWatch(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason) {
-  // TODO(roth): We currently track this ref manually.  Once the new
-  // ClosureRef API is ready and the subchannel_list code has been
-  // converted to a C++ API, find a way to hold the RefCountedPtr<>
-  // somewhere (maybe in the subchannel_data object) instead of doing
-  // this manually.
-  auto self = Ref(DEBUG_LOCATION, reason);
-  self.release();
-  grpc_lb_subchannel_list_ref(subchannel_list, reason);
-}
-
-void RoundRobin::SubchannelListUnrefForConnectivityWatch(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason) {
-  Unref(DEBUG_LOCATION, reason);
-  grpc_lb_subchannel_list_unref(subchannel_list, reason);
-}
-
 void RoundRobin::StartPickingLocked() {
   started_picking_ = true;
-  for (size_t i = 0; i < subchannel_list_->num_subchannels; i++) {
-    if (subchannel_list_->subchannels[i].subchannel != nullptr) {
-      SubchannelListRefForConnectivityWatch(subchannel_list_,
-                                            "connectivity_watch");
-      grpc_lb_subchannel_data_start_connectivity_watch(
-          &subchannel_list_->subchannels[i]);
+  for (size_t i = 0; i < subchannel_list_->num_subchannels(); i++) {
+    if (subchannel_list_->subchannel(i)->subchannel() != nullptr) {
+      subchannel_list_->RefForConnectivityWatch("connectivity_watch");
+      subchannel_list_->subchannel(i)->StartConnectivityWatchLocked();
     }
   }
 }
@@ -317,21 +359,21 @@ bool RoundRobin::PickLocked(PickState* pick) {
   GPR_ASSERT(!shutdown_);
   if (subchannel_list_ != nullptr) {
     const size_t next_ready_index = GetNextReadySubchannelIndexLocked();
-    if (next_ready_index < subchannel_list_->num_subchannels) {
+    if (next_ready_index < subchannel_list_->num_subchannels()) {
       /* readily available, report right away */
-      grpc_lb_subchannel_data* sd =
-          &subchannel_list_->subchannels[next_ready_index];
-      pick->connected_subchannel = sd->connected_subchannel;
+      RoundRobinSubchannelData* sd =
+          subchannel_list_->subchannel(next_ready_index);
+      pick->connected_subchannel = sd->connected_subchannel()->Ref();
       if (pick->user_data != nullptr) {
-        *pick->user_data = sd->user_data;
+        *pick->user_data = sd->user_data();
       }
       if (grpc_lb_round_robin_trace.enabled()) {
         gpr_log(
             GPR_DEBUG,
             "[RR %p] Picked target <-- Subchannel %p (connected %p) (sl %p, "
             "index %" PRIuPTR ")",
-            this, sd->subchannel, pick->connected_subchannel.get(),
-            sd->subchannel_list, next_ready_index);
+            this, sd->subchannel(), pick->connected_subchannel.get(),
+            sd->subchannel_list(), next_ready_index);
       }
       /* only advance the last picked pointer if the selection was used */
       UpdateLastReadySubchannelIndexLocked(next_ready_index);
@@ -347,36 +389,12 @@ bool RoundRobin::PickLocked(PickState* pick) {
   return false;
 }
 
-void UpdateStateCountersLocked(grpc_lb_subchannel_data* sd) {
-  grpc_lb_subchannel_list* subchannel_list = sd->subchannel_list;
-  GPR_ASSERT(sd->prev_connectivity_state != GRPC_CHANNEL_SHUTDOWN);
-  GPR_ASSERT(sd->curr_connectivity_state != GRPC_CHANNEL_SHUTDOWN);
-  if (sd->prev_connectivity_state == GRPC_CHANNEL_READY) {
-    GPR_ASSERT(subchannel_list->num_ready > 0);
-    --subchannel_list->num_ready;
-  } else if (sd->prev_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
-    GPR_ASSERT(subchannel_list->num_transient_failures > 0);
-    --subchannel_list->num_transient_failures;
-  } else if (sd->prev_connectivity_state == GRPC_CHANNEL_IDLE) {
-    GPR_ASSERT(subchannel_list->num_idle > 0);
-    --subchannel_list->num_idle;
-  }
-  sd->prev_connectivity_state = sd->curr_connectivity_state;
-  if (sd->curr_connectivity_state == GRPC_CHANNEL_READY) {
-    ++subchannel_list->num_ready;
-  } else if (sd->curr_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
-    ++subchannel_list->num_transient_failures;
-  } else if (sd->curr_connectivity_state == GRPC_CHANNEL_IDLE) {
-    ++subchannel_list->num_idle;
-  }
-}
-
 /** Sets the policy's connectivity status based on that of the passed-in \a sd
  * (the grpc_lb_subchannel_data associated with the updated subchannel) and the
  * subchannel list \a sd belongs to (sd->subchannel_list). \a error will be used
  * only if the policy transitions to state TRANSIENT_FAILURE. */
-void RoundRobin::UpdateConnectivityStatusLocked(grpc_lb_subchannel_data* sd,
-                                                grpc_error* error) {
+void RoundRobin::UpdateConnectivityStateLocked(grpc_connectivity_state state,
+                                               grpc_error* error) {
   /* In priority order. The first rule to match terminates the search (ie, if we
    * are on rule n, all previous rules were unfulfilled).
    *
@@ -391,18 +409,16 @@ void RoundRobin::UpdateConnectivityStatusLocked(grpc_lb_subchannel_data* sd,
    *    CHECK: subchannel_list->num_transient_failures ==
    *           subchannel_list->num_subchannels.
    */
-  grpc_lb_subchannel_list* subchannel_list = sd->subchannel_list;
-  GPR_ASSERT(sd->curr_connectivity_state != GRPC_CHANNEL_IDLE);
-  if (subchannel_list->num_ready > 0) {
+  if (subchannel_list_->num_ready() > 0) {
     /* 1) READY */
     grpc_connectivity_state_set(&state_tracker_, GRPC_CHANNEL_READY,
                                 GRPC_ERROR_NONE, "rr_ready");
-  } else if (sd->curr_connectivity_state == GRPC_CHANNEL_CONNECTING) {
+  } else if (state == GRPC_CHANNEL_CONNECTING) {
     /* 2) CONNECTING */
     grpc_connectivity_state_set(&state_tracker_, GRPC_CHANNEL_CONNECTING,
                                 GRPC_ERROR_NONE, "rr_connecting");
-  } else if (subchannel_list->num_transient_failures ==
-             subchannel_list->num_subchannels) {
+  } else if (subchannel_list_->num_transient_failure() ==
+             subchannel_list_->num_subchannels()) {
     /* 3) TRANSIENT_FAILURE */
     grpc_connectivity_state_set(&state_tracker_, GRPC_CHANNEL_TRANSIENT_FAILURE,
                                 GRPC_ERROR_REF(error),
@@ -411,99 +427,134 @@ void RoundRobin::UpdateConnectivityStatusLocked(grpc_lb_subchannel_data* sd,
   GRPC_ERROR_UNREF(error);
 }
 
-void RoundRobin::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
-  grpc_lb_subchannel_data* sd = static_cast<grpc_lb_subchannel_data*>(arg);
-  RoundRobin* p = static_cast<RoundRobin*>(sd->subchannel_list->policy);
+void RoundRobin::RoundRobinSubchannelList::RefForConnectivityWatch(
+    const char* reason) {
+  // TODO(roth): We currently track these refs manually.  Once the new
+  // ClosureRef API is ready, find a way to pass the RefCountedPtr<>
+  // along with the closures instead of doing this manually.
