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+// Copyright 2017 The Abseil 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.
+
+#ifndef ABSL_CONTAINER_INTERNAL_TEST_INSTANCE_TRACKER_H_
+#define ABSL_CONTAINER_INTERNAL_TEST_INSTANCE_TRACKER_H_
+
+#include <cstdlib>
+#include <ostream>
+
+namespace absl {
+namespace test_internal {
+
+// A type that counts number of occurences of the type, the live occurrences of
+// the type, as well as the number of copies, moves, and swaps that have
+// occurred on the type. This is used as a base class for the copyable,
+// copyable+movable, and movable types below that are used in actual tests. Use
+// InstanceTracker in tests to track the number of instances.
+class BaseCountedInstance {
+ public:
+  explicit BaseCountedInstance(int x) : value_(x) {
+    ++num_instances_;
+    ++num_live_instances_;
+  }
+  BaseCountedInstance(const BaseCountedInstance& x)
+      : value_(x.value_), is_live_(x.is_live_) {
+    ++num_instances_;
+    if (is_live_) ++num_live_instances_;
+    ++num_copies_;
+  }
+  BaseCountedInstance(BaseCountedInstance&& x)
+      : value_(x.value_), is_live_(x.is_live_) {
+    x.is_live_ = false;
+    ++num_instances_;
+    ++num_moves_;
+  }
+  ~BaseCountedInstance() {
+    --num_instances_;
+    if (is_live_) --num_live_instances_;
+  }
+
+  BaseCountedInstance& operator=(const BaseCountedInstance& x) {
+    value_ = x.value_;
+    if (is_live_) --num_live_instances_;
+    is_live_ = x.is_live_;
+    if (is_live_) ++num_live_instances_;
+    ++num_copies_;
+    return *this;
+  }
+  BaseCountedInstance& operator=(BaseCountedInstance&& x) {
+    value_ = x.value_;
+    if (is_live_) --num_live_instances_;
+    is_live_ = x.is_live_;
+    x.is_live_ = false;
+    ++num_moves_;
+    return *this;
+  }
+
+  int value() const {
+    if (!is_live_) std::abort();
+    return value_;
+  }
+
+  friend std::ostream& operator<<(std::ostream& o,
+                                  const BaseCountedInstance& v) {
+    return o << "[value:" << v.value() << "]";
+  }
+
+  // Implementation of efficient swap() that counts swaps.
+  static void SwapImpl(
+      BaseCountedInstance& lhs,    // NOLINT(runtime/references)
+      BaseCountedInstance& rhs) {  // NOLINT(runtime/references)
+    using std::swap;
+    swap(lhs.value_, rhs.value_);
+    swap(lhs.is_live_, rhs.is_live_);
+    ++BaseCountedInstance::num_swaps_;
+  }
+
+ private:
+  friend class InstanceTracker;
+
+  int value_;
+
+  // Indicates if the value is live, ie it hasn't been moved away from.
+  bool is_live_ = true;
+
+  // Number of instances.
+  static int num_instances_;
+
+  // Number of live instances (those that have not been moved away from.)
+  static int num_live_instances_;
+
+  // Number of times that BaseCountedInstance objects were moved.
+  static int num_moves_;
+
+  // Number of times that BaseCountedInstance objects were copied.
+  static int num_copies_;
+
+  // Number of times that BaseCountedInstance objects were swapped.
+  static int num_swaps_;
+};
+
+// Helper to track the BaseCountedInstance instance counters. Expects that the
+// number of instances and live_instances are the same when it is constructed
+// and when it is destructed.
