copy_on_write.h

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/*
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http:///www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


#ifndef PAGESPEED_KERNEL_UTIL_COPY_ON_WRITE_H_
#define PAGESPEED_KERNEL_UTIL_COPY_ON_WRITE_H_

#include "pagespeed/kernel/base/ref_counted_ptr.h"

namespace net_instaweb {

template<class T>
class CopyOnWrite {
 public:
  CopyOnWrite() {}

  explicit CopyOnWrite(const T& obj) : reference_(obj) {}
  ~CopyOnWrite() {}

  const T* get() const { return reference_.get(); }
  const T* operator->() const { return reference_.get(); }
  const T& operator*() const { return *reference_.get(); }

  T* MakeWriteable() {
    if (!reference_.unique()) {
      reference_.reset(*get());
    }
    return reference_.get();
  }

  CopyOnWrite& operator=(const CopyOnWrite& src) {
    if (&src != this) {
      reference_ = src.reference_;
    }
    return *this;
  }

  CopyOnWrite(const CopyOnWrite& src) : reference_(src.reference_) {}

  void MergeOrShare(const CopyOnWrite& src) {
    if (!src->empty()) {
      if ((*this)->empty()) {
        *this = src;
      } else {
        MakeWriteable()->Merge(*src.get());
      }
    }
  }

 private:
  typedef RefCountedObj<T> Reference;

  Reference reference_;

};

}  

#endif