1 /*
2 * Copyright 2006 The Android Open Source Project
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8 #ifndef SkRefCnt_DEFINED
9 #define SkRefCnt_DEFINED
10
11 #include "../private/SkAtomics.h"
12 #include "../private/SkUniquePtr.h"
13 #include "SkTypes.h"
14
15 /** \class SkRefCntBase
16
17 SkRefCntBase is the base class for objects that may be shared by multiple
18 objects. When an existing owner wants to share a reference, it calls ref().
19 When an owner wants to release its reference, it calls unref(). When the
20 shared object's reference count goes to zero as the result of an unref()
21 call, its (virtual) destructor is called. It is an error for the
22 destructor to be called explicitly (or via the object going out of scope on
23 the stack or calling delete) if getRefCnt() > 1.
24 */
25 class SK_API SkRefCntBase : SkNoncopyable {
26 public:
27 /** Default construct, initializing the reference count to 1.
28 */
SkRefCntBase()29 SkRefCntBase() : fRefCnt(1) {}
30
31 /** Destruct, asserting that the reference count is 1.
32 */
~SkRefCntBase()33 virtual ~SkRefCntBase() {
34 #ifdef SK_DEBUG
35 SkASSERTF(fRefCnt == 1, "fRefCnt was %d", fRefCnt);
36 fRefCnt = 0; // illegal value, to catch us if we reuse after delete
37 #endif
38 }
39
40 #ifdef SK_DEBUG
41 /** Return the reference count. Use only for debugging. */
getRefCnt()42 int32_t getRefCnt() const { return fRefCnt; }
43 #endif
44
45 /** May return true if the caller is the only owner.
46 * Ensures that all previous owner's actions are complete.
47 */
unique()48 bool unique() const {
49 if (1 == sk_atomic_load(&fRefCnt, sk_memory_order_acquire)) {
50 // The acquire barrier is only really needed if we return true. It
51 // prevents code conditioned on the result of unique() from running
52 // until previous owners are all totally done calling unref().
53 return true;
54 }
55 return false;
56 }
57
58 /** Increment the reference count. Must be balanced by a call to unref().
59 */
ref()60 void ref() const {
61 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
62 // Android employs some special subclasses that enable the fRefCnt to
63 // go to zero, but not below, prior to reusing the object. This breaks
64 // the use of unique() on such objects and as such should be removed
65 // once the Android code is fixed.
66 SkASSERT(fRefCnt >= 0);
67 #else
68 SkASSERT(fRefCnt > 0);
69 #endif
70 (void)sk_atomic_fetch_add(&fRefCnt, +1, sk_memory_order_relaxed); // No barrier required.
71 }
72
73 /** Decrement the reference count. If the reference count is 1 before the
74 decrement, then delete the object. Note that if this is the case, then
75 the object needs to have been allocated via new, and not on the stack.
76 */
unref()77 void unref() const {
78 SkASSERT(fRefCnt > 0);
79 // A release here acts in place of all releases we "should" have been doing in ref().
80 if (1 == sk_atomic_fetch_add(&fRefCnt, -1, sk_memory_order_acq_rel)) {
81 // Like unique(), the acquire is only needed on success, to make sure
82 // code in internal_dispose() doesn't happen before the decrement.
83 this->internal_dispose();
84 }
85 }
86
87 #ifdef SK_DEBUG
validate()88 void validate() const {
89 SkASSERT(fRefCnt > 0);
90 }
91 #endif
92
93 protected:
94 /**
95 * Allow subclasses to call this if they've overridden internal_dispose
96 * so they can reset fRefCnt before the destructor is called. Should only
97 * be called right before calling through to inherited internal_dispose()
98 * or before calling the destructor.
99 */
internal_dispose_restore_refcnt_to_1()100 void internal_dispose_restore_refcnt_to_1() const {
101 #ifdef SK_DEBUG
102 SkASSERT(0 == fRefCnt);
103 fRefCnt = 1;
104 #endif
105 }
106
107 private:
108 /**
109 * Called when the ref count goes to 0.
110 */
internal_dispose()111 virtual void internal_dispose() const {
112 this->internal_dispose_restore_refcnt_to_1();
113 delete this;
114 }
115
116 // The following friends are those which override internal_dispose()
117 // and conditionally call SkRefCnt::internal_dispose().
118 friend class SkWeakRefCnt;
119
120 mutable int32_t fRefCnt;
121
122 typedef SkNoncopyable INHERITED;
123 };
124
125 #ifdef SK_REF_CNT_MIXIN_INCLUDE
126 // It is the responsibility of the following include to define the type SkRefCnt.
