mirror of
https://github.com/mozilla/gecko-dev.git
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273 lines
8.1 KiB
C++
273 lines
8.1 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Weak pointer functionality, implemented as a mixin for use with any class. */
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/**
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* SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
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* its lifetime. It works by creating a single shared reference counted object
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* (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
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* clear the pointer in the WeakReference without having to know about all of
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* the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
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* of 'Foo'.
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*
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* PLEASE NOTE: This weak pointer implementation is not thread-safe.
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*
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* Note that when deriving from SupportsWeakPtr you should add
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* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(ClassName) to the public section of your
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* class, where ClassName is the name of your class.
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*
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* The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
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* dereference, and an additional heap allocated pointer sized object shared
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* between all of the WeakPtrs.
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*
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* Example of usage:
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*
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* // To have a class C support weak pointers, inherit from
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* // SupportsWeakPtr<C>.
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* class C : public SupportsWeakPtr<C>
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* {
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* public:
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* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(C)
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* int mNum;
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* void act();
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* };
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*
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* C* ptr = new C();
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*
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* // Get weak pointers to ptr. The first time a weak pointer
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* // is obtained, a reference counted WeakReference object is created that
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* // can live beyond the lifetime of 'ptr'. The WeakReference
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* // object will be notified of 'ptr's destruction.
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* WeakPtr<C> weak = ptr;
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* WeakPtr<C> other = ptr;
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*
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* // Test a weak pointer for validity before using it.
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* if (weak) {
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* weak->mNum = 17;
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* weak->act();
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* }
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*
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* // Destroying the underlying object clears weak pointers to it.
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* delete ptr;
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*
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* MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
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* MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
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*
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* WeakPtr is typesafe and may be used with any class. It is not required that
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* the class be reference-counted or allocated in any particular way.
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*
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* The API was loosely inspired by Chromium's weak_ptr.h:
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* http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
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*/
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#ifndef mozilla_WeakPtr_h
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#define mozilla_WeakPtr_h
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#include "mozilla/ArrayUtils.h"
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/RefCounted.h"
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#include "mozilla/RefPtr.h"
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#include "mozilla/TypeTraits.h"
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#include <string.h>
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// Weak referencing is not implemeted as thread safe. When a WeakPtr
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// is created or dereferenced on thread A but the real object is just
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// being Released() on thread B, there is a possibility of a race
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// when the proxy object (detail::WeakReference) is notified about
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// the real object destruction just between when thread A is storing
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// the object pointer locally and is about to add a reference to it.
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//
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// Hence, a non-null weak proxy object is considered to have a single
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// "owning thread". It means that each query for a weak reference,
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// its dereference, and destruction of the real object must all happen
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// on a single thread. The following macros implement assertions for
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// checking these conditions.
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#if defined(DEBUG) || (defined(NIGHTLY_BUILD) && !defined(MOZ_PROFILING))
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#include <thread>
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#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK \
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std::thread::id _owningThread; \
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bool _empty; // If it was initialized as a placeholder with mPtr = nullptr.
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#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() \
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do { \
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_owningThread = std::this_thread::get_id(); \
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_empty = !p; \
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} while (false)
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#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() \
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MOZ_DIAGNOSTIC_ASSERT(_empty || _owningThread == std::this_thread::get_id(), \
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"WeakPtr used on multiple threads")
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#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) \
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(that)->AssertThreadSafety();
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#define MOZ_WEAKPTR_THREAD_SAFETY_CHECKING 1
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#else
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#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
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#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() do { } while (false)
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#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() do { } while (false)
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#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) do { } while (false)
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#endif
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namespace mozilla {
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template <typename T> class WeakPtr;
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template <typename T> class SupportsWeakPtr;
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T) \
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static const char* weakReferenceTypeName() { return "WeakReference<" #T ">"; }
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#else
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#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T)
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#endif
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namespace detail {
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// This can live beyond the lifetime of the class derived from
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// SupportsWeakPtr.
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template<class T>
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class WeakReference : public ::mozilla::RefCounted<WeakReference<T> >
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{
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public:
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explicit WeakReference(T* p) : mPtr(p)
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{
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MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK();
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}
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T* get() const {
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MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
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return mPtr;
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}
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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const char* typeName() const
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{
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// The first time this is called mPtr is null, so don't
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// invoke any methods on mPtr.
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return T::weakReferenceTypeName();
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}
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size_t typeSize() const { return sizeof(*this); }
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#endif
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#ifdef MOZ_WEAKPTR_THREAD_SAFETY_CHECKING
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void AssertThreadSafety() { MOZ_WEAKPTR_ASSERT_THREAD_SAFETY(); }
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#endif
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private:
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friend class mozilla::SupportsWeakPtr<T>;
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void detach() {
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MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
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mPtr = nullptr;
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}
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T* MOZ_NON_OWNING_REF mPtr;
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MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
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};
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} // namespace detail
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template <typename T>
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class SupportsWeakPtr
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{
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protected:
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~SupportsWeakPtr()
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{
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static_assert(IsBaseOf<SupportsWeakPtr<T>, T>::value,
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"T must derive from SupportsWeakPtr<T>");
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if (mSelfReferencingWeakPtr) {
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mSelfReferencingWeakPtr.mRef->detach();
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}
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}
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private:
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const WeakPtr<T>& SelfReferencingWeakPtr()
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{
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if (!mSelfReferencingWeakPtr) {
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mSelfReferencingWeakPtr.mRef = new detail::WeakReference<T>(static_cast<T*>(this));
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} else {
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MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mSelfReferencingWeakPtr.mRef);
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}
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return mSelfReferencingWeakPtr;
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}
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const WeakPtr<const T>& SelfReferencingWeakPtr() const
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{
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const WeakPtr<T>& p = const_cast<SupportsWeakPtr*>(this)->SelfReferencingWeakPtr();
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return reinterpret_cast<const WeakPtr<const T>&>(p);
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}
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friend class WeakPtr<T>;
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friend class WeakPtr<const T>;
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WeakPtr<T> mSelfReferencingWeakPtr;
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};
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template <typename T>
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class WeakPtr
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{
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typedef detail::WeakReference<T> WeakReference;
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public:
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WeakPtr& operator=(const WeakPtr& aOther)
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{
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mRef = aOther.mRef;
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MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
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return *this;
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}
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WeakPtr(const WeakPtr& aOther)
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{
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// The thread safety check is performed inside of the operator= method.
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*this = aOther;
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}
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WeakPtr& operator=(T* aOther)
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{
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if (aOther) {
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*this = aOther->SelfReferencingWeakPtr();
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} else if (!mRef || mRef->get()) {
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// Ensure that mRef is dereferenceable in the uninitialized state.
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mRef = new WeakReference(nullptr);
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}
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// The thread safety check happens inside SelfReferencingWeakPtr
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// or is initialized in the WeakReference constructor.
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return *this;
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}
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MOZ_IMPLICIT WeakPtr(T* aOther)
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{
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*this = aOther;
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MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
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}
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// Ensure that mRef is dereferenceable in the uninitialized state.
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WeakPtr() : mRef(new WeakReference(nullptr)) {}
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operator T*() const { return mRef->get(); }
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T& operator*() const { return *mRef->get(); }
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T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return mRef->get(); }
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T* get() const { return mRef->get(); }
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private:
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friend class SupportsWeakPtr<T>;
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explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
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RefPtr<WeakReference> mRef;
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};
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} // namespace mozilla
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#endif /* mozilla_WeakPtr_h */
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