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gecko-dev/mfbt/WeakPtr.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* Weak pointer functionality, implemented as a mixin for use with any class. */
/**
* SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
* its lifetime. It works by creating a single shared reference counted object
* (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
* clear the pointer in the WeakReference without having to know about all of
* the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
* of 'Foo'.
*
* PLEASE NOTE: This weak pointer implementation is not thread-safe.
*
* The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
* dereference, and an additional heap allocated pointer sized object shared
* between all of the WeakPtrs.
*
* Example of usage:
*
* // To have a class C support weak pointers, inherit from
* // SupportsWeakPtr
* class C : public SupportsWeakPtr
* {
* public:
* int mNum;
* void act();
* };
*
* C* ptr = new C();
*
* // Get weak pointers to ptr. The first time a weak pointer
* // is obtained, a reference counted WeakReference object is created that
* // can live beyond the lifetime of 'ptr'. The WeakReference
* // object will be notified of 'ptr's destruction.
* WeakPtr<C> weak = ptr;
* WeakPtr<C> other = ptr;
*
* // Test a weak pointer for validity before using it.
* if (weak) {
* weak->mNum = 17;
* weak->act();
* }
*
* // Destroying the underlying object clears weak pointers to it.
* delete ptr;
*
* MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
* MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
*
* WeakPtr is typesafe and may be used with any class. It is not required that
* the class be reference-counted or allocated in any particular way.
*
* The API was loosely inspired by Chromium's weak_ptr.h:
* http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
*
* Note that multiple base classes inheriting from SupportsWeakPtr is not
* currently supported. We could support it if needed though.
*
* For Gecko-internal usage there is also MainThreadWeakPtr<T>, a version of
* WeakPtr that can be destroyed on any thread, but whose release gets proxied
* to the main thread. This is a similar API to nsMainThreadPtrHandle, but
* without keeping a strong reference to the main-thread object. Said WeakPtr
* can't be accessed from any other thread that isn't the main thread.
*/
#ifndef mozilla_WeakPtr_h
#define mozilla_WeakPtr_h
#include "mozilla/ArrayUtils.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Maybe.h"
#include "mozilla/RefCounted.h"
#include "mozilla/RefPtr.h"
#include <string.h>
#include <type_traits>
#if defined(MOZILLA_INTERNAL_API)
// For thread safety checking.
# include "nsISupportsImpl.h"
// For main thread destructor behavior.
# include "nsProxyRelease.h"
#endif
#if defined(MOZILLA_INTERNAL_API) && \
defined(MOZ_THREAD_SAFETY_OWNERSHIP_CHECKS_SUPPORTED)
// Weak referencing is not implemented as thread safe. When a WeakPtr
// is created or dereferenced on thread A but the real object is just
// being Released() on thread B, there is a possibility of a race
// when the proxy object (detail::WeakReference) is notified about
// the real object destruction just between when thread A is storing
// the object pointer locally and is about to add a reference to it.
//
// Hence, a non-null weak proxy object is considered to have a single
// "owning thread". It means that each query for a weak reference,
// its dereference, and destruction of the real object must all happen
// on a single thread. The following macros implement assertions for
// checking these conditions.
//
// We re-use XPCOM's nsAutoOwningEventTarget checks when they are available.
// This has the advantage that it works with cooperative thread pools.
# define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK \
/* Will be none if mPtr = nullptr. */ \
Maybe<nsAutoOwningEventTarget> _owningThread;
# define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() \
do { \
if (p) { \
_owningThread.emplace(); \
} \
} while (false)
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() \
do { \
MOZ_DIAGNOSTIC_ASSERT( \
!_owningThread || _owningThread->IsCurrentThread(), \
"WeakPtr accessed from multiple threads"); \
} while (false)
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) \
(that)->AssertThreadSafety();
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED_IF(that) \
do { \
if (that) { \
(that)->AssertThreadSafety(); \
} \
} while (false)
# define MOZ_WEAKPTR_THREAD_SAFETY_CHECKING 1
#else
# define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
# define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() \
do { \
} while (false)
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() \
do { \
} while (false)
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) \
do { \
} while (false)
# define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED_IF(that) \
do { \
} while (false)
#endif
namespace mozilla {
namespace detail {
enum class WeakPtrDestructorBehavior {
Normal,
#ifdef MOZILLA_INTERNAL_API
ProxyToMainThread,
#endif
};
} // namespace detail
template <typename T, detail::WeakPtrDestructorBehavior =
detail::WeakPtrDestructorBehavior::Normal>
class WeakPtr;
class SupportsWeakPtr;
namespace detail {
// This can live beyond the lifetime of the class derived from
// SupportsWeakPtr.
