text: /*
   * Copyright (C) 2017 Apple Inc. All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
   * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
   * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
   * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
   * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
   * THE POSSIBILITY OF SUCH DAMAGE.
   */
  
  #pragma once
  
  #include <atomic>
  #include <wtf/NeverDestroyed.h>
  
  namespace WTF {
  
  template <typename IdentifierType, typename ClassType>
  class IdentifiedBase {
  public:
      IdentifierType identifier() const
      {
          return m_identifier;
      }
  
  protected:
      IdentifiedBase(const IdentifiedBase& other)
          : m_identifier(other.m_identifier)
      {
      }
  
      explicit IdentifiedBase(IdentifierType identifier)
          : m_identifier(identifier)
      {
      }
  
  private:
      IdentifierType m_identifier;
  };
  
  template <typename T>
  class Identified : public IdentifiedBase<uint64_t, T> {
  protected:
      Identified()
          : IdentifiedBase<uint64_t, T>(generateIdentifier())
      {
      }
  
      Identified(const Identified&) = default;
  
      explicit Identified(uint64_t identifier)
          : IdentifiedBase<uint64_t, T>(identifier)
      {
      }
  
  private:
      static uint64_t generateIdentifier()
      {
          static uint64_t currentIdentifier;
          return ++currentIdentifier;
      }
  };
  
  template <typename T>
  class ThreadSafeIdentified : public IdentifiedBase<uint64_t, T> {
  protected:
      ThreadSafeIdentified()
          : IdentifiedBase<uint64_t, T>(generateIdentifier())
      {
      }
  
      ThreadSafeIdentified(const ThreadSafeIdentified&) = default;
  
      explicit ThreadSafeIdentified(uint64_t identifier)
          : IdentifiedBase<uint64_t, T>(identifier)
      {
      }
  
  private:
      static uint64_t generateIdentifier()
      {
          static LazyNeverDestroyed<std::atomic<uint64_t>> currentIdentifier;
          static std::once_flag initializeCurrentIdentifier;
          std::call_once(initializeCurrentIdentifier, [] {
              currentIdentifier.construct(0);
          });
          return ++currentIdentifier.get();
      }
  };
  
  } // namespace WTF
  
  using WTF::Identified;
  using WTF::ThreadSafeIdentified;
  