SkRefCnt.h revision 3c850c561fcad1ac35bff4ec2875a40ef2309148 
1
2/*
3 * Copyright 2006 The Android Open Source Project
4 *
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
7 */
8
9
10#ifndef SkRefCnt_DEFINED
11#define SkRefCnt_DEFINED
12
13#include "SkDynamicAnnotations.h"
14#include "SkThread.h"
15#include "SkInstCnt.h"
16#include "SkTemplates.h"
17
18/** \class SkRefCntBase
19
20    SkRefCntBase is the base class for objects that may be shared by multiple
21    objects. When an existing owner wants to share a reference, it calls ref().
22    When an owner wants to release its reference, it calls unref(). When the
23    shared object's reference count goes to zero as the result of an unref()
24    call, its (virtual) destructor is called. It is an error for the
25    destructor to be called explicitly (or via the object going out of scope on
26    the stack or calling delete) if getRefCnt() > 1.
27*/
28class SK_API SkRefCntBase : SkNoncopyable {
29public:
30    SK_DECLARE_INST_COUNT_ROOT(SkRefCntBase)
31
32    /** Default construct, initializing the reference count to 1.
33    */
34    SkRefCntBase() : fRefCnt(1) {}
35
36    /** Destruct, asserting that the reference count is 1.
37    */
38    virtual ~SkRefCntBase() {
39#ifdef SK_DEBUG
40        SkASSERTF(fRefCnt == 1, "fRefCnt was %d", fRefCnt);
41        fRefCnt = 0;    // illegal value, to catch us if we reuse after delete
42#endif
43    }
44
45//#ifdef SK_DEBUG
46    /** Return the reference count. Use only for debugging. */
47    int32_t getRefCnt() const { return fRefCnt; }
48//#endif
49
50    /** May return true if the caller is the only owner.
51     *  Ensures that all previous owner's actions are complete.
52     */
53    bool unique() const {
54        // We believe we're reading fRefCnt in a safe way here, so we stifle the TSAN warning about
55        // an unproctected read.  Generally, don't read fRefCnt, and don't stifle this warning.
56        bool const unique = (1 == sk_acquire_load(&fRefCnt));
57        if (unique) {
58            // Acquire barrier (L/SL), if not provided by load of fRefCnt.
59            // Prevents user's 'unique' code from happening before decrements.
60            //TODO: issue the barrier only when unique is true
61        }
62        return unique;
63    }
64
65    /** Increment the reference count. Must be balanced by a call to unref().
66    */
67    void ref() const {
68        SkASSERT(fRefCnt > 0);
69        sk_atomic_inc(&fRefCnt);  // No barrier required.
70    }
71
72    /** Decrement the reference count. If the reference count is 1 before the
73        decrement, then delete the object. Note that if this is the case, then
74        the object needs to have been allocated via new, and not on the stack.
75    */
76    void unref() const {
77        SkASSERT(fRefCnt > 0);
78        // Release barrier (SL/S), if not provided below.
79        if (sk_atomic_dec(&fRefCnt) == 1) {
80            // Acquire barrier (L/SL), if not provided above.
81            // Prevents code in dispose from happening before the decrement.
82            sk_membar_acquire__after_atomic_dec();
83            internal_dispose();
84        }
85    }
86
87#ifdef SK_DEBUG
88    void validate() const {
89        SkASSERT(fRefCnt > 0);
90    }
91#endif
92
93protected:
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     */
100    void internal_dispose_restore_refcnt_to_1() const {
101#ifdef SK_DEBUG
102        SkASSERT(0 == fRefCnt);
103        fRefCnt = 1;
104#endif
105    }
106
107private:
108    /**
109     *  Called when the ref count goes to 0.
110     */
111    virtual void internal_dispose() const {
112        this->internal_dispose_restore_refcnt_to_1();
113        SkDELETE(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
130class 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 NULL.
148 */
149template <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 */
157template <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 */
166template <typename T> static inline void SkSafeUnref(T* obj) {
167    if (obj) {
168        obj->unref();
169    }
170}
171
172template<typename T> static inline void SkSafeSetNull(T*& obj) {
173    if (obj) {
174        obj->unref();
175        obj = NULL;
176    }
177}
178
179///////////////////////////////////////////////////////////////////////////////
180
181/**
182 *  Utility class that simply unref's its argument in the destructor.