+class InstanceTracker {
+ public:
+  InstanceTracker()
+      : start_instances_(BaseCountedInstance::num_instances_),
+        start_live_instances_(BaseCountedInstance::num_live_instances_) {
+    ResetCopiesMovesSwaps();
+  }
+  ~InstanceTracker() {
+    if (instances() != 0) std::abort();
+    if (live_instances() != 0) std::abort();
+  }
+
+  // Returns the number of BaseCountedInstance instances both containing valid
+  // values and those moved away from compared to when the InstanceTracker was
+  // constructed
+  int instances() const {
+    return BaseCountedInstance::num_instances_ - start_instances_;
+  }
+
+  // Returns the number of live BaseCountedInstance instances compared to when
+  // the InstanceTracker was constructed
+  int live_instances() const {
+    return BaseCountedInstance::num_live_instances_ - start_live_instances_;
+  }
+
+  // Returns the number of moves on BaseCountedInstance objects since
+  // construction or since the last call to ResetCopiesMovesSwaps().
+  int moves() const { return BaseCountedInstance::num_moves_ - start_moves_; }
+
+  // Returns the number of copies on BaseCountedInstance objects since
+  // construction or the last call to ResetCopiesMovesSwaps().
+  int copies() const {
+    return BaseCountedInstance::num_copies_ - start_copies_;
+  }
+
+  // Returns the number of swaps on BaseCountedInstance objects since
+  // construction or the last call to ResetCopiesMovesSwaps().
+  int swaps() const { return BaseCountedInstance::num_swaps_ - start_swaps_; }
+
+  // Resets the base values for moves, copies and swaps to the current values,
+  // so that subsequent Get*() calls for moves, copies and swaps will compare to
+  // the situation at the point of this call.
+  void ResetCopiesMovesSwaps() {
+    start_moves_ = BaseCountedInstance::num_moves_;
+    start_copies_ = BaseCountedInstance::num_copies_;
+    start_swaps_ = BaseCountedInstance::num_swaps_;
+  }
+
+ private:
+  int start_instances_;
+  int start_live_instances_;
+  int start_moves_;
+  int start_copies_;
+  int start_swaps_;
+};
+
+// Copyable, not movable.
+class CopyableOnlyInstance : public BaseCountedInstance {
+ public:
+  explicit CopyableOnlyInstance(int x) : BaseCountedInstance(x) {}
+  CopyableOnlyInstance(const CopyableOnlyInstance& rhs) = default;
+  CopyableOnlyInstance& operator=(const CopyableOnlyInstance& rhs) = default;
+
+  friend void swap(CopyableOnlyInstance& lhs, CopyableOnlyInstance& rhs) {
+    BaseCountedInstance::SwapImpl(lhs, rhs);
+  }
+
+  static bool supports_move() { return false; }
+};
+
+// Copyable and movable.
+class CopyableMovableInstance : public BaseCountedInstance {
+ public:
+  explicit CopyableMovableInstance(int x) : BaseCountedInstance(x) {}
+  CopyableMovableInstance(const CopyableMovableInstance& rhs) = default;
+  CopyableMovableInstance(CopyableMovableInstance&& rhs) = default;
+  CopyableMovableInstance& operator=(const CopyableMovableInstance& rhs) =
+      default;
+  CopyableMovableInstance& operator=(CopyableMovableInstance&& rhs) = default;
+
+  friend void swap(CopyableMovableInstance& lhs, CopyableMovableInstance& rhs) {
+    BaseCountedInstance::SwapImpl(lhs, rhs);
+  }
+
+  static bool supports_move() { return true; }
+};
+
+// Only movable, not default-constructible.
+class MovableOnlyInstance : public BaseCountedInstance {
+ public:
+  explicit MovableOnlyInstance(int x) : BaseCountedInstance(x) {}
+  MovableOnlyInstance(MovableOnlyInstance&& other) = default;
+  MovableOnlyInstance& operator=(MovableOnlyInstance&& other) = default;
+
+  friend void swap(MovableOnlyInstance& lhs, MovableOnlyInstance& rhs) {
+    BaseCountedInstance::SwapImpl(lhs, rhs);
+  }
+
+  static bool supports_move() { return true; }
+};
+
+}  // namespace test_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_TEST_INSTANCE_TRACKER_H_