Mojo C++ bindings: custom type mapping of map

mojo/public/cpp/bindings/array_traits.h

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_H_
 
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as
 // a mojom array.
 //
 // Usually you would like to do a partial specialization for an array/vector
 // template. Imagine you want to specialize it for CustomArray<>, you need to
 // implement:
 //
 //   template <typename T>
 //   struct ArrayTraits<CustomArray<T>> {
 //     using Element = T;
 //
 //     // These two methods are optional. Please see comments in struct_traits.h
 //     static bool IsNull(const CustomArray<T>& input);
 //     static void SetToNull(CustomArray<T>* output);
 //
 //     static size_t GetSize(const CustomArray<T>& input);
 //
+//     // These two methods are optional. They are used to access the
+//     // underlying storage of the array to speed up copy of POD types.
 //     static T* GetData(CustomArray<T>& input);
 //     static const T* GetData(const CustomArray<T>& input);
 //
 //     static T& GetAt(CustomArray<T>& input, size_t index);
 //     static const T& GetAt(const CustomArray<T>& input, size_t index);
 //
 //     static void Resize(CustomArray<T>& input, size_t size);
 //   };
 //
 template <typename T>
 struct ArrayTraits;
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_H_