pagespeed/kernel/base/manually_ref_counted.h

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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_BASE_MANUALLY_REF_COUNTED_H_
#define PAGESPEED_KERNEL_BASE_MANUALLY_REF_COUNTED_H_

#include "base/logging.h"
#include "pagespeed/kernel/base/atomic_int32.h"
#include "pagespeed/kernel/base/basictypes.h"

namespace net_instaweb {

// Class that assists with manual reference counting.  Other classes should
// inherit from this one, and users of those classes should call IncrementRefs
// when making copies and DecrementRefs when they finish with those copies.
class ManuallyRefCounted {
 public:
  ManuallyRefCounted() : n_active(1) {}

  // Call this when duplicating a pointer to subclass instances:
  //   Example* e = other->GetExample();
  //   e->IncrementRefs();
  void IncrementRefs() { n_active.NoBarrierIncrement(1); }

  // Call this when finished with a pointer to a subclass instance:
  //   class ExampleHolder {
  //     Example* e;
  //     ~ExampleHolder() {
  //       if (e != NULL) {
  //         // If e is the last pointer to *e then *e will be deleted.
  //         e->DecrementRefs();
  //       }
  //     }
  //   }
  void DecrementRefs() {
    int n_others_active = n_active.BarrierIncrement(-1);
    if (n_others_active == 0) {
      delete this;
    }
  }

 protected:
  // The destructor isn't public because instances of ManuallyRefCounted
  // subclasses should not be directly deleted.  They should be released via
  // DecrementRefs().
  virtual ~ManuallyRefCounted() { DCHECK_EQ(0, n_active.value()); }

 private:
  AtomicInt32 n_active;

  DISALLOW_COPY_AND_ASSIGN(ManuallyRefCounted);
};

}  // namespace net_instaweb

#endif  // PAGESPEED_KERNEL_BASE_MANUALLY_REF_COUNTED_H_