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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_UTIL_CONVERTER_STRUCTURED_OBJECTWRITER_H__
+#define GOOGLE_PROTOBUF_UTIL_CONVERTER_STRUCTURED_OBJECTWRITER_H__
+
+#include <memory>
+#ifndef _SHARED_PTR_H
+#include <google/protobuf/stubs/shared_ptr.h>
+#endif
+
+#include <google/protobuf/stubs/casts.h>
+#include <google/protobuf/stubs/common.h>
+#include <google/protobuf/util/internal/object_writer.h>
+
+namespace google {
+namespace protobuf {
+namespace util {
+namespace converter {
+
+// An StructuredObjectWriter is an ObjectWriter for writing
+// tree-structured data in a stream of events representing objects
+// and collections. Implementation of this interface can be used to
+// write an object stream to an in-memory structure, protobufs,
+// JSON, XML, or any other output format desired. The ObjectSource
+// interface is typically used as the source of an object stream.
+//
+// See JsonObjectWriter for a sample implementation of
+// StructuredObjectWriter and its use.
+//
+// Derived classes could be thread-unsafe.
+class LIBPROTOBUF_EXPORT StructuredObjectWriter : public ObjectWriter {
+ public:
+  virtual ~StructuredObjectWriter() {}
+
+ protected:
+  // A base element class for subclasses to extend, makes tracking state easier.
+  //
+  // StructuredObjectWriter behaves as a visitor. BaseElement represents a node
+  // in the input tree. Implementation of StructuredObjectWriter should also
+  // extend BaseElement to keep track of the location in the input tree.
+  class LIBPROTOBUF_EXPORT BaseElement {
+   public:
+    // Takes ownership of the parent Element.
+    explicit BaseElement(BaseElement* parent)
+        : parent_(parent), level_(parent == NULL ? 0 : parent->level() + 1) {}
+    virtual ~BaseElement() {}
+
+    // Releases ownership of the parent and returns a pointer to it.
+    template <typename ElementType>
+    ElementType* pop() {
+      return down_cast<ElementType*>(parent_.release());
+    }
+
+    // Returns true if this element is the root.
+    bool is_root() const { return parent_ == NULL; }
+
+    // Returns the number of hops from this element to the root element.
+    int level() const { return level_; }
+
+   protected:
+    // Returns pointer to parent element without releasing ownership.
+    virtual BaseElement* parent() const { return parent_.get(); }
+
+   private:
+    // Pointer to the parent Element.
+    google::protobuf::scoped_ptr<BaseElement> parent_;
+
+    // Number of hops to the root Element.
+    // The root Element has NULL parent_ and a level_ of 0.
+    const int level_;
+
+    GOOGLE_DISALLOW_IMPLICIT_CONSTRUCTORS(BaseElement);
+  };
+
+  StructuredObjectWriter() {}
+
+  // Returns the current element. Used for indentation and name overrides.
+  virtual BaseElement* element() = 0;
+
+ private:
+  // Do not add any data members to this class.
+  GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(StructuredObjectWriter);
+};
+
+}  // namespace converter
+}  // namespace util
+}  // namespace protobuf
+
+}  // namespace google
+#endif  // GOOGLE_PROTOBUF_UTIL_CONVERTER_STRUCTURED_OBJECTWRITER_H__