text: // Copyright (c) 2011 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  // WARNING: Thread local storage is a bit tricky to get right. Please make sure
  // that this is really the proper solution for what you're trying to achieve.
  // Don't prematurely optimize, most likely you can just use a Lock.
  //
  // These classes implement a wrapper around ThreadLocalStorage::Slot. On
  // construction, they will allocate a TLS slot, and free the TLS slot on
  // destruction. No memory management (creation or destruction) is handled. This
  // means for uses of ThreadLocalPointer, you must correctly manage the memory
  // yourself, these classes will not destroy the pointer for you. There are no
  // at-thread-exit actions taken by these classes.
  //
  // ThreadLocalPointer<Type> wraps a Type*. It performs no creation or
  // destruction, so memory management must be handled elsewhere. The first call
  // to Get() on a thread will return NULL. You can update the pointer with a call
  // to Set().
  //
  // ThreadLocalBoolean wraps a bool. It will default to false if it has never
  // been set otherwise with Set().
  //
  // Thread Safety: An instance of ThreadLocalStorage is completely thread safe
  // once it has been created. If you want to dynamically create an instance, you
  // must of course properly deal with safety and race conditions.
  //
  // In Android, the system TLS is limited.
  //
  // Example usage:
  //   // My class is logically attached to a single thread. We cache a pointer
  //   // on the thread it was created on, so we can implement current().
  //   MyClass::MyClass() {
  //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() == NULL);
  //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(this);
  //   }
  //
  //   MyClass::~MyClass() {
  //     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() != NULL);
  //     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(NULL);
  //   }
  //
  //   // Return the current MyClass associated with the calling thread, can be
  //   // NULL if there isn't a MyClass associated.
  //   MyClass* MyClass::current() {
  //     return Singleton<ThreadLocalPointer<MyClass> >::get()->Get();
  //   }
  
  #ifndef BASE_THREADING_THREAD_LOCAL_H_
  #define BASE_THREADING_THREAD_LOCAL_H_
  
  #include <memory>
  
  #include "base/check_op.h"
  #include "base/macros.h"
  #include "base/threading/thread_local_internal.h"
  #include "base/threading/thread_local_storage.h"
  
  namespace base {
  
  template <typename T>
  class ThreadLocalPointer {
   public:
    ThreadLocalPointer() = default;
    ~ThreadLocalPointer() = default;
  
    T* Get() const { return static_cast<T*>(slot_.Get()); }
  
    void Set(T* ptr) {
      slot_.Set(const_cast<void*>(static_cast<const void*>(ptr)));
    }
  
   private:
    ThreadLocalStorage::Slot slot_;
  
    DISALLOW_COPY_AND_ASSIGN(ThreadLocalPointer<T>);
  };
  
  // A ThreadLocalOwnedPointer<T> is like a ThreadLocalPointer<T> except that
  // pointers handed to it are owned and automatically deleted during their
  // associated thread's exit phase (or when replaced if Set() is invoked multiple
  // times on the same thread).
  // The ThreadLocalOwnedPointer instance itself can only be destroyed when no
  // threads, other than the one it is destroyed on, have remaining state set in
  // it. Typically this means that ThreadLocalOwnedPointer instances are held in
  // static storage or at the very least only recycled in the single-threaded
  // phase between tests in the same process.
  #if DCHECK_IS_ON()
  template <typename T>
  using ThreadLocalOwnedPointer = internal::CheckedThreadLocalOwnedPointer<T>;
  #else   // DCHECK_IS_ON()
  template <typename T>
  class ThreadLocalOwnedPointer {
   public:
    ThreadLocalOwnedPointer() = default;
  
    ~ThreadLocalOwnedPointer() {
      // Assume that this thread is the only one with potential state left. This
      // is verified in ~CheckedThreadLocalOwnedPointer().
      Set(nullptr);
    }
  
    T* Get() const { return static_cast<T*>(slot_.Get()); }
  
    void Set(std::unique_ptr<T> ptr) {
      delete Get();
      slot_.Set(const_cast<void*>(static_cast<const void*>(ptr.release())));
    }
  
   private:
    static void DeleteTlsPtr(void* ptr) { delete static_cast<T*>(ptr); }
  
    ThreadLocalStorage::Slot slot_{&DeleteTlsPtr};
  
    DISALLOW_COPY_AND_ASSIGN(ThreadLocalOwnedPointer<T>);
  };
  #endif  // DCHECK_IS_ON()
  
  class ThreadLocalBoolean {
   public:
    ThreadLocalBoolean() = default;
    ~ThreadLocalBoolean() = default;
  
    bool Get() const { return tlp_.Get() != nullptr; }
  
    void Set(bool val) { tlp_.Set(val ? this : nullptr); }
  
   private:
    ThreadLocalPointer<void> tlp_;
  
    DISALLOW_COPY_AND_ASSIGN(ThreadLocalBoolean);
  };
  
  }  // namespace base
  
  #endif  // BASE_THREADING_THREAD_LOCAL_H_
  