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diff --git a/core/fxcrt/shared_copy_on_write.h b/core/fxcrt/shared_copy_on_write.h
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+++ b/core/fxcrt/shared_copy_on_write.h
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+// Copyright 2016 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
+
+#ifndef CORE_FXCRT_SHARED_COPY_ON_WRITE_H_
+#define CORE_FXCRT_SHARED_COPY_ON_WRITE_H_
+
+#include "core/fxcrt/fx_system.h"
+#include "core/fxcrt/retain_ptr.h"
+
+namespace fxcrt {
+
+// A shared object with Copy on Write semantics that makes it appear as
+// if each one were independent.
+template <class ObjClass>
+class SharedCopyOnWrite {
+ public:
+  SharedCopyOnWrite() {}
+  SharedCopyOnWrite(const SharedCopyOnWrite& other)
+      : m_pObject(other.m_pObject) {}
+  ~SharedCopyOnWrite() {}
+
+  template <typename... Args>
+  ObjClass* Emplace(Args... params) {
+    m_pObject.Reset(new CountedObj(params...));
+    return m_pObject.Get();
+  }
+
+  SharedCopyOnWrite& operator=(const SharedCopyOnWrite& that) {
+    if (*this != that)
+      m_pObject = that.m_pObject;
+    return *this;
+  }
+
+  void SetNull() { m_pObject.Reset(); }
+  const ObjClass* GetObject() const { return m_pObject.Get(); }
+
+  template <typename... Args>
+  ObjClass* GetPrivateCopy(Args... params) {
+    if (!m_pObject)
+      return Emplace(params...);
+    if (!m_pObject->HasOneRef())
+      m_pObject.Reset(new CountedObj(*m_pObject));
+    return m_pObject.Get();
+  }
+
+  bool operator==(const SharedCopyOnWrite& that) const {
+    return m_pObject == that.m_pObject;
+  }
+  bool operator!=(const SharedCopyOnWrite& that) const {
+    return !(*this == that);
+  }
+  explicit operator bool() const { return !!m_pObject; }
+
+ private:
+  class CountedObj : public ObjClass {
+   public:
+    template <typename... Args>
+    // NOLINTNEXTLINE(runtime/explicit)
+    CountedObj(Args... params) : ObjClass(params...), m_RefCount(0) {}
+
+    CountedObj(const CountedObj& src) : ObjClass(src), m_RefCount(0) {}
+    ~CountedObj() { m_RefCount = 0; }
+
+    bool HasOneRef() const { return m_RefCount == 1; }
+    void Retain() { m_RefCount++; }
+    void Release() {
+      ASSERT(m_RefCount);
+      if (--m_RefCount == 0)
+        delete this;
+    }
+
+   private:
+    // To ensure ref counts do not overflow, consider the worst possible case:
+    // the entire address space contains nothing but pointers to this object.
+    // Since the count increments with each new pointer, the largest value is
+    // the number of pointers that can fit into the address space. The size of
+    // the address space itself is a good upper bound on it.
+    intptr_t m_RefCount;
+  };
+
+  RetainPtr<CountedObj> m_pObject;
+};
+
+}  // namespace fxcrt
+
+using fxcrt::SharedCopyOnWrite;
+
+#endif  // CORE_FXCRT_SHARED_COPY_ON_WRITE_H_