[gin] fix casting WrappableBase to a derived class

We first need to cast the void pointer to WrappableBase before we
can cast it to the derived class: initially a WrappableBase* is
stored as holder, so we can't directly cast it to the derived class.

To test this, I added a useless base class so the test object this
pointers don't align with the WrappableBase this pointer.

Also update the comments about how to use WrappableBase.

BUG=none
R=aa@chromium.org,dcarney@chromium.org

Review URL: https://codereview.chromium.org/119173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241863 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 20 insertions, 7 deletions

diff --git a/gin/wrappable.h b/gin/wrappable.h
index e20c7f1..ff52b19 100644
--- a/gin/wrappable.h
+++ b/gin/wrappable.h
@@ -31,17 +31,18 @@ GIN_EXPORT void* FromV8Impl(v8::Isolate* isolate,
 //   static WrapperInfo kWrapperInfo;
 //
 //   // Optional, only required if non-empty template should be used.
-//   static v8::Local<v8::ObjectTemplate> GetObjectTemplate(
+//   virtual gin::ObjectTemplateBuilder GetObjectTemplateBuilder(
 //       v8::Isolate* isolate);
 //   ...
 // };
 //
 // // my_class.cc
-// WrapperInfo MyClass::kWrapperInfo = { kEmbedderNativeGin };
+// WrapperInfo MyClass::kWrapperInfo = {kEmbedderNativeGin};
 //
-// v8::Local<v8::ObjectTemplate> MyClass::GetObjectTemplate(
+// gin::ObjectTemplateBuilder MyClass::GetObjectTemplateBuilder(
 //     v8::Isolate* isolate) {
-//   return ObjectTemplateBuilder(isolate).SetValue("foobar", 42).Build();
+//   return Wrappable<MyClass>::GetObjectTemplateBuilder(isolate)
+//       .SetValue("foobar", 42);
 // }
 //
 // Subclasses should also typically have private constructors and expose a
@@ -104,8 +105,8 @@ struct Converter<T*, typename base::enable_if<
   }
 
   static bool FromV8(v8::Isolate* isolate, v8::Handle<v8::Value> val, T** out) {
-    *out = static_cast<T*>(internal::FromV8Impl(isolate, val,
-                                                &T::kWrapperInfo));
+    *out = static_cast<T*>(static_cast<WrappableBase*>(
+        internal::FromV8Impl(isolate, val, &T::kWrapperInfo)));
     return *out != NULL;
   }
 };
diff --git a/gin/wrappable_unittest.cc b/gin/wrappable_unittest.cc
index ad8b6b7..2428dc1 100644
--- a/gin/wrappable_unittest.cc
+++ b/gin/wrappable_unittest.cc
@@ -15,7 +15,19 @@
 
 namespace gin {
 
-class MyObject : public Wrappable<MyObject> {
+class BaseClass {
+ public:
+  BaseClass() : value_(23) {}
+  virtual ~BaseClass() {}
+
+ private:
+  int value_;
+
+  DISALLOW_COPY_AND_ASSIGN(BaseClass);
+};
+
+class MyObject : public BaseClass,
+                 public Wrappable<MyObject> {
  public:
   static WrapperInfo kWrapperInfo;