base/ref_counted.h

   1 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_REF_COUNTED_H_
   6 #define BASE_REF_COUNTED_H_
   7
   8 #include "base/atomic_ref_count.h"
   9 #include "base/thread_collision_warner.h"
  10
  11 namespace base {
  12
  13 namespace subtle {
  14
  15 class RefCountedBase {
  16  public:
  17   static bool ImplementsThreadSafeReferenceCounting() { return false; }
  18
  19   bool HasOneRef() const { return ref_count_ == 1; }
  20
  21  protected:
  22   RefCountedBase();
  23   ~RefCountedBase();
  24
  25   void AddRef();
  26
  27   // Returns true if the object should self-delete.
  28   bool Release();
  29
  30  private:
  31   int ref_count_;
  32 #ifndef NDEBUG
  33   bool in_dtor_;
  34 #endif
  35
  36   DFAKE_MUTEX(add_release_);
  37
  38   DISALLOW_COPY_AND_ASSIGN(RefCountedBase);
  39 };
  40
  41 class RefCountedThreadSafeBase {
  42  public:
  43   static bool ImplementsThreadSafeReferenceCounting() { return true; }
  44
  45   bool HasOneRef() const;
  46
  47  protected:
  48   RefCountedThreadSafeBase();
  49   ~RefCountedThreadSafeBase();
  50
  51   void AddRef();
  52
  53   // Returns true if the object should self-delete.
  54   bool Release();
  55
  56  private:
  57   AtomicRefCount ref_count_;
  58 #ifndef NDEBUG
  59   bool in_dtor_;
  60 #endif
  61
  62   DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafeBase);
  63 };
  64
  65 }  // namespace subtle
  66
  67 //
  68 // A base class for reference counted classes.  Otherwise, known as a cheap
  69 // knock-off of WebKit's RefCounted<T> class.  To use this guy just extend your
  70 // class from it like so:
  71 //
  72 //   class MyFoo : public base::RefCounted<MyFoo> {
  73 //    ...
  74 //    private:
  75 //     friend class base::RefCounted<MyFoo>;
  76 //     ~MyFoo();
  77 //   };
  78 //
  79 // You should always make your destructor private, to avoid any code deleting
  80 // the object accidently while there are references to it.
  81 template <class T>
  82 class RefCounted : public subtle::RefCountedBase {
  83  public:
  84   RefCounted() { }
  85   ~RefCounted() { }
  86
  87   void AddRef() {
  88     subtle::RefCountedBase::AddRef();
  89   }
  90
  91   void Release() {
  92     if (subtle::RefCountedBase::Release()) {
  93       delete static_cast<T*>(this);
  94     }
  95   }
  96
  97  private:
  98   DISALLOW_COPY_AND_ASSIGN(RefCounted<T>);
  99 };
 100
 101 // Forward declaration.
 102 template <class T, typename Traits> class RefCountedThreadSafe;
 103
 104 // Default traits for RefCountedThreadSafe<T>.  Deletes the object when its ref
 105 // count reaches 0.  Overload to delete it on a different thread etc.
 106 template<typename T>
 107 struct DefaultRefCountedThreadSafeTraits {
 108   static void Destruct(T* x) {
 109     // Delete through RefCountedThreadSafe to make child classes only need to be
 110     // friend with RefCountedThreadSafe instead of this struct, which is an
 111     // implementation detail.
 112     RefCountedThreadSafe<T, DefaultRefCountedThreadSafeTraits>::DeleteInternal(x);
 113   }
 114 };
 115
 116 //
 117 // A thread-safe variant of RefCounted<T>
 118 //
 119 //   class MyFoo : public base::RefCountedThreadSafe<MyFoo> {
 120 //    ...
 121 //   };
 122 //
 123 // If you're using the default trait, then you should add compile time
 124 // asserts that no one else is deleting your object.  i.e.
 125 //    private:
 126 //     friend class base::RefCountedThreadSafe<MyFoo>;
 127 //     ~MyFoo();
 128 template <class T, typename Traits = DefaultRefCountedThreadSafeTraits<T> >
 129 class RefCountedThreadSafe : public subtle::RefCountedThreadSafeBase {
 130  public:
 131   RefCountedThreadSafe() { }
 132   ~RefCountedThreadSafe() { }
 133
 134   void AddRef() {
 135     subtle::RefCountedThreadSafeBase::AddRef();
 136   }
 137
 138   void Release() {
 139     if (subtle::RefCountedThreadSafeBase::Release()) {
 140       Traits::Destruct(static_cast<T*>(this));
 141     }
 142   }
 143
 144  private:
 145   friend struct DefaultRefCountedThreadSafeTraits<T>;
 146   static void DeleteInternal(T* x) { delete x; }
 147
 148   DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafe);
 149 };
 150
 151 //
 152 // A wrapper for some piece of data so we can place other things in
 153 // scoped_refptrs<>.
