include/perfetto/protozero/message_handle.h

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * 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 INCLUDE_PERFETTO_PROTOZERO_MESSAGE_HANDLE_H_
#define INCLUDE_PERFETTO_PROTOZERO_MESSAGE_HANDLE_H_

#include <functional>

#include "perfetto/base/export.h"
#include "perfetto/protozero/message.h"

namespace protozero {

class Message;

// MessageHandle allows to decouple the lifetime of a proto message
// from the underlying storage. It gives the following guarantees:
// - The underlying message is finalized (if still alive) if the handle goes
//   out of scope.
// - In Debug / DCHECK_ALWAYS_ON builds, the handle becomes null once the
//   message is finalized. This is to enforce the append-only API. For instance
//   when adding two repeated messages, the addition of the 2nd one forces
//   the finalization of the first.
// Think about this as a WeakPtr<Message> which calls
// Message::Finalize() when going out of scope.

class PERFETTO_EXPORT MessageHandleBase {
 public:
  ~MessageHandleBase();

  // Move-only type.
  MessageHandleBase(MessageHandleBase&&) noexcept;
  MessageHandleBase& operator=(MessageHandleBase&&);
  explicit operator bool() const {
#if PERFETTO_DCHECK_IS_ON()
    PERFETTO_DCHECK(!message_ || generation_ == message_->generation_);
#endif
    return !!message_;
  }

 protected:
  explicit MessageHandleBase(Message* = nullptr);
  Message* operator->() const {
#if PERFETTO_DCHECK_IS_ON()
    PERFETTO_DCHECK(!message_ || generation_ == message_->generation_);
#endif
    return message_;
  }
  Message& operator*() const { return *(operator->()); }

 private:
  friend class Message;
  MessageHandleBase(const MessageHandleBase&) = delete;
  MessageHandleBase& operator=(const MessageHandleBase&) = delete;

  void reset_message() { message_ = nullptr; }
  void Move(MessageHandleBase&&);

  Message* message_;
#if PERFETTO_DCHECK_IS_ON()
  uint32_t generation_;
#endif
};

template <typename T>
class MessageHandle : public MessageHandleBase {
 public:
  MessageHandle() : MessageHandle(nullptr) {}
  explicit MessageHandle(T* message) : MessageHandleBase(message) {}

  T& operator*() const {
    return static_cast<T&>(MessageHandleBase::operator*());
  }

  T* operator->() const {
    return static_cast<T*>(MessageHandleBase::operator->());
  }
};

}  // namespace protozero

#endif  // INCLUDE_PERFETTO_PROTOZERO_MESSAGE_HANDLE_H_