routing: Use a static function to return a const reference to an empty vector.

Author: Mizux

ortools/routing/BUILD.bazel

@@ -305,6 +305,7 @@ cc_library(
         "//ortools/util:time_limit",
         "@abseil-cpp//absl/algorithm:container",
         "@abseil-cpp//absl/base:core_headers",
+        "@abseil-cpp//absl/base:no_destructor",
         "@abseil-cpp//absl/container:btree",
         "@abseil-cpp//absl/container:flat_hash_map",
         "@abseil-cpp//absl/container:flat_hash_set",

ortools/routing/routing.cc

@@ -36,6 +36,7 @@
 #include <vector>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/no_destructor.h"
 #include "absl/container/flat_hash_map.h"
 #include "absl/container/flat_hash_set.h"
 #include "absl/flags/flag.h"
@@ -217,6 +218,12 @@ void Model::SetSweepArranger(SweepArranger* sweep_arranger) {
 
 SweepArranger* Model::sweep_arranger() const { return sweep_arranger_.get(); }
 
+/* static */
+const std::vector<int>& Model::NodeNeighborsByCostClass::GetEmptyNeighbors() {
+  static const absl::NoDestructor<std::vector<int>> kEmptyNeighbors;
+  return *kEmptyNeighbors;
+}
+
 void Model::NodeNeighborsByCostClass::ComputeNeighbors(
     const NodeNeighborsParameters& params) {
   auto [num_neighbors, add_vehicle_starts_to_neighbors,

ortools/routing/routing.h

@@ -1693,7 +1693,7 @@ class OR_DLL Model {
     /// is a neighborhood arc for 'cost_class'.
     const std::vector<int>& GetIncomingNeighborsOfNodeForCostClass(
         int cost_class, int node_index) const {
-      if (routing_model_.IsStart(node_index)) return empty_neighbors_;
+      if (routing_model_.IsStart(node_index)) return GetEmptyNeighbors();
 
       if (node_index_to_incoming_neighbors_by_cost_class_.empty()) {
         DCHECK(IsFullNeighborhood());
@@ -1702,7 +1702,7 @@ class OR_DLL Model {
       const std::vector<std::vector<int>>& node_index_to_incoming_neighbors =
           node_index_to_incoming_neighbors_by_cost_class_[cost_class];
       if (node_index_to_incoming_neighbors.empty()) {
-        return empty_neighbors_;
+        return GetEmptyNeighbors();
       }
       return node_index_to_incoming_neighbors[node_index];
     }
@@ -1712,7 +1712,7 @@ class OR_DLL Model {
     /// arc for 'cost_class'.
     const std::vector<int>& GetOutgoingNeighborsOfNodeForCostClass(
         int cost_class, int node_index) const {
-      if (routing_model_.IsEnd(node_index)) return empty_neighbors_;
+      if (routing_model_.IsEnd(node_index)) return GetEmptyNeighbors();
 
       if (node_index_to_outgoing_neighbors_by_cost_class_.empty()) {
         DCHECK(IsFullNeighborhood());
@@ -1721,7 +1721,7 @@ class OR_DLL Model {
       const std::vector<std::vector<int>>& node_index_to_outgoing_neighbors =
           node_index_to_outgoing_neighbors_by_cost_class_[cost_class];
       if (node_index_to_outgoing_neighbors.empty()) {
-        return empty_neighbors_;
+        return GetEmptyNeighbors();
       }
       return node_index_to_outgoing_neighbors[node_index];
     }
@@ -1744,12 +1744,9 @@ class OR_DLL Model {
     bool IsFullNeighborhood() const { return full_neighborhood_; }
 
    private:
+    static const std::vector<int>& GetEmptyNeighbors();
+
     const Model& routing_model_;
-#if __cplusplus >= 202002L && !defined(__APPLE__)
-    static constexpr std::vector<int> empty_neighbors_ = {};
-#else
-    inline static const std::vector<int> empty_neighbors_ = {};
-#endif
 
     std::vector<std::vector<std::vector<int>>>
         node_index_to_incoming_neighbors_by_cost_class_;