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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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'.
*
* AtomicSupportsWeakPtr can be used for a variant with an atomically updated
* reference counter.
*
* 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:
* int num;
* void act();
* };
*
* C* ptr = new C();
*
* // Get weak pointers to ptr. The first time asWeakPtr is called
* // 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->asWeakPtr();
* WeakPtr<C> other = ptr->asWeakPtr();
*
* // Test a weak pointer for validity before using it.
* if (weak) {
* weak->num = 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/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/NullPtr.h"
#include "mozilla/RefPtr.h"
#include "mozilla/TypeTraits.h"
namespace mozilla {
template <typename T, class WeakReference> class WeakPtrBase;
template <typename T, class WeakReference> class SupportsWeakPtrBase;
namespace detail {
// This can live beyond the lifetime of the class derived from SupportsWeakPtrBase.
template<class T, RefCountAtomicity Atomicity>
class WeakReference : public RefCounted<WeakReference<T, Atomicity>, Atomicity>
{
public:
explicit WeakReference(T* p) : ptr(p) {}
T* get() const {
return ptr;
}
private:
friend class WeakPtrBase<T, WeakReference>;
friend class SupportsWeakPtrBase<T, WeakReference>;
void detach() {
ptr = nullptr;
}
T* ptr;
};
} // namespace detail
template <typename T, class WeakReference>
class SupportsWeakPtrBase
{
public:
WeakPtrBase<T, WeakReference> asWeakPtr() {
if (!weakRef)
weakRef = new WeakReference(static_cast<T*>(this));
return WeakPtrBase<T, WeakReference>(weakRef);
}
protected:
~SupportsWeakPtrBase() {
MOZ_STATIC_ASSERT((IsBaseOf<SupportsWeakPtrBase<T, WeakReference>, T>::value),
"T must derive from SupportsWeakPtrBase<T, WeakReference>");
if (weakRef)
weakRef->detach();
}
private:
friend class WeakPtrBase<T, WeakReference>;
RefPtr<WeakReference> weakRef;
};
template <typename T>
class SupportsWeakPtr
: public SupportsWeakPtrBase<T, detail::WeakReference<T, detail::NonAtomicRefCount> >
{
};
template <typename T>
class AtomicSupportsWeakPtr
: public SupportsWeakPtrBase<T, detail::WeakReference<T, detail::AtomicRefCount> >
{
};
namespace detail {
template <typename T>
struct WeakReferenceCount
{
static const RefCountAtomicity atomicity =
IsBaseOf<AtomicSupportsWeakPtr<T>, T>::value
? AtomicRefCount
: NonAtomicRefCount;
};
}
template <typename T, class WeakReference>
class WeakPtrBase
{
public:
WeakPtrBase(const WeakPtrBase<T, WeakReference>& o) : ref(o.ref) {}
// Ensure that ref is dereferenceable in the uninitialized state
WeakPtrBase() : ref(new WeakReference(nullptr)) {}
operator T*() const {
return ref->get();
}
T& operator*() const {
return *ref->get();
}
T* operator->() const {
return ref->get();
}
T* get() const {
return ref->get();
}
private:
friend class SupportsWeakPtrBase<T, WeakReference>;
explicit WeakPtrBase(const RefPtr<WeakReference> &o) : ref(o) {}
RefPtr<WeakReference> ref;
};
template <typename T>
class WeakPtr : public WeakPtrBase<T, detail::WeakReference<T, detail::WeakReferenceCount<T>::atomicity> >
{
typedef WeakPtrBase<T, detail::WeakReference<T, detail::WeakReferenceCount<T>::atomicity> > Base;
public:
WeakPtr(const WeakPtr<T>& o) : Base(o) {}
WeakPtr(const Base& o) : Base(o) {}
WeakPtr() {}
};
} // namespace mozilla
#endif /* ifdef mozilla_WeakPtr_h_ */
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