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.
*
* Note that when deriving from SupportsWeakPtr you should add
* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(ClassName) to the public section of your
* class, where ClassName is the name of your class.
*
* 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<C>.
* class C : public SupportsWeakPtr<C>
* {
* public:
* MOZ_DECLARE_WEAKREFERENCE_TYPENAME(C)
* 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
*/
#ifndef mozilla_WeakPtr_h
#define mozilla_WeakPtr_h
#include "mozilla/ArrayUtils.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/RefCounted.h"
#include "mozilla/RefPtr.h"
#include "mozilla/TypeTraits.h"
#include <string.h>
namespace mozilla {
template <typename T> class WeakPtr;
template <typename T> class SupportsWeakPtr;
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T) \
static const char* weakReferenceTypeName() { return "WeakReference<" #T ">"; }
#else
#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T)
#endif
namespace detail {
// This can live beyond the lifetime of the class derived from
// SupportsWeakPtr.
template<class T>
class WeakReference : public ::mozilla::RefCounted<WeakReference<T> >
{
public:
explicit WeakReference(T* p) : mPtr(p) {}
T* get() const { return mPtr; }
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
const char* typeName() const
{
// The first time this is called mPtr is null, so don't
// invoke any methods on mPtr.
return T::weakReferenceTypeName();
}
size_t typeSize() const { return sizeof(*this); }
#endif
private:
friend class mozilla::SupportsWeakPtr<T>;
void detach() { mPtr = nullptr; }
T* MOZ_NON_OWNING_REF mPtr;
};
} // namespace detail
template <typename T>
class SupportsWeakPtr
{
protected:
~SupportsWeakPtr()
{
static_assert(IsBaseOf<SupportsWeakPtr<T>, T>::value,
"T must derive from SupportsWeakPtr<T>");
if (mSelfReferencingWeakPtr) {
mSelfReferencingWeakPtr.mRef->detach();
}
}
private:
const WeakPtr<T>& SelfReferencingWeakPtr()
{
if (!mSelfReferencingWeakPtr) {
mSelfReferencingWeakPtr.mRef = new detail::WeakReference<T>(static_cast<T*>(this));
}
return mSelfReferencingWeakPtr;
}
const WeakPtr<const T>& SelfReferencingWeakPtr() const
{
const WeakPtr<T>& p = const_cast<SupportsWeakPtr*>(this)->SelfReferencingWeakPtr();
return reinterpret_cast<const WeakPtr<const T>&>(p);
}
friend class WeakPtr<T>;
friend class WeakPtr<const T>;
WeakPtr<T> mSelfReferencingWeakPtr;
};
template <typename T>
class WeakPtr
{
typedef detail::WeakReference<T> WeakReference;
public:
WeakPtr& operator=(const WeakPtr& aOther)
{
mRef = aOther.mRef;
return *this;
}
WeakPtr(const WeakPtr& aOther)
{
*this = aOther;
}
WeakPtr& operator=(T* aOther)
{
if (aOther) {
*this = aOther->SelfReferencingWeakPtr();
} else if (!mRef || mRef->get()) {
// Ensure that mRef is dereferenceable in the uninitialized state.
mRef = new WeakReference(nullptr);
}
return *this;
}
MOZ_IMPLICIT WeakPtr(T* aOther)
{
*this = aOther;
}
// Ensure that mRef is dereferenceable in the uninitialized state.
WeakPtr() : mRef(new WeakReference(nullptr)) {}
operator T*() const { return mRef->get(); }
T& operator*() const { return *mRef->get(); }
T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return mRef->get(); }
T* get() const { return mRef->get(); }
private:
friend class SupportsWeakPtr<T>;
explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
RefPtr<WeakReference> mRef;
};
} // namespace mozilla
#endif /* mozilla_WeakPtr_h */