mfbt/RefPtr.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 /* Helpers for defining and using refcounted objects. */
   8
   9 #ifndef mozilla_RefPtr_h
  10 #define mozilla_RefPtr_h
  11
  12 #include "mozilla/Assertions.h"
  13 #include "mozilla/Atomics.h"
  14 #include "mozilla/Attributes.h"
  15 #include "mozilla/NullPtr.h"
  16 #include "mozilla/RefCountType.h"
  17 #include "mozilla/TypeTraits.h"
  18 #if defined(MOZILLA_INTERNAL_API)
  19 #include "nsXPCOM.h"
  20 #endif
  21
  22 #if defined(MOZILLA_INTERNAL_API) && \
  23     (defined(DEBUG) || defined(FORCE_BUILD_REFCNT_LOGGING))
  24 #define MOZ_REFCOUNTED_LEAK_CHECKING
  25 #endif
  26
  27 namespace mozilla {
  28
  29 template<typename T> class RefCounted;
  30 template<typename T> class RefPtr;
  31 template<typename T> class TemporaryRef;
  32 template<typename T> class OutParamRef;
  33 template<typename T> OutParamRef<T> byRef(RefPtr<T>&);
  34
  35 /**
  36  * RefCounted<T> is a sort of a "mixin" for a class T.  RefCounted
  37  * manages, well, refcounting for T, and because RefCounted is
  38  * parameterized on T, RefCounted<T> can call T's destructor directly.
  39  * This means T doesn't need to have a virtual dtor and so doesn't
  40  * need a vtable.
  41  *
  42  * RefCounted<T> is created with refcount == 0.  Newly-allocated
  43  * RefCounted<T> must immediately be assigned to a RefPtr to make the
  44  * refcount > 0.  It's an error to allocate and free a bare
  45  * RefCounted<T>, i.e. outside of the RefPtr machinery.  Attempts to
  46  * do so will abort DEBUG builds.
  47  *
  48  * Live RefCounted<T> have refcount > 0.  The lifetime (refcounts) of
  49  * live RefCounted<T> are controlled by RefPtr<T> and
  50  * RefPtr<super/subclass of T>.  Upon a transition from refcounted==1
  51  * to 0, the RefCounted<T> "dies" and is destroyed.  The "destroyed"
  52  * state is represented in DEBUG builds by refcount==0xffffdead.  This
  53  * state distinguishes use-before-ref (refcount==0) from
  54  * use-after-destroy (refcount==0xffffdead).
  55  *
  56  * Note that when deriving from RefCounted or AtomicRefCounted, you
  57  * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
  58  * section of your class, where ClassName is the name of your class.
  59  */
  60 namespace detail {
  61 #ifdef DEBUG
  62 const MozRefCountType DEAD = 0xffffdead;
  63 #endif
  64
  65 // When building code that gets compiled into Gecko, try to use the
  66 // trace-refcount leak logging facilities.
  67 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
  68 class RefCountLogger
  69 {
  70 public:
  71   static void logAddRef(const void* aPointer, MozRefCountType aRefCount,
  72                         const char* aTypeName, uint32_t aInstanceSize)
  73   {
  74     MOZ_ASSERT(aRefCount != DEAD);
  75     NS_LogAddRef(const_cast<void*>(aPointer), aRefCount, aTypeName,
  76                  aInstanceSize);
  77   }
  78
  79   static void logRelease(const void* aPointer, MozRefCountType aRefCount,
  80                          const char* aTypeName)
  81   {
  82     MOZ_ASSERT(aRefCount != DEAD);
  83     NS_LogRelease(const_cast<void*>(aPointer), aRefCount, aTypeName);
  84   }
  85 };
  86 #endif
  87
  88 // This is used WeakPtr.h as well as this file.
  89 enum RefCountAtomicity
  90 {
  91   AtomicRefCount,
  92   NonAtomicRefCount
  93 };
  94
  95 template<typename T, RefCountAtomicity Atomicity>
  96 class RefCounted
  97 {
  98   friend class RefPtr<T>;
  99
 100 protected:
 101   RefCounted() : mRefCnt(0) {}
 102   ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }
 103
 104 public:
 105   // Compatibility with nsRefPtr.
 106   void AddRef() const
 107   {
 108     // Note: this method must be thread safe for AtomicRefCounted.
 109     MOZ_ASSERT(int32_t(mRefCnt) >= 0);
 110 #ifndef MOZ_REFCOUNTED_LEAK_CHECKING
 111     ++mRefCnt;
 112 #else
 113     const char* type = static_cast<const T*>(this)->typeName();
 114     uint32_t size = static_cast<const T*>(this)->typeSize();
 115     const void* ptr = static_cast<const T*>(this);
 116     MozRefCountType cnt = ++mRefCnt;
 117     detail::RefCountLogger::logAddRef(ptr, cnt, type, size);
 118 #endif
 119   }
 120
 121   void Release() const
 122   {
 123     // Note: this method must be thread safe for AtomicRefCounted.