+  // Ref subchannel list.
+  Ref(DEBUG_LOCATION, reason).release();
+  // Ref LB policy.
+  RoundRobin* p = static_cast<RoundRobin*>(policy());
+  p->Ref(DEBUG_LOCATION, reason).release();
+}
+
+void RoundRobin::RoundRobinSubchannelList::UnrefForConnectivityWatch(
+    const char* reason) {
+  // Unref LB policy.
+  RoundRobin* p = static_cast<RoundRobin*>(policy());
+  p->Unref(DEBUG_LOCATION, reason);
+  // Unref subchannel list.
+  Unref(DEBUG_LOCATION, reason);
+}
+
+void RoundRobin::RoundRobinSubchannelList::UpdateStateCountersLocked(
+    grpc_connectivity_state old_state, grpc_connectivity_state new_state) {
+  GPR_ASSERT(old_state != GRPC_CHANNEL_SHUTDOWN);
+  GPR_ASSERT(new_state != GRPC_CHANNEL_SHUTDOWN);
+  if (old_state == GRPC_CHANNEL_READY) {
+    GPR_ASSERT(num_ready_ > 0);
+    --num_ready_;
+  } else if (old_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
+    GPR_ASSERT(num_transient_failure_ > 0);
+    --num_transient_failure_;
+  } else if (old_state == GRPC_CHANNEL_IDLE) {
+    GPR_ASSERT(num_idle_ > 0);
+    --num_idle_;
+  }
+  if (new_state == GRPC_CHANNEL_READY) {
+    ++num_ready_;
+  } else if (new_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
+    ++num_transient_failure_;
+  } else if (new_state == GRPC_CHANNEL_IDLE) {
+    ++num_idle_;
+  }
+}
+
+void RoundRobin::RoundRobinSubchannelData::ProcessConnectivityChangeLocked(
+    grpc_error* error) {
+  RoundRobin* p = static_cast<RoundRobin*>(subchannel_list()->policy());
   if (grpc_lb_round_robin_trace.enabled()) {
     gpr_log(
         GPR_DEBUG,
         "[RR %p] connectivity changed for subchannel %p, subchannel_list %p: "
         "prev_state=%s new_state=%s p->shutdown=%d "
         "sd->subchannel_list->shutting_down=%d error=%s",
-        p, sd->subchannel, sd->subchannel_list,
-        grpc_connectivity_state_name(sd->prev_connectivity_state),
-        grpc_connectivity_state_name(sd->pending_connectivity_state_unsafe),
-        p->shutdown_, sd->subchannel_list->shutting_down,
+        p, subchannel(), subchannel_list(),
+        grpc_connectivity_state_name(prev_connectivity_state_),
+        grpc_connectivity_state_name(connectivity_state()),
+        p->shutdown_, subchannel_list()->shutting_down(),
         grpc_error_string(error));
   }
-  GPR_ASSERT(sd->subchannel != nullptr);
+  GPR_ASSERT(subchannel() != nullptr);
   // If the policy is shutting down, unref and return.
   if (p->shutdown_) {
-    grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-    grpc_lb_subchannel_data_unref_subchannel(sd, "rr_shutdown");
-    p->SubchannelListUnrefForConnectivityWatch(sd->subchannel_list,
-                                               "rr_shutdown");
+    StopConnectivityWatchLocked();
+    UnrefSubchannelLocked("rr_shutdown");
+    subchannel_list()->UnrefForConnectivityWatch("rr_shutdown");
     return;
   }
   // If the subchannel list is shutting down, stop watching.
-  if (sd->subchannel_list->shutting_down || error == GRPC_ERROR_CANCELLED) {
-    grpc_lb_subchannel_data_stop_connectivity_watch(sd);
-    grpc_lb_subchannel_data_unref_subchannel(sd, "rr_sl_shutdown");
-    p->SubchannelListUnrefForConnectivityWatch(sd->subchannel_list,
-                                               "rr_sl_shutdown");
+  if (subchannel_list()->shutting_down() || error == GRPC_ERROR_CANCELLED) {
+    StopConnectivityWatchLocked();
+    UnrefSubchannelLocked("rr_sl_shutdown");
+    subchannel_list()->UnrefForConnectivityWatch("rr_sl_shutdown");
     return;
   }
+  GPR_ASSERT(connectivity_state() != GRPC_CHANNEL_SHUTDOWN);
   // If we're still here, the notification must be for a subchannel in
   // either the current or latest pending subchannel lists.
-  GPR_ASSERT(sd->subchannel_list == p->subchannel_list_ ||
-             sd->subchannel_list == p->latest_pending_subchannel_list_);
-  GPR_ASSERT(sd->pending_connectivity_state_unsafe != GRPC_CHANNEL_SHUTDOWN);
-  // Now that we're inside the combiner, copy the pending connectivity
-  // state (which was set by the connectivity state watcher) to
-  // curr_connectivity_state, which is what we use inside of the combiner.
-  sd->curr_connectivity_state = sd->pending_connectivity_state_unsafe;
+  GPR_ASSERT(p->subchannel_list_ == subchannel_list() ||
+             p->latest_pending_subchannel_list_ == subchannel_list());
   // If the sd's new state is TRANSIENT_FAILURE, unref the *connected*
   // subchannel, if any.
-  switch (sd->curr_connectivity_state) {
+  switch (connectivity_state()) {
     case GRPC_CHANNEL_TRANSIENT_FAILURE: {
-      sd->connected_subchannel.reset();
+      clear_connected_subchannel();
       if (grpc_lb_round_robin_trace.enabled()) {
         gpr_log(GPR_DEBUG,
                 "[RR %p] Subchannel %p has gone into TRANSIENT_FAILURE. "
                 "Requesting re-resolution",
-                p, sd->subchannel);
+                p, subchannel());
       }
       p->TryReresolutionLocked(&grpc_lb_round_robin_trace, GRPC_ERROR_NONE);
       break;
     }
     case GRPC_CHANNEL_READY: {
-      if (sd->connected_subchannel == nullptr) {
-        sd->connected_subchannel =
-            grpc_subchannel_get_connected_subchannel(sd->subchannel);
+      if (connected_subchannel() == nullptr) {
+        GetConnectedSubchannelFromSubchannelLocked();
       }
-      if (sd->subchannel_list != p->subchannel_list_) {
-        // promote sd->subchannel_list to p->subchannel_list_.
-        // sd->subchannel_list must be equal to
+      if (p->subchannel_list_ != subchannel_list()) {
+        // promote subchannel_list() to p->subchannel_list_.
+        // subchannel_list() must be equal to
         // p->latest_pending_subchannel_list_ because we have already filtered
-        // for sds belonging to outdated subchannel lists.
-        GPR_ASSERT(sd->subchannel_list == p->latest_pending_subchannel_list_);
-        GPR_ASSERT(!sd->subchannel_list->shutting_down);
+        // for subchannels belonging to outdated subchannel lists.
