Inside Bitcoin

1// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file. See the AUTHORS file for names of contributors.
4
5#ifndef STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
6#define STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
7
8#include <cstddef>
9#include <type_traits>
10#include <utility>
11
12namespace leveldb {
13
14// Wraps an instance whose destructor is never called.
15//
16// This is intended for use with function-level static variables.
17template <typename InstanceType>
18class NoDestructor {
19 public:
20  template <typename... ConstructorArgTypes>
21  explicit NoDestructor(ConstructorArgTypes&&... constructor_args) {
22    static_assert(sizeof(instance_storage_) >= sizeof(InstanceType),
23                  "instance_storage_ is not large enough to hold the instance");
24    static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
25    static_assert(
26        offsetof(NoDestructor, instance_storage_) % alignof(InstanceType) == 0,
27        "instance_storage_ does not meet the instance's alignment requirement");
28    static_assert(
29        alignof(NoDestructor<InstanceType>) % alignof(InstanceType) == 0,
30        "instance_storage_ does not meet the instance's alignment requirement");
31    new (instance_storage_)
32        InstanceType(std::forward<ConstructorArgTypes>(constructor_args)...);
33  }
34
35  ~NoDestructor() = default;
36
37  NoDestructor(const NoDestructor&) = delete;
38  NoDestructor& operator=(const NoDestructor&) = delete;
39
40  InstanceType* get() {
41    return reinterpret_cast<InstanceType*>(&instance_storage_);
42  }
43
44 private:
45  alignas(InstanceType) char instance_storage_[sizeof(InstanceType)];
46};
47
48}  // namespace leveldb
49
50#endif  // STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
51