183 */
184template <typename T> class SkAutoTUnref : SkNoncopyable {
185public:
186    explicit SkAutoTUnref(T* obj = NULL) : fObj(obj) {}
187    ~SkAutoTUnref() { SkSafeUnref(fObj); }
188
189    T* get() const { return fObj; }
190
191    T* reset(T* obj) {
192        SkSafeUnref(fObj);
193        fObj = obj;
194        return obj;
195    }
196
197    void swap(SkAutoTUnref* other) {
198        T* tmp = fObj;
199        fObj = other->fObj;
200        other->fObj = tmp;
201    }
202
203    /**
204     *  Return the hosted object (which may be null), transferring ownership.
205     *  The reference count is not modified, and the internal ptr is set to NULL
206     *  so unref() will not be called in our destructor. A subsequent call to
207     *  detach() will do nothing and return null.
208     */
209    T* detach() {
210        T* obj = fObj;
211        fObj = NULL;
212        return obj;
213    }
214
215    /**
216     *  BlockRef<B> is a type which inherits from B, cannot be created,
217     *  cannot be deleted, and makes ref and unref private.
218     */
219    template<typename B> class BlockRef : public B {
220    private:
221        BlockRef();
222        ~BlockRef();
223        void ref() const;
224        void unref() const;
225    };
226
227    /** If T is const, the type returned from operator-> will also be const. */
228    typedef typename SkTConstType<BlockRef<T>, SkTIsConst<T>::value>::type BlockRefType;
229
230    /**
231     *  SkAutoTUnref assumes ownership of the ref. As a result, it is an error
232     *  for the user to ref or unref through SkAutoTUnref. Therefore
233     *  SkAutoTUnref::operator-> returns BlockRef<T>*. This prevents use of
234     *  skAutoTUnrefInstance->ref() and skAutoTUnrefInstance->unref().
235     */
236    BlockRefType *operator->() const {
237        return static_cast<BlockRefType*>(fObj);
238    }
239    operator T*() const { return fObj; }
240
241private:
242    T*  fObj;
243};
244// Can't use the #define trick below to guard a bare SkAutoTUnref(...) because it's templated. :(
245
246class SkAutoUnref : public SkAutoTUnref<SkRefCnt> {
247public:
248    SkAutoUnref(SkRefCnt* obj) : SkAutoTUnref<SkRefCnt>(obj) {}
249};
250#define SkAutoUnref(...) SK_REQUIRE_LOCAL_VAR(SkAutoUnref)
251
252// This is a variant of SkRefCnt that's Not Virtual, so weighs 4 bytes instead of 8 or 16.
253// There's only benefit to using this if the deriving class does not otherwise need a vtable.
254template <typename Derived>
255class SkNVRefCnt : SkNoncopyable {
256public:
257    SkNVRefCnt() : fRefCnt(1) {}
258    ~SkNVRefCnt() { SkASSERTF(1 == fRefCnt, "NVRefCnt was %d", fRefCnt); }
259
260    // Implementation is pretty much the same as SkRefCntBase. All required barriers are the same:
261    //   - unique() needs acquire when it returns true, and no barrier if it returns false;
262    //   - ref() doesn't need any barrier;
263    //   - unref() needs a release barrier, and an acquire if it's going to call delete.
264
265    bool unique() const { return 1 == sk_acquire_load(&fRefCnt); }
266    void    ref() const { sk_atomic_inc(&fRefCnt); }
267    void  unref() const {
268        int32_t prevValue = sk_atomic_dec(&fRefCnt);
269        SkASSERT(prevValue >= 1);
270        if (1 == prevValue) {
271            SkDEBUGCODE(fRefCnt = 1;)   // restore the 1 for our destructor's assert
272            SkDELETE((const Derived*)this);
273        }
274    }
275    void  deref() const { this->unref(); }  // Chrome prefers to call deref().
276
277private:
278    mutable int32_t fRefCnt;
279};
280
281#endif
282