Implement unions as members of structs.

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

 {%- macro validate(struct, class_name) %}
   if (!data)
     return true;
 
   if (!ValidateStructHeader(
           data, sizeof({{class_name}}),
           {{struct.packed.packed_fields|length}}, bounds_checker)) {
     return false;
   }
 
   const {{class_name}}* object = static_cast<const {{class_name}}*>(data);
   MOJO_ALLOW_UNUSED_LOCAL(object);
 
 {%-   for packed_field in struct.packed.packed_fields %}
 {%-     set name = packed_field.field.name %}
 {%-     set kind = packed_field.field.kind %}
-{%-     if kind|is_object_kind %}
+{%-     if kind|is_union_kind %}
+{%-       if not kind|is_nullable_kind %}

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

(Please note that this part has been under significant change recently.
Especially after https://codereview.chromium.org/917333003/. Besides, I am
planning to move tmpl code into C++ template magic. :))

[azani, 2015/02/18 00:27:57]

Done.

+  if (object->{{name}}.is_null()) {
+    ReportValidationError(
+        mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
+        "null {{name}} field in {{struct.name}} struct");
+    return false;
+  }
+{%-       endif %}
+  // We already claimed the union's memory as part of the struct, so we don't
+  // try to claim the union's memory when validating the union.
+  if (!{{kind.name}}::Data_::Validate(&object->{{name}}, bounds_checker, false)) {

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

Do you think it make sense to make |false| a template argument? That seems to be
more consistent with other Validate().

[azani, 2015/02/18 00:27:57]

It seems like we would just shift the inconsistency from the argument list to
the template arguments. I am somewhat disinclined to using templates for this
purpose, but only slightly so.

+    return false;
+  }
+{%-     elif kind|is_object_kind %}
 {%-       set wrapper_type = kind|cpp_wrapper_type %}
 {%-       if not kind|is_nullable_kind %}
   if (!object->{{name}}.offset) {
     ReportValidationError(
         mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
         "null {{name}} field in {{struct.name}} struct");
     return false;
   }
 {%-       endif %}
   if (!mojo::internal::ValidateEncodedPointer(&object->{{name}}.offset)) {
@@ skipping 70 matching lines @@
 {%-     set offset = last_field.offset + last_field.size %}
 {%-     set pad = offset|get_pad(8) -%}
 {%-     if pad > 0 %}
   uint8_t padfinal_[{{pad}}];
 {%-     endif %}
 {%-   endif %}
 {%- endmacro %}
 
 {%- macro encodes(struct) -%}
 {%-   for pf in struct.packed.packed_fields %}
-{%-     if pf.field.kind|is_object_kind %}
+{%-     if pf.field.kind|is_union_kind %}
+// TODO(azani): Encode pointers and handles.
+{%-     elif pf.field.kind|is_object_kind %}
 mojo::internal::Encode(&{{pf.field.name}}, handles);
 {%-     elif pf.field.kind|is_any_handle_kind %}
 mojo::internal::EncodeHandle(&{{pf.field.name}}, handles);
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro -%}
 
 {%- macro decodes(struct) -%}
 {%-   for pf in struct.packed.packed_fields %}
-{%-     if pf.field.kind|is_object_kind %}
+{%-     if pf.field.kind|is_union_kind %}
+// TODO(azani): Decode pointers and handles.
+{%-     elif pf.field.kind|is_object_kind %}
 mojo::internal::Decode(&{{pf.field.name}}, handles);
 {%-     elif pf.field.kind|is_any_handle_kind %}
 mojo::internal::DecodeHandle(&{{pf.field.name}}, handles);
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro -%}