 154 //
 155 template<typename T>
 156 class RefCountedData : public base::RefCounted< base::RefCountedData<T> > {
 157  public:
 158   RefCountedData() : data() {}
 159   RefCountedData(const T& in_value) : data(in_value) {}
 160
 161   T data;
 162 };
 163
 164 }  // namespace base
 165
 166 //
 167 // A smart pointer class for reference counted objects.  Use this class instead
 168 // of calling AddRef and Release manually on a reference counted object to
 169 // avoid common memory leaks caused by forgetting to Release an object
 170 // reference.  Sample usage:
 171 //
 172 //   class MyFoo : public RefCounted<MyFoo> {
 173 //    ...
 174 //   };
 175 //
 176 //   void some_function() {
 177 //     scoped_refptr<MyFoo> foo = new MyFoo();
 178 //     foo->Method(param);
 179 //     // |foo| is released when this function returns
 180 //   }
 181 //
 182 //   void some_other_function() {
 183 //     scoped_refptr<MyFoo> foo = new MyFoo();
 184 //     ...
 185 //     foo = NULL;  // explicitly releases |foo|
 186 //     ...
 187 //     if (foo)
 188 //       foo->Method(param);
 189 //   }
 190 //
 191 // The above examples show how scoped_refptr<T> acts like a pointer to T.
 192 // Given two scoped_refptr<T> classes, it is also possible to exchange
 193 // references between the two objects, like so:
 194 //
 195 //   {
 196 //     scoped_refptr<MyFoo> a = new MyFoo();
 197 //     scoped_refptr<MyFoo> b;
 198 //
 199 //     b.swap(a);
 200 //     // now, |b| references the MyFoo object, and |a| references NULL.
 201 //   }
 202 //
 203 // To make both |a| and |b| in the above example reference the same MyFoo
 204 // object, simply use the assignment operator:
 205 //
 206 //   {
 207 //     scoped_refptr<MyFoo> a = new MyFoo();
 208 //     scoped_refptr<MyFoo> b;
 209 //
 210 //     b = a;
 211 //     // now, |a| and |b| each own a reference to the same MyFoo object.
 212 //   }
 213 //
 214 template <class T>
 215 class scoped_refptr {
 216  public:
 217   scoped_refptr() : ptr_(NULL) {
 218   }
 219
 220   scoped_refptr(T* p) : ptr_(p) {
 221     if (ptr_)
 222       ptr_->AddRef();
 223   }
 224
 225   scoped_refptr(const scoped_refptr<T>& r) : ptr_(r.ptr_) {
 226     if (ptr_)
 227       ptr_->AddRef();
 228   }
 229
 230   template <typename U>
 231   scoped_refptr(const scoped_refptr<U>& r) : ptr_(r.get()) {
 232     if (ptr_)
 233       ptr_->AddRef();
 234   }
 235
 236   ~scoped_refptr() {
 237     if (ptr_)
 238       ptr_->Release();
 239   }
 240
 241   T* get() const { return ptr_; }
 242   operator T*() const { return ptr_; }
 243   T* operator->() const { return ptr_; }
 244
 245   // Release a pointer.
 246   // The return value is the current pointer held by this object.
 247   // If this object holds a NULL pointer, the return value is NULL.
 248   // After this operation, this object will hold a NULL pointer,
 249   // and will not own the object any more.
 250   T* release() {
 251     T* retVal = ptr_;
 252     ptr_ = NULL;
 253     return retVal;
 254   }
 255
 256   scoped_refptr<T>& operator=(T* p) {
 257     // AddRef first so that self assignment should work
 258     if (p)
 259       p->AddRef();
 260     if (ptr_ )
 261       ptr_ ->Release();
 262     ptr_ = p;
 263     return *this;
 264   }
 265
 266   scoped_refptr<T>& operator=(const scoped_refptr<T>& r) {
 267     return *this = r.ptr_;
 268   }
 269
 270   template <typename U>
 271   scoped_refptr<T>& operator=(const scoped_refptr<U>& r) {
 272     return *this = r.get();
 273   }
 274
 275   void swap(T** pp) {
 276     T* p = ptr_;
 277     ptr_ = *pp;
 278     *pp = p;
 279   }
 280
 281   void swap(scoped_refptr<T>& r) {
 282     swap(&r.ptr_);
 283   }
 284
 285  protected:
 286   T* ptr_;
 287 };
 288
 289 // Handy utility for creating a scoped_refptr<T> out of a T* explicitly without
 290 // having to retype all the template arguments
 291 template <typename T>
 292 scoped_refptr<T> make_scoped_refptr(T* t) {
 293   return scoped_refptr<T>(t);
 294 }
 295
 296 #endif  // BASE_REF_COUNTED_H_