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..04c4e5ca8f574cf9cdd3866641a1fa74cb4e6294 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
@@ -21,10 +21,16 @@ size_t GetSerializedSize_(const {{union.name}}Ptr& input) {
 void Serialize_({{union.name}}Ptr input, mojo::internal::Buffer* buf,
                 internal::{{union.name}}_Data** output) {
   if (input) {
+    internal::{{union.name}}_Data* result = nullptr;
+    if (*output) {

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

Unfortunately, we don't initialize |*output|. (For example, if you look at
struct_definition.tmpl, the constructor doesn't do anything about the fields.)
So it may be a random non-zero value.

That is why we have line 49.


As a comparison, we do initialization for the wrapper class, because that one is
user facing.

[azani, 2015/02/18 00:27:58]

The problem I'm trying to solve is again related to inlining.

If we are serializing a free-standing union, we need to allocate some new memory
for the union. But if the union is inlined, then memory has already been
allocated for it.

The code above works when dealing with inlined unions since |*output| is the
location where the union needs to be inlined. Is your concern that if the union
is free-standing, *output might not be initialized to nullptr?

+      result = *output;
+    } else {
+      result = internal::{{union.name}}_Data::New(buf);
+    }
     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->flags = 0x1;

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

This is the size for the non-inlined case.

     result->tag = input->which();
     switch (input->which()) {
 {%  for field in union.fields %}