bindings: Introduces ScriptWrappableBase to provide the internal pointer.

Source/bindings/core/v8/ScriptWrappable.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 28 matching lines @@
 // Helper to call webCoreInitializeScriptWrappableForInterface in the global namespace.
 template <class C> inline void initializeScriptWrappableHelper(C* object)
 {
     void webCoreInitializeScriptWrappableForInterface(C*);
     webCoreInitializeScriptWrappableForInterface(object);
 }
 
 namespace blink {
 
 /**
+ * The base class of all wrappable objects.
+ *
+ * This class provides the internal pointer to be stored in the wrapper objects,
+ * and its conversions from / to the DOM instances.
+ *
+ * Note that this class must not have vtbl (any virtual function) or any member
+ * variable which increase the size of instances.  Some of the classes sensitive
+ * to the size inherit from this class.  So this class must be zero size.
+ */
+class ScriptWrappableBase {
+public:
+    template <class T> static T* fromInternalPointer(void* internalPointer)
+    {
+        // Check if T* is castable to ScriptWrappableBase*, which means T
+        // doesn't have two or more ScriptWrappableBase as superclasses.
+        // If T has two ScriptWrappableBase as superclasses, conversions
+        // from T* to ScriptWrappableBase* are ambiguous.
+        ASSERT(static_cast<ScriptWrappableBase*>(static_cast<T*>(static_cast<ScriptWrappableBase*>(internalPointer))));
+        return static_cast<T*>(static_cast<ScriptWrappableBase*>(internalPointer));
+    }
+    void* toInternalPointer() { return this; }
+};
+
+/**
  * ScriptWrappable wraps a V8 object and its WrapperTypeInfo.
  *
  * ScriptWrappable acts much like a v8::Persistent<> in that it keeps a
  * V8 object alive. Under the hood, however, it keeps either a TypeInfo
  * object or an actual v8 persistent (or is empty).
  *
  * The physical state space of ScriptWrappable is:
  * - uintptr_t m_wrapperOrTypeInfo;
  *   - if 0: the ScriptWrappable is uninitialized/empty.
  *   - if even: a pointer to blink::TypeInfo
  *   - if odd: a pointer to v8::Persistent<v8::Object> + 1.
  *
  * In other words, one integer represents one of two object pointers,
  * depending on its least signficiant bit, plus an uninitialized state.
  * This class is meant to mask the logistics behind this.
  *
  * typeInfo() and newLocalWrapper will return appropriate values (possibly
  * 0/empty) in all physical states.
  *
  *  The state transitions are:
  *  - new: an empty and invalid ScriptWrappable.
  *  - init (to be called by all subclasses in their constructor):
  *        needs to call setTypeInfo
  *  - setTypeInfo: install a WrapperTypeInfo
  *  - setWrapper: install a v8::Persistent (or empty)
  *  - disposeWrapper (via setWeakCallback, triggered by V8 garbage collecter):
  *        remove v8::Persistent and install a TypeInfo of the previous value.
  */
-class ScriptWrappable {
+class ScriptWrappable : public ScriptWrappableBase {
 public:
     ScriptWrappable() : m_wrapperOrTypeInfo(0) { }
 
     // Wrappables need to be initialized with their most derrived type for which
     // bindings exist, in much the same way that certain other types need to be
     // adopted and so forth. The overloaded initializeScriptWrappableForInterface()
     // functions are implemented by the generated V8 bindings code. Declaring the
     // extern function in the template avoids making a centralized header of all
     // the bindings in the universe. C++11's extern template feature may provide
     // a cleaner solution someday.
@@ skipping 187 matching lines @@
         // FIXME: I noticed that 50%~ of minor GC cycle times can be consumed
         // inside data.GetParameter()->deref(), which causes Node destructions. We should
         // make Node destructions incremental.
         releaseObject(data.GetValue());
     }
 };
 
 } // namespace blink
 
 #endif // ScriptWrappable_h