Implement unions as members of structs.

mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
index 8b82f7f404206e492e905bf85c96a6f486628fb8..9eadee4ca079a176fb6714025169fb012a1b6e4b 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
@@ -1,8 +1,8 @@
 size_t GetSerializedSize_(const {{union.name}}Ptr& input) {
+  size_t size = sizeof(internal::{{union.name}}_Data);
   if (!input)
-    return 0;
+    return size;
 
-  size_t size = sizeof(internal::{{union.name}}_Data);
   switch (input->which()) {
 {%  for field in union.fields %}
 {%    if field.kind|is_string_kind %}
@@ -17,14 +17,19 @@ size_t GetSerializedSize_(const {{union.name}}Ptr& input) {
   return size;
 }
 
-
-void Serialize_({{union.name}}Ptr input, mojo::internal::Buffer* buf,
-                internal::{{union.name}}_Data** output) {
+void SerializeUnion_({{union.name}}Ptr input, mojo::internal::Buffer* buf,
+                     internal::{{union.name}}_Data** output, bool inlined) {
+  internal::{{union.name}}_Data* result = nullptr;
+  if (inlined) {
+    result = *output;
+  } else {
+    result = internal::{{union.name}}_Data::New(buf);

[yzshen1, 2015/03/27 16:26:33]

If it is non-inlined, and input is nullptr, you don't need to allocate |result|,
right?

How about 

if (input) {
  // declare |result|.
  ...
  *output = result;
} else {
  *output = nullptr;
}

[azani, 2015/03/27 20:50:14]

I went with something a bit different, but same idea.

+  }
   if (input) {
     mojo::internal::UnionAccessor<{{union.name}}> input_acc(input.get());
-    internal::{{union.name}}_Data* result =
-        internal::{{union.name}}_Data::New(buf);
     // TODO(azani): Handle unknown and objects.
+    // Set the not-null flag.
+    result->size = 16;
     result->tag = input->which();
     switch (input->which()) {
 {%  for field in union.fields %}
@@ -38,15 +43,17 @@ void Serialize_({{union.name}}Ptr input, mojo::internal::Buffer* buf,
         break;
 {%- endfor %}
     }
-    *output = result;
+  } else if (inlined) {
+    result->set_null();
   } else {
-    *output = nullptr;
+    result = nullptr;
   }
+  *output = result;
 }
 
 void Deserialize_(internal::{{union.name}}_Data* input,
                   {{union.name}}Ptr* output) {
-  if (input) {
+  if (input && !input->is_null()) {
     {{union.name}}Ptr result({{union.name}}::New());
     mojo::internal::UnionAccessor<{{union.name}}> result_acc(result.get());
     switch (input->tag) {