Fix IsBaseOf in output_traits.h.

Previously, it may be used with forward declarations unintentionally and does wrong things silently.

BUG=None
TEST=None


Review URL: http://codereview.chromium.org/10068004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@132088 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 8 insertions, 5 deletions

diff --git a/ppapi/cpp/dev/audio_input_dev.h b/ppapi/cpp/dev/audio_input_dev.h
index 2b2b6bb..1c80f4d 100644
--- a/ppapi/cpp/dev/audio_input_dev.h
+++ b/ppapi/cpp/dev/audio_input_dev.h
@@ -59,11 +59,6 @@ class AudioInput_Dev : public Resource {
   /// struct.
   const AudioConfig& config() const { return config_; }
 
-  // TODO(yzshen, brettw): If we forward declare DeviceRef_Dev (as opposed to
-  // including its .h file), it still compiles. However, it is not recognized as
-  // a derived class of Resource and does the wrong thing!
-  // This is due to the limitation of IsBaseOf in ppapi/cpp/output_traits.h. We
-  // need to figure out a way to overcome this problem.
   int32_t EnumerateDevices(
       const CompletionCallbackWithOutput<std::vector<DeviceRef_Dev> >&
           callback);
diff --git a/ppapi/cpp/output_traits.h b/ppapi/cpp/output_traits.h
index 963f296..5b45aa47e 100644
--- a/ppapi/cpp/output_traits.h
+++ b/ppapi/cpp/output_traits.h
@@ -35,9 +35,17 @@ template<typename A> struct IsSame<A, A> {
   static bool const value = true;
 };
 template<typename Base, typename Derived> struct IsBaseOf {
+ private:
+  // This class doesn't work correctly with forward declarations.
+  // Because sizeof cannot be applied to incomplete types, this line prevents us
+  // from passing in forward declarations.
+  typedef char (*EnsureTypesAreComplete)[sizeof(Base) + sizeof(Derived)];
+
   static Derived* CreateDerived();
   static char (&Check(Base*))[1];
   static char (&Check(...))[2];
+
+ public:
   static bool const value = sizeof Check(CreateDerived()) == 1 &&
                             !IsSame<Base const, void const>::value;
 };