+        GPR_ASSERT(p->latest_pending_subchannel_list_ == subchannel_list());
+        GPR_ASSERT(!subchannel_list()->shutting_down());
         if (grpc_lb_round_robin_trace.enabled()) {
           const size_t num_subchannels =
               p->subchannel_list_ != nullptr
-                  ? p->subchannel_list_->num_subchannels
+                  ? p->subchannel_list_->num_subchannels()
                   : 0;
           gpr_log(GPR_DEBUG,
                   "[RR %p] phasing out subchannel list %p (size %" PRIuPTR
                   ") in favor of %p (size %" PRIuPTR ")",
-                  p, p->subchannel_list_, num_subchannels, sd->subchannel_list,
-                  num_subchannels);
+                  p, p->subchannel_list_.get(), num_subchannels,
+                  subchannel_list(), subchannel_list()->num_subchannels());
         }
         if (p->subchannel_list_ != nullptr) {
           // dispose of the current subchannel_list
-          grpc_lb_subchannel_list_shutdown_and_unref(p->subchannel_list_,
-                                                     "sl_phase_out_shutdown");
+          p->subchannel_list_->ShutdownLocked("sl_phase_out_shutdown");
         }
-        p->subchannel_list_ = p->latest_pending_subchannel_list_;
-        p->latest_pending_subchannel_list_ = nullptr;
+        p->subchannel_list_ = std::move(p->latest_pending_subchannel_list_);
       }
       /* at this point we know there's at least one suitable subchannel. Go
        * ahead and pick one and notify the pending suitors in
        * p->pending_picks. This preemptively replicates rr_pick()'s actions. */
       const size_t next_ready_index = p->GetNextReadySubchannelIndexLocked();
-      GPR_ASSERT(next_ready_index < p->subchannel_list_->num_subchannels);
-      grpc_lb_subchannel_data* selected =
-          &p->subchannel_list_->subchannels[next_ready_index];
+      GPR_ASSERT(next_ready_index < p->subchannel_list_->num_subchannels());
+      RoundRobinSubchannelData* selected =
+          p->subchannel_list_->subchannel(next_ready_index);
       if (p->pending_picks_ != nullptr) {
         // if the selected subchannel is going to be used for the pending
         // picks, update the last picked pointer
@@ -512,15 +563,15 @@ void RoundRobin::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
       PickState* pick;
       while ((pick = p->pending_picks_)) {
         p->pending_picks_ = pick->next;
-        pick->connected_subchannel = selected->connected_subchannel;
+        pick->connected_subchannel = selected->connected_subchannel()->Ref();
         if (pick->user_data != nullptr) {
-          *pick->user_data = selected->user_data;
+          *pick->user_data = selected->user_data();
         }
         if (grpc_lb_round_robin_trace.enabled()) {
           gpr_log(GPR_DEBUG,
                   "[RR %p] Fulfilling pending pick. Target <-- subchannel %p "
                   "(subchannel_list %p, index %" PRIuPTR ")",
-                  p, selected->subchannel, p->subchannel_list_,
+                  p, selected->subchannel(), p->subchannel_list_.get(),
                   next_ready_index);
         }
         GRPC_CLOSURE_SCHED(pick->on_complete, GRPC_ERROR_NONE);
@@ -533,13 +584,16 @@ void RoundRobin::OnConnectivityChangedLocked(void* arg, grpc_error* error) {
     case GRPC_CHANNEL_IDLE:;  // fallthrough
   }
   // Update state counters.
-  UpdateStateCountersLocked(sd);
+  subchannel_list()->UpdateStateCountersLocked(prev_connectivity_state_,
+                                               connectivity_state());
+  prev_connectivity_state_ = connectivity_state();
   // Only update connectivity based on the selected subchannel list.
-  if (sd->subchannel_list == p->subchannel_list_) {
-    p->UpdateConnectivityStatusLocked(sd, GRPC_ERROR_REF(error));
+  if (p->subchannel_list_ == subchannel_list()) {
+    p->UpdateConnectivityStateLocked(connectivity_state(),
+                                     GRPC_ERROR_REF(error));
   }
   // Renew notification.
-  grpc_lb_subchannel_data_start_connectivity_watch(sd);
+  StartConnectivityWatchLocked();
 }
 
 grpc_connectivity_state RoundRobin::CheckConnectivityLocked(
@@ -556,10 +610,10 @@ void RoundRobin::NotifyOnStateChangeLocked(grpc_connectivity_state* current,
 void RoundRobin::PingOneLocked(grpc_closure* on_initiate,
                                grpc_closure* on_ack) {
   const size_t next_ready_index = GetNextReadySubchannelIndexLocked();
-  if (next_ready_index < subchannel_list_->num_subchannels) {
-    grpc_lb_subchannel_data* selected =
-        &subchannel_list_->subchannels[next_ready_index];
-    selected->connected_subchannel->Ping(on_initiate, on_ack);
+  if (next_ready_index < subchannel_list_->num_subchannels()) {
+    RoundRobinSubchannelData* selected =
+        subchannel_list_->subchannel(next_ready_index);
+    selected->connected_subchannel()->Ping(on_initiate, on_ack);
   } else {
     GRPC_CLOSURE_SCHED(on_initiate, GRPC_ERROR_CREATE_FROM_STATIC_STRING(
                                         "Round Robin not connected"));
@@ -587,30 +641,29 @@ void RoundRobin::UpdateLocked(const grpc_channel_args& args) {
     gpr_log(GPR_DEBUG, "[RR %p] received update with %" PRIuPTR " addresses",
             this, addresses->num_addresses);
   }
-  grpc_lb_subchannel_list* subchannel_list = grpc_lb_subchannel_list_create(
+  auto subchannel_list = MakeRefCounted<RoundRobinSubchannelList>(
       this, &grpc_lb_round_robin_trace, addresses, combiner(),
-      client_channel_factory(), args, &RoundRobin::OnConnectivityChangedLocked);
-  if (subchannel_list->num_subchannels == 0) {
+      client_channel_factory(), args);
+  if (subchannel_list->num_subchannels() == 0) {
     grpc_connectivity_state_set(
         &state_tracker_, GRPC_CHANNEL_TRANSIENT_FAILURE,
         GRPC_ERROR_CREATE_FROM_STATIC_STRING("Empty update"),
         "rr_update_empty");
     if (subchannel_list_ != nullptr) {
-      grpc_lb_subchannel_list_shutdown_and_unref(subchannel_list_,
-                                                 "sl_shutdown_empty_update");
+      subchannel_list_->ShutdownLocked("sl_shutdown_empty_update");
     }
-    subchannel_list_ = subchannel_list;  // empty list
+    subchannel_list_ = std::move(subchannel_list);  // empty list
     return;
   }
   if (started_picking_) {
-    for (size_t i = 0; i < subchannel_list->num_subchannels; ++i) {
+    for (size_t i = 0; i < subchannel_list->num_subchannels(); ++i) {
       const grpc_connectivity_state subchannel_state =
-          grpc_subchannel_check_connectivity(
-              subchannel_list->subchannels[i].subchannel, nullptr);
+          subchannel_list->subchannel(i)->CheckConnectivityStateLocked();
       // Override the default setting of IDLE for connectivity notification
       // purposes if the subchannel is already in transient failure. Otherwise
       // we'd be immediately notified of the IDLE-TRANSIENT_FAILURE
-      // discrepancy, attempt to re-resolve and end up here again.
