WeakPtr.h

/* -*- 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 */