127 // This SkRefCnt should normally derive from SkRefCntBase.
128 #include SK_REF_CNT_MIXIN_INCLUDE
129 #else
130 class SK_API SkRefCnt : public SkRefCntBase { };
131 #endif
132
133 ///////////////////////////////////////////////////////////////////////////////
134
135 /** Helper macro to safely assign one SkRefCnt[TS]* to another, checking for
136 null in on each side of the assignment, and ensuring that ref() is called
137 before unref(), in case the two pointers point to the same object.
138 */
139 #define SkRefCnt_SafeAssign(dst, src) \
140 do { \
141 if (src) src->ref(); \
142 if (dst) dst->unref(); \
143 dst = src; \
144 } while (0)
145
146
147 /** Call obj->ref() and return obj. The obj must not be nullptr.
148 */
SkRef(T * obj)149 template <typename T> static inline T* SkRef(T* obj) {
150 SkASSERT(obj);
151 obj->ref();
152 return obj;
153 }
154
155 /** Check if the argument is non-null, and if so, call obj->ref() and return obj.
156 */
SkSafeRef(T * obj)157 template <typename T> static inline T* SkSafeRef(T* obj) {
158 if (obj) {
159 obj->ref();
160 }
161 return obj;
162 }
163
164 /** Check if the argument is non-null, and if so, call obj->unref()
165 */
SkSafeUnref(T * obj)166 template <typename T> static inline void SkSafeUnref(T* obj) {
167 if (obj) {
168 obj->unref();
169 }
170 }
171
SkSafeSetNull(T * & obj)172 template<typename T> static inline void SkSafeSetNull(T*& obj) {
173 if (obj) {
174 obj->unref();
175 obj = nullptr;
176 }
177 }
178
179 ///////////////////////////////////////////////////////////////////////////////
180
181 template <typename T> struct SkTUnref {
operatorSkTUnref182 void operator()(T* t) { t->unref(); }
183 };
184
185 /**
186 * Utility class that simply unref's its argument in the destructor.
187 */
188 template <typename T> class SkAutoTUnref : public skstd::unique_ptr<T, SkTUnref<T>> {
189 public:
190 explicit SkAutoTUnref(T* obj = nullptr) : skstd::unique_ptr<T, SkTUnref<T>>(obj) {}
191
detach()192 T* detach() { return this->release(); }
193 operator T*() const { return this->get(); }
194 };
195 // Can't use the #define trick below to guard a bare SkAutoTUnref(...) because it's templated. :(
196
197 class SkAutoUnref : public SkAutoTUnref<SkRefCnt> {
198 public:
SkAutoUnref(SkRefCnt * obj)199 SkAutoUnref(SkRefCnt* obj) : SkAutoTUnref<SkRefCnt>(obj) {}
200 };
201 #define SkAutoUnref(...) SK_REQUIRE_LOCAL_VAR(SkAutoUnref)
202
203 // This is a variant of SkRefCnt that's Not Virtual, so weighs 4 bytes instead of 8 or 16.
204 // There's only benefit to using this if the deriving class does not otherwise need a vtable.
205 template <typename Derived>
206 class SkNVRefCnt : SkNoncopyable {
207 public:
SkNVRefCnt()208 SkNVRefCnt() : fRefCnt(1) {}
~SkNVRefCnt()209 ~SkNVRefCnt() { SkASSERTF(1 == fRefCnt, "NVRefCnt was %d", fRefCnt); }
210
211 // Implementation is pretty much the same as SkRefCntBase. All required barriers are the same:
212 // - unique() needs acquire when it returns true, and no barrier if it returns false;
213 // - ref() doesn't need any barrier;
214 // - unref() needs a release barrier, and an acquire if it's going to call delete.
215
unique()216 bool unique() const { return 1 == sk_atomic_load(&fRefCnt, sk_memory_order_acquire); }
ref()217 void ref() const { (void)sk_atomic_fetch_add(&fRefCnt, +1, sk_memory_order_relaxed); }
unref()218 void unref() const {
219 if (1 == sk_atomic_fetch_add(&fRefCnt, -1, sk_memory_order_acq_rel)) {
220 SkDEBUGCODE(fRefCnt = 1;) // restore the 1 for our destructor's assert
221 delete (const Derived*)this;
222 }
223 }
deref()224 void deref() const { this->unref(); }
225
226 private:
227 mutable int32_t fRefCnt;
228 };
229
230 #endif
231