+      // discrepancy, attempt to re-resolve, and end up here again.
+// FIXME
       // TODO(roth): As part of C++-ifying the subchannel_list API, design a
       // better API for notifying the LB policy of subchannel states, which can
       // be used both for the subchannel's initial state and for subsequent
@@ -619,43 +672,36 @@ void RoundRobin::UpdateLocked(const grpc_channel_args& args) {
       // pending picks across all READY subchannels rather than sending them all
       // to the first one).
       if (subchannel_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
-        subchannel_list->subchannels[i].pending_connectivity_state_unsafe =
-            subchannel_list->subchannels[i].curr_connectivity_state =
-                subchannel_list->subchannels[i].prev_connectivity_state =
-                    subchannel_state;
-        --subchannel_list->num_idle;
-        ++subchannel_list->num_transient_failures;
+        subchannel_list->UpdateStateCountersLocked(GRPC_CHANNEL_IDLE,
+                                                   subchannel_state);
       }
     }
+    for (size_t i = 0; i < subchannel_list->num_subchannels(); ++i) {
+      /* Watch every new subchannel. A subchannel list becomes active the
+       * moment one of its subchannels is READY. At that moment, we swap
+       * p->subchannel_list for sd->subchannel_list, provided the subchannel
+       * list is still valid (ie, isn't shutting down) */
+      subchannel_list->RefForConnectivityWatch("connectivity_watch");
+      subchannel_list->subchannel(i)->StartConnectivityWatchLocked();
+    }
     if (latest_pending_subchannel_list_ != nullptr) {
       if (grpc_lb_round_robin_trace.enabled()) {
         gpr_log(GPR_DEBUG,
                 "[RR %p] Shutting down latest pending subchannel list %p, "
                 "about to be replaced by newer latest %p",
-                this, latest_pending_subchannel_list_, subchannel_list);
+                this, latest_pending_subchannel_list_.get(),
+                subchannel_list.get());
       }
-      grpc_lb_subchannel_list_shutdown_and_unref(
-          latest_pending_subchannel_list_, "sl_outdated");
-    }
-    latest_pending_subchannel_list_ = subchannel_list;
-    for (size_t i = 0; i < subchannel_list->num_subchannels; ++i) {
-      /* Watch every new subchannel. A subchannel list becomes active the
-       * moment one of its subchannels is READY. At that moment, we swap
-       * p->subchannel_list for sd->subchannel_list, provided the subchannel
-       * list is still valid (ie, isn't shutting down) */
-      SubchannelListRefForConnectivityWatch(subchannel_list,
-                                            "connectivity_watch");
-      grpc_lb_subchannel_data_start_connectivity_watch(
-          &subchannel_list->subchannels[i]);
+      latest_pending_subchannel_list_->ShutdownLocked("sl_outdated");
     }
+    latest_pending_subchannel_list_ = std::move(subchannel_list);
   } else {
     // The policy isn't picking yet. Save the update for later, disposing of
     // previous version if any.
     if (subchannel_list_ != nullptr) {
-      grpc_lb_subchannel_list_shutdown_and_unref(
-          subchannel_list_, "rr_update_before_started_picking");
+      subchannel_list_->ShutdownLocked("rr_update_before_started_picking");
     }
-    subchannel_list_ = subchannel_list;
+    subchannel_list_ = std::move(subchannel_list);
   }
 }
 
diff --git a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.cc b/src/core/ext/filters/client_channel/lb_policy/subchannel_list.cc
deleted file mode 100644
index 79cb64c6c6..0000000000
--- a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.cc
+++ /dev/null
@@ -1,253 +0,0 @@
-/*
- *
- * Copyright 2015 gRPC authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- *
- */
-
-#include <grpc/support/port_platform.h>
-
-#include <string.h>
-
-#include <grpc/support/alloc.h>
-
-#include "src/core/ext/filters/client_channel/lb_policy/subchannel_list.h"
-#include "src/core/lib/channel/channel_args.h"
-#include "src/core/lib/debug/trace.h"
-#include "src/core/lib/iomgr/closure.h"
-#include "src/core/lib/iomgr/combiner.h"
-#include "src/core/lib/iomgr/sockaddr_utils.h"
-#include "src/core/lib/transport/connectivity_state.h"
-
-void grpc_lb_subchannel_data_unref_subchannel(grpc_lb_subchannel_data* sd,
-                                              const char* reason) {
-  if (sd->subchannel != nullptr) {
-    if (sd->subchannel_list->tracer->enabled()) {
-      gpr_log(GPR_DEBUG,
-              "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
-              " (subchannel %p): unreffing subchannel",
-              sd->subchannel_list->tracer->name(), sd->subchannel_list->policy,
-              sd->subchannel_list,
-              static_cast<size_t>(sd - sd->subchannel_list->subchannels),
-              sd->subchannel_list->num_subchannels, sd->subchannel);
-    }
-    GRPC_SUBCHANNEL_UNREF(sd->subchannel, reason);
-    sd->subchannel = nullptr;
-    sd->connected_subchannel.reset();
-    if (sd->user_data != nullptr) {
-      GPR_ASSERT(sd->user_data_vtable != nullptr);
-      sd->user_data_vtable->destroy(sd->user_data);
-      sd->user_data = nullptr;
-    }
-  }
-}
-
-void grpc_lb_subchannel_data_start_connectivity_watch(
-    grpc_lb_subchannel_data* sd) {
-  if (sd->subchannel_list->tracer->enabled()) {
-    gpr_log(
-        GPR_DEBUG,
-        "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
-        " (subchannel %p): requesting connectivity change "
-        "notification (from %s)",
-        sd->subchannel_list->tracer->name(), sd->subchannel_list->policy,
-        sd->subchannel_list,
-        static_cast<size_t>(sd - sd->subchannel_list->subchannels),
-        sd->subchannel_list->num_subchannels, sd->subchannel,
-        grpc_connectivity_state_name(sd->pending_connectivity_state_unsafe));
-  }
-  sd->connectivity_notification_pending = true;
-  grpc_subchannel_notify_on_state_change(
-      sd->subchannel, sd->subchannel_list->policy->interested_parties(),
-      &sd->pending_connectivity_state_unsafe,
-      &sd->connectivity_changed_closure);
-}
-
-void grpc_lb_subchannel_data_stop_connectivity_watch(
-    grpc_lb_subchannel_data* sd) {
-  if (sd->subchannel_list->tracer->enabled()) {
-    gpr_log(GPR_DEBUG,
-            "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
-            " (subchannel %p): stopping connectivity watch",
-            sd->subchannel_list->tracer->name(), sd->subchannel_list->policy,
-            sd->subchannel_list,
-            static_cast<size_t>(sd - sd->subchannel_list->subchannels),
-            sd->subchannel_list->num_subchannels, sd->subchannel);
-  }
-  GPR_ASSERT(sd->connectivity_notification_pending);
-  sd->connectivity_notification_pending = false;
-}
-
-grpc_lb_subchannel_list* grpc_lb_subchannel_list_create(
-    grpc_core::LoadBalancingPolicy* p, grpc_core::TraceFlag* tracer,
-    const grpc_lb_addresses* addresses, grpc_combiner* combiner,
-    grpc_client_channel_factory* client_channel_factory,
-    const grpc_channel_args& args, grpc_iomgr_cb_func connectivity_changed_cb) {
-  grpc_lb_subchannel_list* subchannel_list =
-      static_cast<grpc_lb_subchannel_list*>(
-          gpr_zalloc(sizeof(*subchannel_list)));
-  if (tracer->enabled()) {
-    gpr_log(GPR_DEBUG,
-            "[%s %p] Creating subchannel list %p for %" PRIuPTR " subchannels",
-            tracer->name(), p, subchannel_list, addresses->num_addresses);
-  }
-  subchannel_list->policy = p;
-  subchannel_list->tracer = tracer;
-  gpr_ref_init(&subchannel_list->refcount, 1);
-  subchannel_list->subchannels = static_cast<grpc_lb_subchannel_data*>(
-      gpr_zalloc(sizeof(grpc_lb_subchannel_data) * addresses->num_addresses));
-  // We need to remove the LB addresses in order to be able to compare the
-  // subchannel keys of subchannels from a different batch of addresses.
