// 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 <cstddef>

#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(std::is_standard_layout_v<NoDestructor<InstanceType>>);

    static_assert(

        offsetof(NoDestructor, instance_storage_) % alignof(InstanceType) == 0,

        "instance_storage_ does not meet the instance's alignment requirement");

    static_assert(

        alignof(NoDestructor<InstanceType>) % alignof(InstanceType) == 0,

        "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:

  alignas(InstanceType) char instance_storage_[sizeof(InstanceType)];

};

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_