mfbt/WeakPtr.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Weak pointer functionality, implemented as a mixin for use with any class. */
   8
   9 /**
  10  * SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
  11  * its lifetime. It works by creating a single shared reference counted object
  12  * (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
  13  * clear the pointer in the WeakReference without having to know about all of
  14  * the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
  15  * of 'Foo'.
  16  *
  17  * PLEASE NOTE: This weak pointer implementation is not thread-safe.
  18  *
  19  * Note that when deriving from SupportsWeakPtr you should add
  20  * MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public section of your
  21  * class, where ClassName is the name of your class.
  22  *
  23  * The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
  24  * dereference, and an additional heap allocated pointer sized object shared
  25  * between all of the WeakPtrs.
  26  *
  27  * Example of usage:
  28  *
  29  *   // To have a class C support weak pointers, inherit from
  30  *   // SupportsWeakPtr<C>.
  31  *   class C : public SupportsWeakPtr<C>
  32  *   {
  33  *   public:
  34  *     MOZ_DECLARE_REFCOUNTED_TYPENAME(C)
  35  *     int mNum;
  36  *     void act();
  37  *   };
  38  *
  39  *   C* ptr = new C();
  40  *
  41  *   // Get weak pointers to ptr. The first time a weak pointer
  42  *   // is obtained, a reference counted WeakReference object is created that
  43  *   // can live beyond the lifetime of 'ptr'. The WeakReference
  44  *   // object will be notified of 'ptr's destruction.
  45  *   WeakPtr<C> weak = ptr;
  46  *   WeakPtr<C> other = ptr;
  47  *
  48  *   // Test a weak pointer for validity before using it.
  49  *   if (weak) {
  50  *     weak->mNum = 17;
  51  *     weak->act();
  52  *   }
  53  *
  54  *   // Destroying the underlying object clears weak pointers to it.
  55  *   delete ptr;
  56  *
  57  *   MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
  58  *   MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
  59  *
  60  * WeakPtr is typesafe and may be used with any class. It is not required that
  61  * the class be reference-counted or allocated in any particular way.
  62  *
  63  * The API was loosely inspired by Chromium's weak_ptr.h:
  64  * http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
  65  */
  66
  67 #ifndef mozilla_WeakPtr_h
  68 #define mozilla_WeakPtr_h
  69
  70 #include "mozilla/ArrayUtils.h"
  71 #include "mozilla/Assertions.h"
  72 #include "mozilla/NullPtr.h"
  73 #include "mozilla/RefPtr.h"
  74 #include "mozilla/TypeTraits.h"
  75
  76 #include <string.h>
  77
  78 namespace mozilla {
  79
  80 template <typename T> class WeakPtr;
  81 template <typename T> class SupportsWeakPtr;
  82
  83 namespace detail {
  84
  85 // This can live beyond the lifetime of the class derived from
  86 // SupportsWeakPtr.
  87 template<class T>
  88 class WeakReference : public ::mozilla::RefCounted<WeakReference<T> >
  89 {
  90 public:
  91   explicit WeakReference(T* p) : mPtr(p) {}
  92
  93   T* get() const { return mPtr; }
  94
  95 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
  96 #ifdef XP_WIN
  97 #define snprintf _snprintf
  98 #endif
  99   const char* typeName() const
 100   {
 101     static char nameBuffer[1024];
 102     const char* innerType = mPtr->typeName();
 103     // We could do fancier length checks at runtime, but innerType is
 104     // controlled by us so we can ensure that this never causes a buffer
 105     // overflow by this assertion.
 106     MOZ_ASSERT(strlen(innerType) + sizeof("WeakReference<>") <
 107                ArrayLength(nameBuffer),
 108                "Exceedingly large type name");
 109     snprintf(nameBuffer, ArrayLength(nameBuffer), "WeakReference<%s>",
 110              innerType);
 111     // This is usually not OK, but here we are returning a pointer to a static
 112     // buffer which will immediately be used by the caller.
 113     return nameBuffer;
 114   }
 115
 116   size_t typeSize() const { return sizeof(*this); }
 117 #undef snprintf
 118 #endif
 119
 120 private:
 121   friend class mozilla::SupportsWeakPtr<T>;
 122
 123   void detach() { mPtr = nullptr; }
 124
 125   T* mPtr;
 126 };
 127
 128 } // namespace detail
 129
 130 template <typename T>
 131 class SupportsWeakPtr
 132 {
 133 protected:
 134   ~SupportsWeakPtr()
 135   {
 136     static_assert(IsBaseOf<SupportsWeakPtr<T>, T>::value,
 137                   "T must derive from SupportsWeakPtr<T>");
 138     if (mSelfReferencingWeakPtr) {
 139       mSelfReferencingWeakPtr.mRef->detach();
 140     }
 141   }
 142
 143 private:
 144   const WeakPtr<T>& SelfReferencingWeakPtr()
 145   {
 146     if (!mSelfReferencingWeakPtr) {
 147       mSelfReferencingWeakPtr.mRef = new detail::WeakReference<T>(static_cast<T*>(this));
 148     }
 149     return mSelfReferencingWeakPtr;
 150   }
 151
 152   const WeakPtr<const T>& SelfReferencingWeakPtr() const
 153   {
 154     const WeakPtr<T>& p = const_cast<SupportsWeakPtr*>(this)->SelfReferencingWeakPtr();
 155     return reinterpret_cast<const WeakPtr<const T>&>(p);
 156   }
 157
 158   friend class WeakPtr<T>;
 159   friend class WeakPtr<const T>;
 160
 161   WeakPtr<T> mSelfReferencingWeakPtr;
 162 };
 163
 164 template <typename T>
 165 class WeakPtr
 166 {
 167   typedef detail::WeakReference<T> WeakReference;
 168
 169 public:
 170   WeakPtr& operator=(const WeakPtr& aOther)
 171   {
 172     mRef = aOther.mRef;
 173     return *this;
 174   }
 175
 176   WeakPtr(const WeakPtr& aOther)
 177   {
 178     *this = aOther;
 179   }
 180
 181   WeakPtr& operator=(T* aOther)
 182   {
 183     return *this = aOther->SelfReferencingWeakPtr();
 184   }
 185
 186   MOZ_IMPLICIT WeakPtr(T* aOther)
 187   {
 188     *this = aOther;
 189   }
 190
 191   // Ensure that mRef is dereferenceable in the uninitialized state.
 192   WeakPtr() : mRef(new WeakReference(nullptr)) {}
 193
 194   operator T*() const { return mRef->get(); }
 195   T& operator*() const { return *mRef->get(); }
 196
 197   T* operator->() const { return mRef->get(); }
 198
 199   T* get() const { return mRef->get(); }
 200
 201 private:
 202   friend class SupportsWeakPtr<T>;
 203
 204   explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
 205
 206   RefPtr<WeakReference> mRef;
 207 };
 208
 209 } // namespace mozilla
 210
 211 #endif /* mozilla_WeakPtr_h */