-  static const char* keys_to_remove[] = {GRPC_ARG_SUBCHANNEL_ADDRESS,
-                                         GRPC_ARG_LB_ADDRESSES};
-  // Create a subchannel for each address.
-  grpc_subchannel_args sc_args;
-  size_t subchannel_index = 0;
-  for (size_t i = 0; i < addresses->num_addresses; i++) {
-    // If there were any balancer, we would have chosen grpclb policy instead.
-    GPR_ASSERT(!addresses->addresses[i].is_balancer);
-    memset(&sc_args, 0, sizeof(grpc_subchannel_args));
-    grpc_arg addr_arg =
-        grpc_create_subchannel_address_arg(&addresses->addresses[i].address);
-    grpc_channel_args* new_args = grpc_channel_args_copy_and_add_and_remove(
-        &args, keys_to_remove, GPR_ARRAY_SIZE(keys_to_remove), &addr_arg, 1);
-    gpr_free(addr_arg.value.string);
-    sc_args.args = new_args;
-    grpc_subchannel* subchannel = grpc_client_channel_factory_create_subchannel(
-        client_channel_factory, &sc_args);
-    grpc_channel_args_destroy(new_args);
-    if (subchannel == nullptr) {
-      // Subchannel could not be created.
-      if (tracer->enabled()) {
-        char* address_uri =
-            grpc_sockaddr_to_uri(&addresses->addresses[i].address);
-        gpr_log(GPR_DEBUG,
-                "[%s %p] could not create subchannel for address uri %s, "
-                "ignoring",
-                tracer->name(), subchannel_list->policy, address_uri);
-        gpr_free(address_uri);
-      }
-      continue;
-    }
-    if (tracer->enabled()) {
-      char* address_uri =
-          grpc_sockaddr_to_uri(&addresses->addresses[i].address);
-      gpr_log(GPR_DEBUG,
-              "[%s %p] subchannel list %p index %" PRIuPTR
-              ": Created subchannel %p for address uri %s",
-              tracer->name(), p, subchannel_list, subchannel_index, subchannel,
-              address_uri);
-      gpr_free(address_uri);
-    }
-    grpc_lb_subchannel_data* sd =
-        &subchannel_list->subchannels[subchannel_index++];
-    sd->subchannel_list = subchannel_list;
-    sd->subchannel = subchannel;
-    GRPC_CLOSURE_INIT(&sd->connectivity_changed_closure,
-                      connectivity_changed_cb, sd,
-                      grpc_combiner_scheduler(combiner));
-    // We assume that the current state is IDLE.  If not, we'll get a
-    // callback telling us that.
-    sd->prev_connectivity_state = GRPC_CHANNEL_IDLE;
-    sd->curr_connectivity_state = GRPC_CHANNEL_IDLE;
-    sd->pending_connectivity_state_unsafe = GRPC_CHANNEL_IDLE;
-    sd->user_data_vtable = addresses->user_data_vtable;
-    if (sd->user_data_vtable != nullptr) {
-      sd->user_data =
-          sd->user_data_vtable->copy(addresses->addresses[i].user_data);
-    }
-  }
-  subchannel_list->num_subchannels = subchannel_index;
-  subchannel_list->num_idle = subchannel_index;
-  return subchannel_list;
-}
-
-static void subchannel_list_destroy(grpc_lb_subchannel_list* subchannel_list) {
-  if (subchannel_list->tracer->enabled()) {
-    gpr_log(GPR_DEBUG, "[%s %p] Destroying subchannel_list %p",
-            subchannel_list->tracer->name(), subchannel_list->policy,
-            subchannel_list);
-  }
-  for (size_t i = 0; i < subchannel_list->num_subchannels; i++) {
-    grpc_lb_subchannel_data* sd = &subchannel_list->subchannels[i];
-    grpc_lb_subchannel_data_unref_subchannel(sd, "subchannel_list_destroy");
-  }
-  gpr_free(subchannel_list->subchannels);
-  gpr_free(subchannel_list);
-}
-
-void grpc_lb_subchannel_list_ref(grpc_lb_subchannel_list* subchannel_list,
-                                 const char* reason) {
-  gpr_ref_non_zero(&subchannel_list->refcount);
-  if (subchannel_list->tracer->enabled()) {
-    const gpr_atm count = gpr_atm_acq_load(&subchannel_list->refcount.count);
-    gpr_log(GPR_DEBUG, "[%s %p] subchannel_list %p REF %lu->%lu (%s)",
-            subchannel_list->tracer->name(), subchannel_list->policy,
-            subchannel_list, static_cast<unsigned long>(count - 1),
-            static_cast<unsigned long>(count), reason);
-  }
-}
-
-void grpc_lb_subchannel_list_unref(grpc_lb_subchannel_list* subchannel_list,
-                                   const char* reason) {
-  const bool done = gpr_unref(&subchannel_list->refcount);
-  if (subchannel_list->tracer->enabled()) {
-    const gpr_atm count = gpr_atm_acq_load(&subchannel_list->refcount.count);
-    gpr_log(GPR_DEBUG, "[%s %p] subchannel_list %p UNREF %lu->%lu (%s)",
-            subchannel_list->tracer->name(), subchannel_list->policy,
-            subchannel_list, static_cast<unsigned long>(count + 1),
-            static_cast<unsigned long>(count), reason);
-  }
-  if (done) {
-    subchannel_list_destroy(subchannel_list);
-  }
-}
-
-static void subchannel_data_cancel_connectivity_watch(
-    grpc_lb_subchannel_data* sd, const char* reason) {
-  if (sd->subchannel_list->tracer->enabled()) {
-    gpr_log(GPR_DEBUG,
-            "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
-            " (subchannel %p): canceling connectivity watch (%s)",
-            sd->subchannel_list->tracer->name(), sd->subchannel_list->policy,
-            sd->subchannel_list,
-            static_cast<size_t>(sd - sd->subchannel_list->subchannels),
-            sd->subchannel_list->num_subchannels, sd->subchannel, reason);
-  }
-  grpc_subchannel_notify_on_state_change(sd->subchannel, nullptr, nullptr,
-                                         &sd->connectivity_changed_closure);
-}
-
-void grpc_lb_subchannel_list_shutdown_and_unref(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason) {
-  if (subchannel_list->tracer->enabled()) {
-    gpr_log(GPR_DEBUG, "[%s %p] Shutting down subchannel_list %p (%s)",
-            subchannel_list->tracer->name(), subchannel_list->policy,
-            subchannel_list, reason);
-  }
-  GPR_ASSERT(!subchannel_list->shutting_down);
-  subchannel_list->shutting_down = true;
-  for (size_t i = 0; i < subchannel_list->num_subchannels; i++) {
-    grpc_lb_subchannel_data* sd = &subchannel_list->subchannels[i];
-    // If there's a pending notification for this subchannel, cancel it;
-    // the callback is responsible for unreffing the subchannel.
