chromium/base/threading/sequence_local_storage_slot.h

// Copyright 2017 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_THREADING_SEQUENCE_LOCAL_STORAGE_SLOT_H_
#define BASE_THREADING_SEQUENCE_LOCAL_STORAGE_SLOT_H_

#include <memory>
#include <type_traits>
#include <utility>

#include "base/base_export.h"
#include "base/threading/sequence_local_storage_map.h"
#include "third_party/abseil-cpp/absl/meta/type_traits.h"

namespace base {

namespace internal {
BASE_EXPORT int GetNextSequenceLocalStorageSlotNumber();
}

// SequenceLocalStorageSlot allows arbitrary values to be stored and retrieved
// from a sequence. Values are deleted when the sequence is deleted.
//
// Example usage:
//
// int& GetSequenceLocalStorage()
//     static SequenceLocalStorageSlot<int> sls_value;
//     return sls_value->GetOrCreateValue();
// }
//
// void Read() {
//   int value = GetSequenceLocalStorage();
//   ...
// }
//
// void Write() {
//   GetSequenceLocalStorage() = 42;
// }
//
// void PostTasks() {
//   // Since Read() runs on the same sequence as Write(), it
//   // will read the value "42". A Read() running on a different
//   // sequence would not see that value.
//   scoped_refptr<base::SequencedTaskRunner> task_runner = ...;
//   task_runner->PostTask(FROM_HERE, base::BindOnce(&Write));
//   task_runner->PostTask(FROM_HERE, base::BindOnce(&Read));
// }
//
// SequenceLocalStorageSlot must be used within the scope of a
// ScopedSetSequenceLocalStorageMapForCurrentThread object.
// Note: this is true on all ThreadPool workers and on threads bound to a
// MessageLoop.
// SequenceLocalStorageSlot is implemented by either [Generic/Small]
// variants depending on the type. SequenceLocalStorageSlot itself
// doesn't support forward declared types and thus the variant
// [Generic/Small] needs to be specified explicitly.

// Generic implementation for SequenceLocalStorageSlot.
template <typename T, typename Deleter = std::default_delete<T>>
class GenericSequenceLocalStorageSlot {
 public:
  GenericSequenceLocalStorageSlot()
      : … { … }

  GenericSequenceLocalStorageSlot(const GenericSequenceLocalStorageSlot&) =
      delete;
  GenericSequenceLocalStorageSlot& operator=(
      const GenericSequenceLocalStorageSlot&) = delete;

  ~GenericSequenceLocalStorageSlot() = default;

  explicit operator bool() const {
    return internal::SequenceLocalStorageMap::GetForCurrentThread().Has(
        slot_id_);
  }

  // Default-constructs the value for the current sequence if not
  // already constructed. Then, returns the value.
  T& GetOrCreateValue() { … }

  // Returns a pointer to the value for the current sequence. May be
  // nullptr if the value was not constructed on the current sequence.
  T* GetValuePointer() { … }
  const T* GetValuePointer() const { … }

  T* operator->() { … }
  const T* operator->() const { … }

  T& operator*() { … }
  const T& operator*() const { … }

  void reset() { … }

  // Constructs this slot's sequence-local value with |args...| and returns a
  // pointer to the created object.
  template <class... Args>
  T& emplace(Args&&... args) { … }

 private:
  // Takes ownership of |value_ptr|.
  void Adopt(T* value_ptr) { … }

  // |slot_id_| is used as a key in SequenceLocalStorageMap
  const int slot_id_;
};

// Implementation for SequenceLocalStorageSlot optimized for small and trivial
// objects.
template <class T>
class SmallSequenceLocalStorageSlot { … };

template <typename T,
          typename Deleter = std::default_delete<T>,
          bool IsSmall =
              sizeof(T) <= sizeof(void*) && absl::is_trivially_relocatable<T>()>
struct SequenceLocalStorageSlot;

SequenceLocalStorageSlot<T, Deleter, false>;

SequenceLocalStorageSlot<T, std::default_delete<T>, true>;

}  // namespace base
#endif  // BASE_THREADING_SEQUENCE_LOCAL_STORAGE_SLOT_H_