 124     MOZ_ASSERT(int32_t(mRefCnt) > 0);
 125 #ifndef MOZ_REFCOUNTED_LEAK_CHECKING
 126     MozRefCountType cnt = --mRefCnt;
 127 #else
 128     const char* type = static_cast<const T*>(this)->typeName();
 129     const void* ptr = static_cast<const T*>(this);
 130     MozRefCountType cnt = --mRefCnt;
 131     // Note: it's not safe to touch |this| after decrementing the refcount,
 132     // except for below.
 133     detail::RefCountLogger::logRelease(ptr, cnt, type);
 134 #endif
 135     if (0 == cnt) {
 136       // Because we have atomically decremented the refcount above, only
 137       // one thread can get a 0 count here, so as long as we can assume that
 138       // everything else in the system is accessing this object through
 139       // RefPtrs, it's safe to access |this| here.
 140 #ifdef DEBUG
 141       mRefCnt = detail::DEAD;
 142 #endif
 143       delete static_cast<const T*>(this);
 144     }
 145   }
 146
 147   // Compatibility with wtf::RefPtr.
 148   void ref() { AddRef(); }
 149   void deref() { Release(); }
 150   MozRefCountType refCount() const { return mRefCnt; }
 151   bool hasOneRef() const
 152   {
 153     MOZ_ASSERT(mRefCnt > 0);
 154     return mRefCnt == 1;
 155   }
 156
 157 private:
 158   mutable typename Conditional<Atomicity == AtomicRefCount,
 159                                Atomic<MozRefCountType>,
 160                                MozRefCountType>::Type mRefCnt;
 161 };
 162
 163 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
 164 #define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T) \
 165   virtual const char* typeName() const { return #T; } \
 166   virtual size_t typeSize() const { return sizeof(*this); }
 167 #else
 168 #define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T)
 169 #endif
 170
 171 // Note that this macro is expanded unconditionally because it declares only
 172 // two small inline functions which will hopefully get eliminated by the linker
 173 // in non-leak-checking builds.
 174 #define MOZ_DECLARE_REFCOUNTED_TYPENAME(T) \
 175   const char* typeName() const { return #T; } \
 176   size_t typeSize() const { return sizeof(*this); }
 177
 178 } // namespace detail
 179
 180 template<typename T>
 181 class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount>
 182 {
 183 public:
 184   ~RefCounted()
 185   {
 186     static_assert(IsBaseOf<RefCounted, T>::value,
 187                   "T must derive from RefCounted<T>");
 188   }
 189 };
 190
 191 namespace external {
 192
 193 /**
 194  * AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
 195  * reference counter.
 196  *
 197  * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
 198  * instead.
 199  */
 200 template<typename T>
 201 class AtomicRefCounted :
 202   public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount>
 203 {
 204 public:
 205   ~AtomicRefCounted()
 206   {
 207     static_assert(IsBaseOf<AtomicRefCounted, T>::value,
 208                   "T must derive from AtomicRefCounted<T>");
 209   }
 210 };
 211
 212 } // namespace external
 213
 214 /**
 215  * RefPtr points to a refcounted thing that has AddRef and Release
 216  * methods to increase/decrease the refcount, respectively.  After a
 217  * RefPtr<T> is assigned a T*, the T* can be used through the RefPtr
 218  * as if it were a T*.
 219  *
 220  * A RefPtr can forget its underlying T*, which results in the T*
 221  * being wrapped in a temporary object until the T* is either
 222  * re-adopted from or released by the temporary.