-    // Otherwise, unref the subchannel directly.
-    if (sd->connectivity_notification_pending) {
-      subchannel_data_cancel_connectivity_watch(sd, reason);
-    } else if (sd->subchannel != nullptr) {
-      grpc_lb_subchannel_data_unref_subchannel(sd, reason);
-    }
-  }
-  grpc_lb_subchannel_list_unref(subchannel_list, reason);
-}
diff --git a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h b/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
index 6889d596ac..d717ba0e37 100644
--- a/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
+++ b/src/core/ext/filters/client_channel/lb_policy/subchannel_list.h
@@ -21,116 +21,388 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <string.h>
+
+#include <grpc/support/alloc.h>
+
 #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
 #include "src/core/ext/filters/client_channel/subchannel.h"
+#include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/debug/trace.h"
+#include "src/core/lib/gprpp/abstract.h"
+#include "src/core/lib/gprpp/inlined_vector.h"
+#include "src/core/lib/gprpp/ref_counted.h"
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
+#include "src/core/lib/iomgr/closure.h"
+#include "src/core/lib/iomgr/combiner.h"
+#include "src/core/lib/iomgr/sockaddr_utils.h"
 #include "src/core/lib/transport/connectivity_state.h"
 
-// TODO(roth): This code is intended to be shared between pick_first and
-// round_robin.  However, the interface needs more work to provide clean
-// encapsulation.  For example, the structs here have some fields that are
-// only used in one of the two (e.g., the state counters in
-// grpc_lb_subchannel_list and the prev_connectivity_state field in
-// grpc_lb_subchannel_data are only used in round_robin, and the
-// checking_subchannel field in grpc_lb_subchannel_list is only used by
-// pick_first).  Also, there is probably some code duplication between the
-// connectivity state notification callback code in both pick_first and
-// round_robin that could be refactored and moved here.  In a future PR,
-// need to clean this up.
-
-typedef struct grpc_lb_subchannel_list grpc_lb_subchannel_list;
-
-typedef struct {
-  /** backpointer to owning subchannel list */
-  grpc_lb_subchannel_list* subchannel_list;
-  /** subchannel itself */
-  grpc_subchannel* subchannel;
-  grpc_core::RefCountedPtr<grpc_core::ConnectedSubchannel> connected_subchannel;
-  /** Is a connectivity notification pending? */
-  bool connectivity_notification_pending;
-  /** notification that connectivity has changed on subchannel */
-  grpc_closure connectivity_changed_closure;
-  /** previous and current connectivity states.  Updated by \a
-   * \a connectivity_changed_closure based on
-   * \a pending_connectivity_state_unsafe. */
-  grpc_connectivity_state prev_connectivity_state;
-  grpc_connectivity_state curr_connectivity_state;
-  /** connectivity state to be updated by
-   * grpc_subchannel_notify_on_state_change(), not guarded by
-   * the combiner.  To be copied to \a curr_connectivity_state by
-   * \a connectivity_changed_closure. */
-  grpc_connectivity_state pending_connectivity_state_unsafe;
-  /** the subchannel's target user data */
-  void* user_data;
-  /** vtable to operate over \a user_data */
-  const grpc_lb_user_data_vtable* user_data_vtable;
-} grpc_lb_subchannel_data;
-
-/// Unrefs the subchannel contained in sd.
-void grpc_lb_subchannel_data_unref_subchannel(grpc_lb_subchannel_data* sd,
-                                              const char* reason);
-
-/// Starts watching the connectivity state of the subchannel.
-/// The connectivity_changed_cb callback must invoke either
-/// grpc_lb_subchannel_data_stop_connectivity_watch() or again call
-/// grpc_lb_subchannel_data_start_connectivity_watch().
-void grpc_lb_subchannel_data_start_connectivity_watch(
-    grpc_lb_subchannel_data* sd);
-
-/// Stops watching the connectivity state of the subchannel.
-void grpc_lb_subchannel_data_stop_connectivity_watch(
-    grpc_lb_subchannel_data* sd);
-
-struct grpc_lb_subchannel_list {
-  /** backpointer to owning policy */
-  grpc_core::LoadBalancingPolicy* policy;
-
-  grpc_core::TraceFlag* tracer;
-
-  /** all our subchannels */
-  size_t num_subchannels;
-  grpc_lb_subchannel_data* subchannels;
-
-  /** Index into subchannels of the one we're currently checking.
-   * Used when connecting to subchannels serially instead of in parallel. */
-  // TODO(roth): When we have time, we can probably make this go away
-  // and compute the index dynamically by subtracting
-  // subchannel_list->subchannels from the subchannel_data pointer.
-  size_t checking_subchannel;
-
-  /** how many subchannels are in state READY */
-  size_t num_ready;
-  /** how many subchannels are in state TRANSIENT_FAILURE */
-  size_t num_transient_failures;
-  /** how many subchannels are in state IDLE */
-  size_t num_idle;
-
-  /** There will be one ref for each entry in subchannels for which there is a
-   * pending connectivity state watcher callback. */
-  gpr_refcount refcount;
-
-  /** Is this list shutting down? This may be true due to the shutdown of the
-   * policy itself or because a newer update has arrived while this one hadn't
-   * finished processing. */
-  bool shutting_down;
+// FIXME: add comments
+
+namespace grpc_core {
+
+template <typename SubchannelListType, typename SubchannelDataType>
+class SubchannelData {
+ public:
+  // Returns the index into subchannel_list_ of this object.
+  size_t Index() const {
+    return static_cast<size_t>(static_cast<const SubchannelDataType*>(this) -
+                               subchannel_list_->subchannel(0));
+  }
+
+  SubchannelListType* subchannel_list() const { return subchannel_list_; }
+
+  grpc_subchannel* subchannel() const { return subchannel_; }
+
+  ConnectedSubchannel* connected_subchannel() const {
+    return connected_subchannel_.get();
+  }
+
+// FIXME: maybe do this automatically in OnConnectivityChangedLocked()
+// when the new state is TRANSIENT_FAILURE?
+  void clear_connected_subchannel() { connected_subchannel_.reset(); }
+
+  void GetConnectedSubchannelFromSubchannelLocked() {
+    connected_subchannel_ =
+        grpc_subchannel_get_connected_subchannel(subchannel_);
+  }
+
+  void SetConnectedSubchannelFromLocked(SubchannelData* other) {
+    connected_subchannel_ = other->connected_subchannel_;  // Adds ref.
