WeakRefCnt

http://codereview.appspot.com/5649046/


git-svn-id: http://skia.googlecode.com/svn/trunk@3978 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 26 insertions, 13 deletions

diff --git a/include/core/SkRefCnt.h b/include/core/SkRefCnt.h
index 71a2443584..384b3be2eb 100644
--- a/include/core/SkRefCnt.h
+++ b/include/core/SkRefCnt.h
@@ -15,12 +15,12 @@
 /** \class SkRefCnt
 
     SkRefCnt is the base class for objects that may be shared by multiple
-    objects. When a new owner wants a reference, it calls ref(). When an owner
-    wants to release its reference, it calls unref(). When the shared object's
-    reference count goes to zero as the result of an unref() call, its (virtual)
-    destructor is called. It is an error for the destructor to be called
-    explicitly (or via the object going out of scope on the stack or calling
-    delete) if getRefCnt() > 1.
+    objects. When an existing owner wants to share a reference, it calls ref().
+    When an owner wants to release its reference, it calls unref(). When the
+    shared object's reference count goes to zero as the result of an unref()
+    call, its (virtual) destructor is called. It is an error for the
+    destructor to be called explicitly (or via the object going out of scope on
+    the stack or calling delete) if getRefCnt() > 1.
 */
 class SK_API SkRefCnt : SkNoncopyable {
 public:
@@ -28,7 +28,7 @@ public:
     */
     SkRefCnt() : fRefCnt(1) {}
 
-    /**  Destruct, asserting that the reference count is 1.
+    /** Destruct, asserting that the reference count is 1.
     */
     virtual ~SkRefCnt() {
 #ifdef SK_DEBUG
@@ -45,19 +45,21 @@ public:
     */
     void ref() const {
         SkASSERT(fRefCnt > 0);
-        sk_atomic_inc(&fRefCnt);
+        sk_atomic_inc(&fRefCnt);  // No barrier required.
     }
 
     /** Decrement the reference count. If the reference count is 1 before the
-        decrement, then call delete on the object. Note that if this is the
-        case, then the object needs to have been allocated via new, and not on
-        the stack.
+        decrement, then delete the object. Note that if this is the case, then
+        the object needs to have been allocated via new, and not on the stack.
     */
     void unref() const {
         SkASSERT(fRefCnt > 0);
+        // Release barrier (SL/S), if not provided below.
         if (sk_atomic_dec(&fRefCnt) == 1) {
-            fRefCnt = 1;    // so our destructor won't complain
-            SkDELETE(this);
+            // Aquire barrier (L/SL), if not provided above.
+            // Prevents code in dispose from happening before the decrement.
+            sk_membar_aquire__after_atomic_dec();
+            internal_dispose();
         }
     }
 
@@ -66,6 +68,17 @@ public:
     }
 
 private:
+    /** Called when the ref count goes to 0.
+    */
+    virtual void internal_dispose() const {
+#ifdef SK_DEBUG
+        // so our destructor won't complain
+        fRefCnt = 1;
+#endif
+        SkDELETE(this);
+    }
+    friend class SkWeakRefCnt;
+
     mutable int32_t fRefCnt;
 };