refcount.h

// Copyright (c) 2013, Kenton Varda <temporal@gmail.com>
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this
//    list of conditions and the following disclaimer.
// 2. 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.
//
// 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.

#include "memory.h"

#ifndef KJ_REFCOUNT_H_
#define KJ_REFCOUNT_H_

namespace kj {

class Refcounted: private Disposer {
  // Subclass this to create a class that contains an atomic reference count.  Then, use
  // `kj::refcounted<T>()` to allocate a new refcounted pointer.
  //
  // Do NOT use this lightly.  Refcounting is a crutch.  Good designs should strive to make object
  // ownership clear, so that refcounting is not necessary.  All that said, reference counting can
  // sometimes simplify code that would otherwise become convoluted with explicit ownership, even
  // when ownership relationships are clear at an abstract level.
  //
  // NOT THREADSAFE:  This refcounting implementation assumes that an object's references are
  // manipulated only in one thread, because atomic (thread-safe) refcounting is surprisingly slow.
  //
  // In general, abstract classes should _not_ subclass this.  The concrete class at the bottom
  // of the heirarchy should be the one to decide how it implements refcounting.  Interfaces should
  // expose only an `addRef()` method that returns `Own<InterfaceType>`.  There are two reasons for
  // this rule:
  // 1. Interfaces would need to virtually inherit Refcounted, otherwise two refcounted interfaces
  //    could not be inherited by the same subclass.  Virtual inheritance is awkward and
  //    inefficient.
  // 2. An implementation may decide that it would rather return a copy than a refcount, or use
  //    some other strategy.
  //
  // TODO(cleanup):  Rethink above.  Virtual inheritance is not necessarily that bad.  OTOH, a
  //   virtual function call for every refcount is sad in its own way.  A Ref<T> type to replace
  //   Own<T> could also be nice.

public:
  virtual ~Refcounted() noexcept(false);

private:
  mutable uint refcount = 0;
  // "mutable" because disposeImpl() is const.  Bleh.

  void disposeImpl(void* pointer) const override;
  template <typename T>
  static Own<T> addRefInternal(T* object);

  template <typename T>
  friend Own<T> addRef(T& object);
  template <typename T, typename... Params>
  friend Own<T> refcounted(Params&&... params);
};

template <typename T, typename... Params>
inline Own<T> refcounted(Params&&... params) {
  // Allocate a new refcounted instance of T, passing `params` to its constructor.  Returns an
  // initial reference to the object.  More references can be created with `kj::addRef()`.

  return Refcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
}

template <typename T>
Own<T> addRef(T& object) {
  // Return a new reference to `object`, which must subclass Refcounted and have been allocated
  // using `kj::refcounted<>()`.  It is suggested that subclasses implement a non-static addRef()
  // method which wraps this and returns the appropriate type.

  KJ_IREQUIRE(object.Refcounted::refcount > 0, "Object not allocated with kj::refcounted().");
  return Refcounted::addRefInternal(&object);
}

template <typename T>
Own<T> Refcounted::addRefInternal(T* object) {
  Refcounted* refcounted = object;
  ++refcounted->refcount;
  return Own<T>(object, *refcounted);
}

}  // namespace kj

#endif  // KJ_REFCOUNT_H_