chromium/base/memory/weak_ptr.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Weak pointers are pointers to an object that do not affect its lifetime,
// and which may be invalidated (i.e. reset to nullptr) by the object, or its
// owner, at any time, most commonly when the object is about to be deleted.

// Weak pointers are useful when an object needs to be accessed safely by one
// or more objects other than its owner, and those callers can cope with the
// object vanishing and e.g. tasks posted to it being silently dropped.
// Reference-counting such an object would complicate the ownership graph and
// make it harder to reason about the object's lifetime.

// EXAMPLE:
//
//  class Controller {
//   public:
//    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
//    void WorkComplete(const Result& result) { ... }
//   private:
//    // Member variables should appear before the WeakPtrFactory, to ensure
//    // that any WeakPtrs to Controller are invalidated before its members
//    // variable's destructors are executed, rendering them invalid.
//    WeakPtrFactory<Controller> weak_factory_{this};
//  };
//
//  class Worker {
//   public:
//    static void StartNew(WeakPtr<Controller> controller) {
//      // Move WeakPtr when possible to avoid atomic refcounting churn on its
//      // internal state.
//      Worker* worker = new Worker(std::move(controller));
//      // Kick off asynchronous processing...
//    }
//   private:
//    Worker(WeakPtr<Controller> controller)
//        : controller_(std::move(controller)) {}
//    void DidCompleteAsynchronousProcessing(const Result& result) {
//      if (controller_)
//        controller_->WorkComplete(result);
//    }
//    WeakPtr<Controller> controller_;
//  };
//
// With this implementation a caller may use SpawnWorker() to dispatch multiple
// Workers and subsequently delete the Controller, without waiting for all
// Workers to have completed.

// ------------------------- IMPORTANT: Thread-safety -------------------------

// Weak pointers may be passed safely between sequences, but must always be
// dereferenced and invalidated on the same SequencedTaskRunner otherwise
// checking the pointer would be racey.
//
// To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
// is dereferenced, the factory and its WeakPtrs become bound to the calling
// sequence or current SequencedWorkerPool token, and cannot be dereferenced or
// invalidated on any other task runner. Bound WeakPtrs can still be handed
// off to other task runners, e.g. to use to post tasks back to object on the
// bound sequence.
//
// If all WeakPtr objects are destroyed or invalidated then the factory is
// unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
// destroyed, or new WeakPtr objects may be used, from a different sequence.
//
// Thus, at least one WeakPtr object must exist and have been dereferenced on
// the correct sequence to enforce that other WeakPtr objects will enforce they
// are used on the desired sequence.

#ifndef BASE_MEMORY_WEAK_PTR_H_
#define BASE_MEMORY_WEAK_PTR_H_

#include <cstddef>
#include <type_traits>
#include <utility>

#include "base/base_export.h"
#include "base/check.h"
#include "base/compiler_specific.h"
#include "base/dcheck_is_on.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/ref_counted.h"
#include "base/memory/safe_ref_traits.h"
#include "base/sequence_checker.h"
#include "base/synchronization/atomic_flag.h"

namespace performance_manager {
class FrameNodeImpl;
class PageNodeImpl;
class ProcessNodeImpl;
class WorkerNodeImpl;
}  // namespace performance_manager

namespace base {

namespace sequence_manager::internal {
class TaskQueueImpl;
}

template <typename T> class WeakPtr;

namespace internal {
// These classes are part of the WeakPtr implementation.
// DO NOT USE THESE CLASSES DIRECTLY YOURSELF.

class BASE_EXPORT TRIVIAL_ABI WeakReference { … };

class BASE_EXPORT WeakReferenceOwner { … };

// Forward declaration from safe_ptr.h.
template <typename T>
SafeRef<T> MakeSafeRefFromWeakPtrInternals(internal::WeakReference&& ref,
                                           T* ptr);

}  // namespace internal

template <typename T> class WeakPtrFactory;

// The WeakPtr class holds a weak reference to |T*|.
//
// This class is designed to be used like a normal pointer.  You should always
// null-test an object of this class before using it or invoking a method that
// may result in the underlying object being destroyed.
//
// EXAMPLE:
//
//   class Foo { ... };
//   WeakPtr<Foo> foo;
//   if (foo)
//     foo->method();
//
template <typename T>
class TRIVIAL_ABI WeakPtr { … };

// Allow callers to compare WeakPtrs against nullptr to test validity.
template <class T>
bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) { … }
template <class T>
bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) { … }
template <class T>
bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) { … }
template <class T>
bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) { … }

namespace internal {
class BASE_EXPORT WeakPtrFactoryBase { … };
}  // namespace internal

namespace subtle {

// Restricts access to WeakPtrFactory::BindToCurrentSequence() to authorized
// callers.
class BASE_EXPORT BindWeakPtrFactoryPassKey { … };

}  // namespace subtle

// A class may be composed of a WeakPtrFactory and thereby
// control how it exposes weak pointers to itself.  This is helpful if you only
// need weak pointers within the implementation of a class.  This class is also
// useful when working with primitive types.  For example, you could have a
// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
template <class T>
class WeakPtrFactory : public internal::WeakPtrFactoryBase { … };

}  // namespace base

#endif  // BASE_MEMORY_WEAK_PTR_H_