class WeakReference : public ::mozilla::RefCounted<WeakReference> {
public:
explicit WeakReference(const SupportsWeakPtr* p)
: mPtr(const_cast<SupportsWeakPtr*>(p)) {
MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK();
}
SupportsWeakPtr* get() const {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
return mPtr;
}
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
const char* typeName() const { return "WeakReference"; }
size_t typeSize() const { return sizeof(*this); }
#endif
#ifdef MOZ_WEAKPTR_THREAD_SAFETY_CHECKING
void AssertThreadSafety() { MOZ_WEAKPTR_ASSERT_THREAD_SAFETY(); }
#endif
private:
friend class mozilla::SupportsWeakPtr;
void detach() {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
mPtr = nullptr;
}
SupportsWeakPtr* MOZ_NON_OWNING_REF mPtr;
MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
};
} // namespace detail
class SupportsWeakPtr {
using WeakReference = detail::WeakReference;
protected:
~SupportsWeakPtr() { DetachWeakPtr(); }
protected:
void DetachWeakPtr() {
if (mSelfReferencingWeakReference) {
mSelfReferencingWeakReference->detach();
}
}
private:
WeakReference* SelfReferencingWeakReference() const {
if (!mSelfReferencingWeakReference) {
mSelfReferencingWeakReference = new WeakReference(this);
} else {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mSelfReferencingWeakReference);
}
return mSelfReferencingWeakReference.get();
}
template <typename U, detail::WeakPtrDestructorBehavior>
friend class WeakPtr;
mutable RefPtr<WeakReference> mSelfReferencingWeakReference;
};
template <typename T, detail::WeakPtrDestructorBehavior Destruct>
class WeakPtr {
using WeakReference = detail::WeakReference;
public:
WeakPtr& operator=(const WeakPtr& aOther) {
// We must make sure the reference we have now is safe to be dereferenced
// before we throw it away... (this can be called from a ctor)
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED_IF(mRef);
// ...and make sure the new reference is used on a single thread as well.
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(aOther.mRef);
mRef = aOther.mRef;
return *this;
}
WeakPtr(const WeakPtr& aOther) {
// The thread safety check is performed inside of the operator= method.
*this = aOther;
}
WeakPtr& operator=(decltype(nullptr)) {
// We must make sure the reference we have now is safe to be dereferenced
// before we throw it away.
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED_IF(mRef);
if (!mRef || mRef->get()) {
// Ensure that mRef is dereferenceable in the uninitialized state.
mRef = new WeakReference(nullptr);
}
return *this;
}
WeakPtr& operator=(const T* aOther) {
// We must make sure the reference we have now is safe to be dereferenced
// before we throw it away.
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED_IF(mRef);
if (aOther) {
mRef = aOther->SelfReferencingWeakReference();
} else if (!mRef || mRef->get()) {
// Ensure that mRef is dereferenceable in the uninitialized state.
mRef = new WeakReference(nullptr);
}
// The thread safety check happens inside SelfReferencingWeakPtr
// or is initialized in the WeakReference constructor.
return *this;
}
MOZ_IMPLICIT WeakPtr(T* aOther) {
*this = aOther;
#ifdef MOZILLA_INTERNAL_API
if (Destruct == detail::WeakPtrDestructorBehavior::ProxyToMainThread) {
MOZ_ASSERT(NS_IsMainThread(),
"MainThreadWeakPtr makes no sense on non-main threads");
}
#endif
}
explicit WeakPtr(const RefPtr<T>& aOther) : WeakPtr(aOther.get()) {}
// Ensure that mRef is dereferenceable in the uninitialized state.
WeakPtr() : mRef(new WeakReference(nullptr)) {}
explicit operator bool() const { return mRef->get(); }
T* get() const { return static_cast<T*>(mRef->get()); }
operator T*() const { return get(); }
T& operator*() const { return *get(); }
T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return get(); }
#ifdef MOZILLA_INTERNAL_API
~WeakPtr() {
if (Destruct == detail::WeakPtrDestructorBehavior::ProxyToMainThread) {
NS_ReleaseOnMainThread("WeakPtr::mRef", mRef.forget());
} else {
MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
}
}
#endif
private:
friend class SupportsWeakPtr;
explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
RefPtr<WeakReference> mRef;
};
#ifdef MOZILLA_INTERNAL_API
template <typename T>
using MainThreadWeakPtr =
WeakPtr<T, detail::WeakPtrDestructorBehavior::ProxyToMainThread>;
#endif
#define NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_PTR tmp->DetachWeakPtr();
#define NS_IMPL_CYCLE_COLLECTION_WEAK_PTR(class_, ...) \
NS_IMPL_CYCLE_COLLECTION_CLASS(class_) \
NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(class_) \
NS_IMPL_CYCLE_COLLECTION_UNLINK(__VA_ARGS__) \
NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_PTR \
NS_IMPL_CYCLE_COLLECTION_UNLINK_END \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(class_) \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE(__VA_ARGS__) \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
#define NS_IMPL_CYCLE_COLLECTION_WEAK_PTR_INHERITED(class_, super_, ...) \
NS_IMPL_CYCLE_COLLECTION_CLASS(class_) \
NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(class_, super_) \
NS_IMPL_CYCLE_COLLECTION_UNLINK(__VA_ARGS__) \
NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_PTR \
NS_IMPL_CYCLE_COLLECTION_UNLINK_END \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(class_, super_) \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE(__VA_ARGS__) \
NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
} // namespace mozilla
#endif /* mozilla_WeakPtr_h */
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