+  }
+
+  bool connectivity_notification_pending() const {
+    return connectivity_notification_pending_;
+  }
+  grpc_connectivity_state connectivity_state() const {
+    return curr_connectivity_state_;
+  }
+
+  virtual grpc_connectivity_state CheckConnectivityStateLocked() {
+    pending_connectivity_state_unsafe_ =
+        grpc_subchannel_check_connectivity(subchannel(), nullptr);
+    curr_connectivity_state_ = pending_connectivity_state_unsafe_;
+    return curr_connectivity_state_;
+  }
+
+  // Unrefs the subchannel.
+  virtual void UnrefSubchannelLocked(const char* reason);
+
+  /// Starts watching the connectivity state of the subchannel.
+  /// The connectivity_changed_cb callback must invoke either
+  /// StopConnectivityWatch() or again call StartConnectivityWatch().
+  void StartConnectivityWatchLocked();
+
+  /// Stops watching the connectivity state of the subchannel.
+  void StopConnectivityWatchLocked();
+
+  /// Cancels watching the connectivity state of the subchannel.
+  void CancelConnectivityWatchLocked(const char* reason);
+
+  void ShutdownLocked(const char* reason);
+
+  GRPC_ABSTRACT_BASE_CLASS
+
+ protected:
+  SubchannelData(
+      SubchannelListType* subchannel_list,
+      const grpc_lb_user_data_vtable* user_data_vtable,
+      const grpc_lb_address& address, grpc_subchannel* subchannel,
+      grpc_combiner* combiner);
+
+  virtual ~SubchannelData();
+
+// FIXME: define API
+  virtual void ProcessConnectivityChangeLocked(grpc_error* error) GRPC_ABSTRACT;
+
+ private:
+  static void OnConnectivityChangedLocked(void* arg, grpc_error* error);
+
+  // Backpointer to owning subchannel list.  Not owned.
+  SubchannelListType* subchannel_list_;
+
+  // The subchannel and connected subchannel.
+  grpc_subchannel* subchannel_;
+  RefCountedPtr<ConnectedSubchannel> connected_subchannel_;
+
+  // Notification that connectivity has changed on subchannel.
+  grpc_closure connectivity_changed_closure_;
+  // Is a connectivity notification pending?
+  bool connectivity_notification_pending_;
+  // Connectivity state to be updated by
+  // grpc_subchannel_notify_on_state_change(), not guarded by
+  // the combiner.  Will be copied to \a curr_connectivity_state_ by
+  // \a connectivity_changed_closure_.
+  grpc_connectivity_state pending_connectivity_state_unsafe_;
+  // Current connectivity state.
+  grpc_connectivity_state curr_connectivity_state_;
 };
 
-grpc_lb_subchannel_list* grpc_lb_subchannel_list_create(
-    grpc_core::LoadBalancingPolicy* p, grpc_core::TraceFlag* tracer,
+template <typename SubchannelListType, typename SubchannelDataType>
+class SubchannelList
+    : public RefCountedWithTracing<SubchannelListType> {
+ public:
+  typedef InlinedVector<SubchannelDataType, 10> SubchannelVector;
+
+  size_t num_subchannels() const { return subchannels_.size(); }
+  SubchannelDataType* subchannel(size_t index) { return &subchannels_[index]; }
+
+  // Marks the subchannel_list as discarded. Unsubscribes all its subchannels.
+  void ShutdownLocked(const char* reason);
+
+  bool shutting_down() const { return shutting_down_; }
+
+  LoadBalancingPolicy* policy() const { return policy_; }
+  TraceFlag* tracer() const { return tracer_; }
+
+  GRPC_ABSTRACT_BASE_CLASS
+
+ protected:
+  SubchannelList(LoadBalancingPolicy* policy, TraceFlag* tracer,
+                 const grpc_lb_addresses* addresses, grpc_combiner* combiner,
+                 grpc_client_channel_factory* client_channel_factory,
+                 const grpc_channel_args& args);
+
+  virtual ~SubchannelList();
+
+ private:
+  // So New() can call our private ctor.
+  template <typename T, typename... Args>
+  friend T* New(Args&&... args);
+
+  // Backpointer to owning policy.
+  LoadBalancingPolicy* policy_;
+
+  TraceFlag* tracer_;
+
+  // The list of subchannels.
+  SubchannelVector subchannels_;
+
+  // Is this list shutting down? This may be true due to the shutdown of the
+  // policy itself or because a newer update has arrived while this one hadn't
+  // finished processing.
+  bool shutting_down_ = false;
+};
+
+//
+// implementation -- no user-servicable parts below
+//
+
+//
+// SubchannelData
+//
+
+template <typename SubchannelListType, typename SubchannelDataType>
+SubchannelData<SubchannelListType, SubchannelDataType>::SubchannelData(
+    SubchannelListType* subchannel_list,
+    const grpc_lb_user_data_vtable* user_data_vtable,
+    const grpc_lb_address& address, grpc_subchannel* subchannel,
+    grpc_combiner* combiner)
+    : subchannel_list_(subchannel_list),
+      subchannel_(subchannel),
+      // We assume that the current state is IDLE.  If not, we'll get a
+      // callback telling us that.
+      pending_connectivity_state_unsafe_(GRPC_CHANNEL_IDLE),
+      curr_connectivity_state_(GRPC_CHANNEL_IDLE) {
+  GRPC_CLOSURE_INIT(
+      &connectivity_changed_closure_,
+      (&SubchannelData<SubchannelListType,
+                       SubchannelDataType>::OnConnectivityChangedLocked),
+      this, grpc_combiner_scheduler(combiner));
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+SubchannelData<SubchannelListType, SubchannelDataType>::~SubchannelData() {
+  UnrefSubchannelLocked("subchannel_data_destroy");
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::UnrefSubchannelLocked(
+    const char* reason) {
+  if (subchannel_ != nullptr) {
+    if (subchannel_list_->tracer()->enabled()) {
+      gpr_log(GPR_DEBUG,
+              "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
+              " (subchannel %p): unreffing subchannel",
+              subchannel_list_->tracer()->name(), subchannel_list_->policy(),
+              subchannel_list_, Index(),
+              subchannel_list_->num_subchannels(), subchannel_);
+    }
+    GRPC_SUBCHANNEL_UNREF(subchannel_, reason);
+    subchannel_ = nullptr;
+    connected_subchannel_.reset();
+  }
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::StartConnectivityWatchLocked() {
+  if (subchannel_list_->tracer()->enabled()) {
+    gpr_log(
+        GPR_DEBUG,
+        "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
+        " (subchannel %p): requesting connectivity change "
+        "notification (from %s)",
+        subchannel_list_->tracer()->name(), subchannel_list_->policy(),
+        subchannel_list_, Index(),
+        subchannel_list_->num_subchannels(), subchannel_,
+        grpc_connectivity_state_name(pending_connectivity_state_unsafe_));
+  }
+  connectivity_notification_pending_ = true;
+  grpc_subchannel_notify_on_state_change(
+      subchannel_, subchannel_list_->policy()->interested_parties(),
+      &pending_connectivity_state_unsafe_,
+      &connectivity_changed_closure_);
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::StopConnectivityWatchLocked() {
+  if (subchannel_list_->tracer()->enabled()) {
+    gpr_log(GPR_DEBUG,
+            "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
+            " (subchannel %p): stopping connectivity watch",
+            subchannel_list_->tracer()->name(), subchannel_list_->policy(),
+            subchannel_list_, Index(),
+            subchannel_list_->num_subchannels(), subchannel_);
+  }
+  GPR_ASSERT(connectivity_notification_pending_);
+  connectivity_notification_pending_ = false;
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::CancelConnectivityWatchLocked(
+    const char* reason) {
+  if (subchannel_list_->tracer()->enabled()) {
+    gpr_log(GPR_DEBUG,
+            "[%s %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
+            " (subchannel %p): canceling connectivity watch (%s)",
+            subchannel_list_->tracer()->name(), subchannel_list_->policy(),
+            subchannel_list_, Index(),
+            subchannel_list_->num_subchannels(), subchannel_, reason);
+  }
+  grpc_subchannel_notify_on_state_change(subchannel_, nullptr, nullptr,
+                                         &connectivity_changed_closure_);
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::OnConnectivityChangedLocked(
+    void* arg, grpc_error* error) {
+  SubchannelData* sd = static_cast<SubchannelData*>(arg);
+  // Now that we're inside the combiner, copy the pending connectivity
+  // state (which was set by the connectivity state watcher) to
+  // curr_connectivity_state_, which is what we use inside of the combiner.
