third_party/WebKit/Source/core/style/DataPersistent.h

// Copyright 2016 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.

#ifndef DataPersistent_h
#define DataPersistent_h

#include "platform/heap/Handle.h"
#include "wtf/Allocator.h"
#include "wtf/PtrUtil.h"
#include <memory>

namespace blink {

// DataPersistent<T> provides the copy-on-modify behavior of DataRef<>,
// but for Persistent<> heap references.
//
// That is, the DataPersistent<T> copy assignment, |a = b;|, makes |a|
// share a reference to the T object that |b| holds until |a| is mutated
// and access() on it is called. Or, dually, if |b| is mutated after the
// assignment that mutation isn't observable to |a| but will be performed
// on a copy of the underlying T object.
//
// DataPersistent<T> does assume that no one keeps non-DataPersistent<> shared
// references to the underlying T object that it manages, and is mutating the
// object via those.
template <typename T>
class DataPersistent {
  USING_FAST_MALLOC(DataPersistent);

 public:
  DataPersistent() : m_ownCopy(false) {}

  DataPersistent(const DataPersistent& other) : m_ownCopy(false) {
    if (other.m_data)
      m_data = WTF::wrapUnique(new Persistent<T>(other.m_data->get()));

    // Invalidated, subsequent mutations will happen on a new copy.
    //
    // (Clearing |m_ownCopy| will not be observable over T, hence
    // the const_cast<> is considered acceptable here.)
    const_cast<DataPersistent&>(other).m_ownCopy = false;
  }

  const T* get() const { return m_data ? m_data->get() : nullptr; }

  const T& operator*() const { return m_data ? *get() : nullptr; }
  const T* operator->() const { return get(); }

  T* access() {
    if (m_data && !m_ownCopy) {
      *m_data = (*m_data)->copy();
      m_ownCopy = true;
    }
    return m_data ? m_data->get() : nullptr;
  }

  void init() {
    ASSERT(!m_data);
    m_data = WTF::wrapUnique(new Persistent<T>(T::create()));
    m_ownCopy = true;
  }

  bool operator==(const DataPersistent<T>& o) const {
    ASSERT(m_data);
    ASSERT(o.m_data);
    return m_data->get() == o.m_data->get() ||
           *m_data->get() == *o.m_data->get();
  }

  bool operator!=(const DataPersistent<T>& o) const {
    ASSERT(m_data);
    ASSERT(o.m_data);
    return m_data->get() != o.m_data->get() &&
           *m_data->get() != *o.m_data->get();
  }

  void operator=(std::nullptr_t) { m_data.clear(); }

 private:
  // Reduce size of DataPersistent<> by delaying creation of Persistent<>.
  std::unique_ptr<Persistent<T>> m_data;
  unsigned m_ownCopy : 1;
};

}  // namespace blink

#endif  // DataPersistent_h