Implement unions as members of structs.

mojo/public/cpp/bindings/lib/bindings_internal.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_
 
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 #include "mojo/public/cpp/bindings/struct_ptr.h"
 #include "mojo/public/cpp/system/core.h"
@@ skipping 58 matching lines @@
   T temp = *ptr;
   *ptr = T();
   return temp;
 }
 
 template <typename H>
 struct IsHandle {
   enum { value = IsBaseOf<Handle, H>::value };
 };
 
+template <typename T>
+struct IsUnionDataType {
+  template <typename U>
+  static YesType Test(const typename U::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value =
+      sizeof(Test<T>(0)) == sizeof(YesType) && !IsConst<T>::value;
+};
+
 template <typename T, bool move_only = IsMoveOnlyType<T>::value>
 struct WrapperTraits;
 
 template <typename T>
 struct WrapperTraits<T, false> {
   typedef T DataType;
 };
 template <typename H>
 struct WrapperTraits<ScopedHandleBase<H>, true> {
   typedef H DataType;
@@ skipping 31 matching lines @@
   static bool Equals(const ScopedHandleBase<T>& a,
                      const ScopedHandleBase<T>& b) {
     return a.get().value() == b.get().value();
   }
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_