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// Copyright (c) 2012 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.
// 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.
// class Controller {
// public:
// Controller() : weak_factory_(this) {}
// 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_;
// };
// class Worker {
// public:
// static void StartNew(const WeakPtr<Controller>& controller) {
// Worker* worker = new Worker(controller);
// // Kick off asynchronous processing...
// }
// private:
// Worker(const WeakPtr<Controller>& controller)
// : controller_(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 threads, 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
// thread 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 thread to enforce that other WeakPtr objects will enforce they
// are used on the desired thread.
#include <cstddef>
#include <type_traits>
#include "base/base_export.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/sequence_checker.h"
namespace base {
template <typename T> class SupportsWeakPtr;
template <typename T> class WeakPtr;
namespace internal {
// These classes are part of the WeakPtr implementation.
class BASE_EXPORT WeakReference {
// Although Flag is bound to a specific SequencedTaskRunner, it may be
// deleted from another via base::WeakPtr::~WeakPtr().
class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> {
void Invalidate();
bool IsValid() const;
friend class base::RefCountedThreadSafe<Flag>;
SequenceChecker sequence_checker_;
bool is_valid_;
WeakReference(const WeakReference& other);
explicit WeakReference(const Flag* flag);
bool is_valid() const;
scoped_refptr<const Flag> flag_;
class BASE_EXPORT WeakReferenceOwner {
WeakReference GetRef() const;
bool HasRefs() const {
return flag_.get() && !flag_->HasOneRef();
void Invalidate();
mutable scoped_refptr<WeakReference::Flag> flag_;
// This class simplifies the implementation of WeakPtr's type conversion
// constructor by avoiding the need for a public accessor for ref_. A
// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
// base class gives us a way to access ref_ in a protected fashion.
class BASE_EXPORT WeakPtrBase {
explicit WeakPtrBase(const WeakReference& ref);
WeakReference ref_;
// This class provides a common implementation of common functions that would
// otherwise get instantiated separately for each distinct instantiation of
// SupportsWeakPtr<>.
class SupportsWeakPtrBase {
// A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
// conversion will only compile if there is exists a Base which inherits
// from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
// function that makes calling this easier.
template<typename Derived>
static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
"AsWeakPtr argument must inherit from SupportsWeakPtr");
return AsWeakPtrImpl<Derived>(t, *t);
// This template function uses type inference to find a Base of Derived
// which is an instance of SupportsWeakPtr<Base>. We can then safely
// static_cast the Base* to a Derived*.
template <typename Derived, typename Base>
static WeakPtr<Derived> AsWeakPtrImpl(
Derived* t, const SupportsWeakPtr<Base>&) {
WeakPtr<Base> ptr = t->Base::AsWeakPtr();
return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.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.
// class Foo { ... };
// WeakPtr<Foo> foo;
// if (foo)
// foo->method();
template <typename T>
class WeakPtr : public internal::WeakPtrBase {
WeakPtr() : ptr_(nullptr) {}
WeakPtr(std::nullptr_t) : ptr_(nullptr) {}
// Allow conversion from U to T provided U "is a" T. Note that this
// is separate from the (implicit) copy constructor.
template <typename U>
WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) {
T* get() const { return ref_.is_valid() ? ptr_ : nullptr; }
T& operator*() const {
DCHECK(get() != nullptr);
return *get();
T* operator->() const {
DCHECK(get() != nullptr);
return get();
void reset() {
ref_ = internal::WeakReference();
ptr_ = nullptr;
// Implement "Safe Bool Idiom"
// Allow WeakPtr<element_type> to be used in boolean expressions such as
// if (weak_ptr_instance)
// But do not become convertible to a real bool (which is dangerous).
// Implementation requires:
// typedef Testable
// operator Testable() const
// operator==
// operator!=
// == and != operators must be declared explicitly or dissallowed, as
// otherwise "ptr1 == ptr2" will compile but do the wrong thing (i.e., convert
// to Testable and then do the comparison).
// C++11 provides for "explicit operator bool()", however it is currently
// banned due to MSVS2013.
typedef T* WeakPtr::*Testable;
operator Testable() const { return get() ? &WeakPtr::ptr_ : nullptr; }
// Explicitly declare comparison operators as required by the "Safe Bool
// Idiom", but keep them private.
template <class U> bool operator==(WeakPtr<U> const&) const;
template <class U> bool operator!=(WeakPtr<U> const&) const;
friend class internal::SupportsWeakPtrBase;
template <typename U> friend class WeakPtr;
friend class SupportsWeakPtr<T>;
friend class WeakPtrFactory<T>;
WeakPtr(const internal::WeakReference& ref, T* ptr)
: WeakPtrBase(ref),
ptr_(ptr) {
// This pointer is only valid when ref_.is_valid() is true. Otherwise, its
// value is undefined (as opposed to nullptr).
T* ptr_;
// 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 {
explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
~WeakPtrFactory() { ptr_ = nullptr; }
WeakPtr<T> GetWeakPtr() {
return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
// Call this method to invalidate all existing weak pointers.
void InvalidateWeakPtrs() {
// Call this method to determine if any weak pointers exist.
bool HasWeakPtrs() const {
return weak_reference_owner_.HasRefs();
internal::WeakReferenceOwner weak_reference_owner_;
T* ptr_;
// A class may extend from SupportsWeakPtr to let others take weak pointers to
// it. This avoids the class itself implementing boilerplate to dispense weak
// pointers. However, since SupportsWeakPtr's destructor won't invalidate
// weak pointers to the class until after the derived class' members have been
// destroyed, its use can lead to subtle use-after-destroy issues.
template <class T>
class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
SupportsWeakPtr() {}
WeakPtr<T> AsWeakPtr() {
return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
~SupportsWeakPtr() {}
internal::WeakReferenceOwner weak_reference_owner_;
// Helper function that uses type deduction to safely return a WeakPtr<Derived>
// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
// extends a Base that extends SupportsWeakPtr<Base>.
// class Base : public base::SupportsWeakPtr<Producer> {};
// class Derived : public Base {};
// Derived derived;
// base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
// Note that the following doesn't work (invalid type conversion) since
// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
// the caller.
// base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails.
template <typename Derived>
WeakPtr<Derived> AsWeakPtr(Derived* t) {
return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
} // namespace base