mirror of
https://github.com/mozilla/gecko-dev.git
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144 lines
3.9 KiB
C++
144 lines
3.9 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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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 file,
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* 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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* 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 SupportsWeakPtr<C>.
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* class C : public SupportsWeakPtr<C>
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* {
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* public:
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* int num;
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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 asWeakPtr is called
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* // 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->asWeakPtr();
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* WeakPtr<C> other = ptr->asWeakPtr();
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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->num = 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/Assertions.h"
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#include "mozilla/NullPtr.h"
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#include "mozilla/RefPtr.h"
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#include "mozilla/TypeTraits.h"
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namespace mozilla {
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template <typename T> class WeakPtr;
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template <typename T>
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class SupportsWeakPtr
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{
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public:
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WeakPtr<T> asWeakPtr() {
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if (!weakRef)
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weakRef = new WeakReference(static_cast<T*>(this));
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return WeakPtr<T>(weakRef);
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}
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protected:
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~SupportsWeakPtr() {
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MOZ_STATIC_ASSERT((IsBaseOf<SupportsWeakPtr<T>, T>::value), "T must derive from SupportsWeakPtr<T>");
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if (weakRef)
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weakRef->detach();
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}
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private:
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friend class WeakPtr<T>;
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// This can live beyond the lifetime of the class derived from SupportsWeakPtr.
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class WeakReference : public RefCounted<WeakReference>
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{
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public:
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explicit WeakReference(T* ptr) : ptr(ptr) {}
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T* get() const {
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return ptr;
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}
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private:
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friend class WeakPtr<T>;
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friend class SupportsWeakPtr<T>;
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void detach() {
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ptr = nullptr;
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}
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T* ptr;
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};
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RefPtr<WeakReference> weakRef;
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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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public:
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WeakPtr(const WeakPtr<T>& o) : ref(o.ref) {}
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WeakPtr() : ref(nullptr) {}
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operator T*() const {
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return ref->get();
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}
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T& operator*() const {
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return *ref->get();
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}
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T* operator->() const {
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return ref->get();
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}
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T* get() const {
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return ref->get();
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}
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private:
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friend class SupportsWeakPtr<T>;
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explicit WeakPtr(const RefPtr<typename SupportsWeakPtr<T>::WeakReference> &o) : ref(o) {}
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RefPtr<typename SupportsWeakPtr<T>::WeakReference> ref;
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};
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} // namespace mozilla
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#endif /* ifdef mozilla_WeakPtr_h_ */
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