add leveldb

lib/leveldb/util/no_destructor.h

@@ -0,0 +1,46 @@
+// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#ifndef STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
+#define STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
+
+#include <type_traits>
+#include <utility>
+
+namespace leveldb {
+
+// Wraps an instance whose destructor is never called.
+//
+// This is intended for use with function-level static variables.
+template <typename InstanceType>
+class NoDestructor {
+ public:
+  template <typename... ConstructorArgTypes>
+  explicit NoDestructor(ConstructorArgTypes&&... constructor_args) {
+    static_assert(sizeof(instance_storage_) >= sizeof(InstanceType),
+                  "instance_storage_ is not large enough to hold the instance");
+    static_assert(
+        alignof(decltype(instance_storage_)) >= alignof(InstanceType),
+        "instance_storage_ does not meet the instance's alignment requirement");
+    new (&instance_storage_)
+        InstanceType(std::forward<ConstructorArgTypes>(constructor_args)...);
+  }
+
+  ~NoDestructor() = default;
+
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  InstanceType* get() {
+    return reinterpret_cast<InstanceType*>(&instance_storage_);
+  }
+
+ private:
+  typename std::aligned_storage<sizeof(InstanceType),
+                                alignof(InstanceType)>::type instance_storage_;
+};
+
+}  // namespace leveldb
+
+#endif  // STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_