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diff --git a/mojo/public/cpp/bindings/string_traits.h b/mojo/public/cpp/bindings/string_traits.h
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+++ b/mojo/public/cpp/bindings/string_traits.h
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+// 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_STRING_TRAITS_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_
+
+#include "mojo/public/cpp/bindings/string_data_view.h"
+
+namespace mojo {
+
+// This must be specialized for any type |T| to be serialized/deserialized as
+// a mojom string.
+//
+// Imagine you want to specialize it for CustomString, usually you need to
+// implement:
+//
+//   template <T>
+//   struct StringTraits<CustomString> {
+//     // These two methods are optional. Please see comments in struct_traits.h
+//     static bool IsNull(const CustomString& input);
+//     static void SetToNull(CustomString* output);
+//
+//     static size_t GetSize(const CustomString& input);
+//     static const char* GetData(const CustomString& input);
+//
+//     // The caller guarantees that |!input.is_null()|.
+//     static bool Read(StringDataView input, CustomString* output);
+//   };
+//
+// In some cases, you may need to do conversion before you can return the size
+// and data as 8-bit characters for serialization. (For example, CustomString is
+// UTF-16 string). In that case, you can add two optional methods:
+//
+//   static void* SetUpContext(const CustomString& input);
+//   static void TearDownContext(const CustomString& input, void* context);
+//
+// And then you append a second parameter, void* context, to GetSize() and
+// GetData():
+//
+//   static size_t GetSize(const CustomString& input, void* context);
+//   static const char* GetData(const CustomString& input, void* context);
+//
+// If a CustomString instance is not null, the serialization code will call
+// SetUpContext() at the beginning, and pass the resulting context pointer to
+// GetSize()/GetData(). After serialization is done, it calls TearDownContext()
+// so that you can do any necessary cleanup.
+//
+template <typename T>
+struct StringTraits;
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_