+  sd->curr_connectivity_state_ = sd->pending_connectivity_state_unsafe_;
+  sd->ProcessConnectivityChangeLocked(error);
+}
+
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelData<SubchannelListType,
+                    SubchannelDataType>::ShutdownLocked(const char* reason) {
+  // If there's a pending notification for this subchannel, cancel it;
+  // the callback is responsible for unreffing the subchannel.
+  // Otherwise, unref the subchannel directly.
+  if (connectivity_notification_pending_) {
+    CancelConnectivityWatchLocked(reason);
+  } else if (subchannel_ != nullptr) {
+    UnrefSubchannelLocked(reason);
+  }
+}
+
+//
+// SubchannelList
+//
+
+template <typename SubchannelListType, typename SubchannelDataType>
+SubchannelList<SubchannelListType, SubchannelDataType>::SubchannelList(
+    LoadBalancingPolicy* policy, TraceFlag* tracer,
     const grpc_lb_addresses* addresses, grpc_combiner* combiner,
     grpc_client_channel_factory* client_channel_factory,
-    const grpc_channel_args& args, grpc_iomgr_cb_func connectivity_changed_cb);
+    const grpc_channel_args& args)
+    : RefCountedWithTracing<SubchannelListType>(tracer),
+      policy_(policy),
+      tracer_(tracer) {
+  if (tracer_->enabled()) {
+    gpr_log(GPR_DEBUG,
+            "[%s %p] Creating subchannel list %p for %" PRIuPTR " subchannels",
+            tracer_->name(), policy, this, addresses->num_addresses);
+  }
+  subchannels_.reserve(addresses->num_addresses);
+  // We need to remove the LB addresses in order to be able to compare the
+  // subchannel keys of subchannels from a different batch of addresses.
+  static const char* keys_to_remove[] = {GRPC_ARG_SUBCHANNEL_ADDRESS,
+                                         GRPC_ARG_LB_ADDRESSES};
+  // Create a subchannel for each address.
+  grpc_subchannel_args sc_args;
+  for (size_t i = 0; i < addresses->num_addresses; i++) {
+    // If there were any balancer, we would have chosen grpclb policy instead.
+    GPR_ASSERT(!addresses->addresses[i].is_balancer);
+    memset(&sc_args, 0, sizeof(grpc_subchannel_args));
+    grpc_arg addr_arg =
+        grpc_create_subchannel_address_arg(&addresses->addresses[i].address);
+    grpc_channel_args* new_args = grpc_channel_args_copy_and_add_and_remove(
+        &args, keys_to_remove, GPR_ARRAY_SIZE(keys_to_remove), &addr_arg, 1);
+    gpr_free(addr_arg.value.string);
+    sc_args.args = new_args;
+    grpc_subchannel* subchannel = grpc_client_channel_factory_create_subchannel(
+        client_channel_factory, &sc_args);
+    grpc_channel_args_destroy(new_args);
+    if (subchannel == nullptr) {
+      // Subchannel could not be created.
+      if (tracer_->enabled()) {
+        char* address_uri =
+            grpc_sockaddr_to_uri(&addresses->addresses[i].address);
+        gpr_log(GPR_DEBUG,
+                "[%s %p] could not create subchannel for address uri %s, "
+                "ignoring",
+                tracer_->name(), policy_, address_uri);
+        gpr_free(address_uri);
+      }
+      continue;
+    }
+    if (tracer_->enabled()) {
+      char* address_uri =
+          grpc_sockaddr_to_uri(&addresses->addresses[i].address);
+      gpr_log(GPR_DEBUG,
+              "[%s %p] subchannel list %p index %" PRIuPTR
+              ": Created subchannel %p for address uri %s",
+              tracer_->name(), policy_, this, subchannels_.size(), subchannel,
+              address_uri);
+      gpr_free(address_uri);
+    }
+    subchannels_.emplace_back(static_cast<SubchannelListType*>(this),
+                              addresses->user_data_vtable,
+                              addresses->addresses[i], subchannel, combiner);
+  }
+}
 
-void grpc_lb_subchannel_list_ref(grpc_lb_subchannel_list* subchannel_list,
-                                 const char* reason);
+template <typename SubchannelListType, typename SubchannelDataType>
+SubchannelList<SubchannelListType, SubchannelDataType>::~SubchannelList() {
+  if (tracer_->enabled()) {
+    gpr_log(GPR_DEBUG, "[%s %p] Destroying subchannel_list %p",
+            tracer_->name(), policy_, this);
+  }
+}
 
-void grpc_lb_subchannel_list_unref(grpc_lb_subchannel_list* subchannel_list,
-                                   const char* reason);
+template <typename SubchannelListType, typename SubchannelDataType>
+void SubchannelList<SubchannelListType,
+                    SubchannelDataType>::ShutdownLocked(const char* reason) {
+  if (tracer_->enabled()) {
+    gpr_log(GPR_DEBUG, "[%s %p] Shutting down subchannel_list %p (%s)",
+            tracer_->name(), policy_, this, reason);
+  }
+  GPR_ASSERT(!shutting_down_);
+  shutting_down_ = true;
+  for (size_t i = 0; i < subchannels_.size(); i++) {
+    SubchannelDataType* sd = &subchannels_[i];
+    sd->ShutdownLocked(reason);
+  }
+}
 
-/// Mark subchannel_list as discarded. Unsubscribes all its subchannels. The
-/// connectivity state notification callback will ultimately unref it.
-void grpc_lb_subchannel_list_shutdown_and_unref(
-    grpc_lb_subchannel_list* subchannel_list, const char* reason);
+}  // namespace grpc_core
 
 #endif /* GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_LB_POLICY_SUBCHANNEL_LIST_H */