 223  */
 224 template<typename T>
 225 class RefPtr
 226 {
 227   // To allow them to use unref()
 228   friend class TemporaryRef<T>;
 229   friend class OutParamRef<T>;
 230
 231   struct DontRef {};
 232
 233 public:
 234   RefPtr() : mPtr(0) {}
 235   RefPtr(const RefPtr& aOther) : mPtr(ref(aOther.mPtr)) {}
 236   MOZ_IMPLICIT RefPtr(const TemporaryRef<T>& aOther) : mPtr(aOther.drop()) {}
 237   MOZ_IMPLICIT RefPtr(T* aVal) : mPtr(ref(aVal)) {}
 238
 239   template<typename U>
 240   RefPtr(const RefPtr<U>& aOther) : mPtr(ref(aOther.get())) {}
 241
 242   ~RefPtr() { unref(mPtr); }
 243
 244   RefPtr& operator=(const RefPtr& aOther)
 245   {
 246     assign(ref(aOther.mPtr));
 247     return *this;
 248   }
 249   RefPtr& operator=(const TemporaryRef<T>& aOther)
 250   {
 251     assign(aOther.drop());
 252     return *this;
 253   }
 254   RefPtr& operator=(T* aVal)
 255   {
 256     assign(ref(aVal));
 257     return *this;
 258   }
 259
 260   template<typename U>
 261   RefPtr& operator=(const RefPtr<U>& aOther)
 262   {
 263     assign(ref(aOther.get()));
 264     return *this;
 265   }
 266
 267   TemporaryRef<T> forget()
 268   {
 269     T* tmp = mPtr;
 270     mPtr = nullptr;
 271     return TemporaryRef<T>(tmp, DontRef());
 272   }
 273
 274   T* get() const { return mPtr; }
 275   operator T*() const { return mPtr; }
 276   T* operator->() const { return mPtr; }
 277   T& operator*() const { return *mPtr; }
 278   template<typename U>
 279   operator TemporaryRef<U>() { return TemporaryRef<U>(mPtr); }
 280
 281 private:
 282   void assign(T* aVal)
 283   {
 284     unref(mPtr);
 285     mPtr = aVal;
 286   }
 287
 288   T* mPtr;
 289
 290   static MOZ_ALWAYS_INLINE T* ref(T* aVal)
 291   {
 292     if (aVal) {
 293       aVal->AddRef();
 294     }
 295     return aVal;
 296   }
 297
 298   static MOZ_ALWAYS_INLINE void unref(T* aVal)
 299   {
 300     if (aVal) {
 301       aVal->Release();
 302     }
 303   }
 304 };
 305
 306 /**
 307  * TemporaryRef<T> represents an object that holds a temporary
 308  * reference to a T.  TemporaryRef objects can't be manually ref'd or
 309  * unref'd (being temporaries, not lvalues), so can only relinquish
 310  * references to other objects, or unref on destruction.
 311  */
 312 template<typename T>
 313 class TemporaryRef
 314 {
 315   // To allow it to construct TemporaryRef from a bare T*
 316   friend class RefPtr<T>;
 317
 318   typedef typename RefPtr<T>::DontRef DontRef;
 319
 320 public:
 321   MOZ_IMPLICIT TemporaryRef(T* aVal) : mPtr(RefPtr<T>::ref(aVal)) {}
 322   TemporaryRef(const TemporaryRef& aOther) : mPtr(aOther.drop()) {}
 323
 324   template<typename U>
 325   TemporaryRef(const TemporaryRef<U>& aOther) : mPtr(aOther.drop()) {}
 326
 327   ~TemporaryRef() { RefPtr<T>::unref(mPtr); }
 328
 329   T* drop() const
 330   {
 331     T* tmp = mPtr;
 332     mPtr = nullptr;
 333     return tmp;
 334   }
 335
 336 private:
 337   TemporaryRef(T* aVal, const DontRef&) : mPtr(aVal) {}
 338
 339   mutable T* mPtr;
 340
 341   TemporaryRef() MOZ_DELETE;
 342   void operator=(const TemporaryRef&) MOZ_DELETE;
 343 };
 344
 345 /**
 346  * OutParamRef is a wrapper that tracks a refcounted pointer passed as
 347  * an outparam argument to a function.  OutParamRef implements COM T**
 348  * outparam semantics: this requires the callee to AddRef() the T*
 349  * returned through the T** outparam on behalf of the caller.  This
 350  * means the caller (through OutParamRef) must Release() the old
 351  * object contained in the tracked RefPtr.  It's OK if the callee
 352  * returns the same T* passed to it through the T** outparam, as long
 353  * as the callee obeys the COM discipline.
 354  *
 355  * Prefer returning TemporaryRef<T> from functions over creating T**
 356  * outparams and passing OutParamRef<T> to T**.  Prefer RefPtr<T>*
 357  * outparams over T** outparams.
 358  */
 359 template<typename T>
 360 class OutParamRef
 361 {
 362   friend OutParamRef byRef<T>(RefPtr<T>&);
 363
 364 public:
 365   ~OutParamRef()
 366   {
 367     RefPtr<T>::unref(mRefPtr.mPtr);
 368     mRefPtr.mPtr = mTmp;
 369   }
 370
 371   operator T**() { return &mTmp; }
 372
 373 private:
 374   explicit OutParamRef(RefPtr<T>& p) : mRefPtr(p), mTmp(p.get()) {}
 375
 376   RefPtr<T>& mRefPtr;
 377   T* mTmp;
 378
 379   OutParamRef() MOZ_DELETE;
 380   OutParamRef& operator=(const OutParamRef&) MOZ_DELETE;
 381 };
 382
 383 /**
 384  * byRef cooperates with OutParamRef to implement COM outparam semantics.
 385  */
 386 template<typename T>
 387 OutParamRef<T>
 388 byRef(RefPtr<T>& aPtr)
 389 {
 390   return OutParamRef<T>(aPtr);
 391 }
 392
 393 } // namespace mozilla
 394
 395 #endif /* mozilla_RefPtr_h */