Implement unions as members of structs.

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

 size_t GetSerializedSize_(const {{struct.name}}Ptr& input) {
   if (!input)
     return 0;
   size_t size = sizeof(internal::{{struct.name}}_Data);
 {%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
   size += GetSerializedSize_(input->{{pf.field.name}});
 {%- endfor %}
   return size;
 }
 
 void Serialize_({{struct.name}}Ptr input, mojo::internal::Buffer* buf,
                 internal::{{struct.name}}_Data** output) {
   if (input) {
     internal::{{struct.name}}_Data* result =
         internal::{{struct.name}}_Data::New(buf);
 {%- for pf in struct.packed.packed_fields %}
-{%-   if pf.field.kind|is_object_kind %}
+{%-   if pf.field.kind|is_union_kind %}

[yzshen1, 2015/02/17 19:34:35]

It seems nicer to move this into the "is_object_kind" branch.

+      internal::{{pf.field.kind.name}}_Data* {{pf.field.name}}_ptr = &result->{{pf.field.name}};
+    Serialize_(mojo::internal::Forward(input->{{pf.field.name}}), buf, &{{pf.field.name}}_ptr);
+{%-   elif pf.field.kind|is_object_kind %}
 {%-     if pf.field.kind|is_array_kind %}
     mojo::SerializeArray_<{{pf.field.kind|get_array_validate_params|indent(26)}}>(
         mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
 {%-     elif pf.field.kind|is_map_kind %}
     mojo::SerializeMap_<{{pf.field.kind.value_kind|get_map_validate_params|indent(26)}}>(
         mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
 {%-     else %}
     Serialize_(mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
 {%-     endif %}
 {%-     if not pf.field.kind|is_nullable_kind %}
@@ skipping 22 matching lines @@
   } else {
     *output = nullptr;
   }
 }
 
 void Deserialize_(internal::{{struct.name}}_Data* input,
                   {{struct.name}}Ptr* output) {
   if (input) {
     {{struct.name}}Ptr result({{struct.name}}::New());
 {%- for pf in struct.packed.packed_fields %}
-{%-   if pf.field.kind|is_object_kind %}
+{%-   if pf.field.kind|is_union_kind %}
+    Deserialize_(&input->{{pf.field.name}}, &result->{{pf.field.name}});
+{%-   elif pf.field.kind|is_object_kind %}
     Deserialize_(input->{{pf.field.name}}.ptr, &result->{{pf.field.name}});
 {%-   elif pf.field.kind|is_interface_kind %}
     if (input->{{pf.field.name}}.is_valid())
       result->{{pf.field.name}}.Bind(mojo::MakeScopedHandle(mojo::internal::FetchAndReset(&input->{{pf.field.name}})));
 {%-   elif pf.field.kind|is_any_handle_kind %}
     result->{{pf.field.name}}.reset(mojo::internal::FetchAndReset(&input->{{pf.field.name}}));
 {%-   elif pf.field.kind|is_enum_kind %}
     result->{{pf.field.name}} = static_cast<{{pf.field.kind|cpp_wrapper_type}}>(
       input->{{pf.field.name}});
 {%-   else %}
     result->{{pf.field.name}} = input->{{pf.field.name}};
 {%-   endif %}
 {%- endfor %}
     *output = result.Pass();
   } else {
     output->reset();
   }
 }