Mojo C++ bindings: support attaching context object for StructTraits.

mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h"
 
 namespace mojo {
+namespace {
+
+struct Context {
+  int32_t value;
+};
+
+}  // namespace
+
+// static
+void* StructTraits<test::NestedStructWithTraits,
+                   test::NestedStructWithTraitsImpl>::
+    SetUpContext(const test::NestedStructWithTraitsImpl& input) {
+  Context* context = new Context;
+  context->value = input.value;
+  return context;
+}
+
+// static
+void StructTraits<test::NestedStructWithTraits,
+                  test::NestedStructWithTraitsImpl>::
+    TearDownContext(const test::NestedStructWithTraitsImpl& input,
+                    void* context) {
+  Context* context_obj = static_cast<Context*>(context);
+  CHECK_EQ(context_obj->value, input.value);
+  delete context_obj;
+}
+
+// static
+int32_t StructTraits<test::NestedStructWithTraits,
+                     test::NestedStructWithTraitsImpl>::
+    value(const test::NestedStructWithTraitsImpl& input, void* context) {
+  Context* context_obj = static_cast<Context*>(context);
+  CHECK_EQ(context_obj->value, input.value);
+  return input.value;
+}
 
 // static
 bool StructTraits<test::NestedStructWithTraits,
                   test::NestedStructWithTraitsImpl>::
     Read(test::NestedStructWithTraitsDataView data,
          test::NestedStructWithTraitsImpl* output) {
   output->value = data.value();
   return true;
 }
 
@@ skipping 19 matching lines @@
   if (!data.ReadFStruct(&out->get_mutable_struct()))
     return false;
 
   if (!data.ReadFStructArray(&out->get_mutable_struct_array()))
     return false;
 
   return true;
 }
 
 }  // namespace mojo