chromium/mojo/public/tools/bindings/generators/cpp_templates/struct_unserialized_message_context.tmpl

// Used by {{struct.name}}::WrapAsMessage to lazily serialize the struct.
template <typename UserType, typename DataView>
struct {{struct.name}}_UnserializedMessageContext
    : public mojo::internal::UnserializedMessageContext {
 public:
  static const mojo::internal::UnserializedMessageContext::Tag kMessageTag;

  {{struct.name}}_UnserializedMessageContext(
    uint32_t message_name,
    uint32_t message_flags,
    UserType input)
      : mojo::internal::UnserializedMessageContext(&kMessageTag, message_name, message_flags)
      , user_data_(std::move(input)) {}
  ~{{struct.name}}_UnserializedMessageContext() override = default;

  UserType TakeData() {
    return std::move(user_data_);
  }

 private:
  // mojo::internal::UnserializedMessageContext:
  void Serialize(mojo::Message& message) override {
    mojo::internal::MessageFragment<{{struct.name}}_Data> fragment(message);
    mojo::internal::Serialize<DataView>(user_data_, fragment);
  }

  UserType user_data_;
};

template <typename UserType, typename DataView>
const mojo::internal::UnserializedMessageContext::Tag
    {{struct.name}}_UnserializedMessageContext<UserType, DataView>::kMessageTag = {};