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, bool move_only = IsMoveOnlyType<T>::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>
+struct IsWrappedUnionPtr {
+  static const bool value = false;
+};
+
+template <typename S>
+struct IsWrappedUnionPtr<StructPtr<S>> {

[yzshen1, 2015/02/25 21:07:00]

I feel that you could use IsUnionDataType and WrapperTraits  to implement this
IsWrappedUnionPtr, instead of doing the overloaded Test() trick again.

[azani, 2015/03/03 00:44:15]

Done.

+  template <typename U>
+  static YesType Test(const typename U::Data_::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value = sizeof(Test<S>(0)) == sizeof(YesType);
+};
+
+template <typename S>
+struct IsWrappedUnionPtr<InlinedStructPtr<S>> {

[yzshen1, 2015/02/25 21:07:00]

ditto.

[azani, 2015/03/03 00:44:15]

Done.

+  template <typename U>
+  static YesType Test(const typename U::Data_::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value = sizeof(Test<S>(0)) == sizeof(YesType);
+};
+
+template <typename T,
+          bool move_only = IsMoveOnlyType<T>::value,
+          bool is_union_wrapper = IsWrappedUnionPtr<T>::value>
 struct WrapperTraits;
 
 template <typename T>
-struct WrapperTraits<T, false> {
+struct WrapperTraits<T, false, false> {
   typedef T DataType;
 };
 template <typename H>
-struct WrapperTraits<ScopedHandleBase<H>, true> {
+struct WrapperTraits<ScopedHandleBase<H>, true, false> {
   typedef H DataType;
 };
 template <typename S>
-struct WrapperTraits<StructPtr<S>, true> {
+struct WrapperTraits<StructPtr<S>, true, false> {
   typedef typename S::Data_* DataType;
 };
 template <typename S>
-struct WrapperTraits<InlinedStructPtr<S>, true> {
+struct WrapperTraits<InlinedStructPtr<S>, true, false> {
   typedef typename S::Data_* DataType;
 };
 template <typename S>
-struct WrapperTraits<S, true> {
+struct WrapperTraits<StructPtr<S>, true, true> {
+  typedef typename S::Data_ DataType;

[yzshen1, 2015/02/25 21:07:00]

I looked at array_serialization, and didn't see why we need it to be S::Data_
instead of S::Data_* (but I didn't look very hard). Would you please point me to
it?

[azani, 2015/03/03 00:44:15]

Done.

+};
+template <typename S>
+struct WrapperTraits<InlinedStructPtr<S>, true, true> {
+  typedef typename S::Data_ DataType;
+};
+template <typename S>
+struct WrapperTraits<S, true, false> {
   typedef typename S::Data_* DataType;
 };
 
 template <typename T, typename Enable = void>
 struct ValueTraits {
   static bool Equals(const T& a, const T& b) { return a == b; }
 };
 
 template <typename T>
 struct ValueTraits<
@@ skipping 10 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_