Line data    Source code

       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              : 
      12              : namespace leveldb {
      13              : 
      14              : // Wraps an instance whose destructor is never called.
      15              : //
      16              : // This is intended for use with function-level static variables.
      17              : template <typename InstanceType>
      18              : class NoDestructor {
      19              :  public:
      20              :   template <typename... ConstructorArgTypes>
      21           50 :   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           50 :     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         1301 :   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_