gz/utils/NeverDestroyed.hh

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/*
 * Copyright (C) 2021 Open Source Robotics Foundation
 *
 * 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 GZ_UTILS_NEVERDESTROYED_HH_
#define GZ_UTILS_NEVERDESTROYED_HH_
 
#include <new>
#include <type_traits>
#include <utility>
 
namespace gz
{
namespace utils
{
//
// The above examples are repeated in the unit test; keep them in sync.
template <typename T>
class NeverDestroyed
{
  public: template <typename... Args>
          explicit NeverDestroyed(Args &&... args)
  {
    // Uses "placement new" to construct a `T` in `storage_`.
    new (&this->storage) T(std::forward<Args>(args)...);
  }
 
  public: ~NeverDestroyed() = default;
 
  public: NeverDestroyed(const NeverDestroyed &) = delete;
 
  public: NeverDestroyed(NeverDestroyed &&) = delete;
 
  public: NeverDestroyed &operator=(const NeverDestroyed &) = delete;
 
  public: NeverDestroyed &operator=(NeverDestroyed &&) noexcept = delete;
 
  public: T &Access()
  {
    return *reinterpret_cast<T *>(&this->storage);
  }
 
  public: const T &Access() const
  {
    return *reinterpret_cast<const T *>(&this->storage);
  }
 
  private: typename std::aligned_storage<sizeof(T), alignof(T)>::type storage;
};
}  // namespace utils
}  // namespace gz
 
#endif  // GZ_UTILS_NEVERDESTROYED_HH_