include/tscore/Ptr.h

/** @file

  Reference-counting shared pointer, like std::shared_ptr.

  @section license License

  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.
 */

#pragma once

#include <atomic>
#include <cstddef>

////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////
//////              ATOMIC VERSIONS
////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////

struct ForceVFPTToTop {
  virtual ~ForceVFPTToTop() {}
};

////////////////////////////////////////////////////////////////////////
//
// class RefCountObj
// prototypical class for reference counting
// not necessarily allocated with new
//
////////////////////////////////////////////////////////////////////////
class RefCountObj : public ForceVFPTToTop
{
public:
  RefCountObj() {}
  RefCountObj(const RefCountObj &) = delete;
  ~RefCountObj() override {}

  RefCountObj &operator=(const RefCountObj &) = delete;

  // Increment the reference count, returning the new count.
  int
  refcount_inc()
  {
    return ++m_refcount;
  }

  // Decrement the reference count, returning the new count.
  int
  refcount_dec()
  {
    return --m_refcount;
  }

  int
  refcount() const
  {
    return m_refcount;
  }

  virtual void free() = 0;

private:
  std::atomic<int> m_refcount = 0;
};

////////////////////////////////////////////////////////////////////////
//
// class RefCountObjInHeap
// reference counted object allocated with new
//
////////////////////////////////////////////////////////////////////////
class RefCountObjInHeap : public RefCountObj
{
public:
  void
  free() override
  {
    delete this;
  }
};

////////////////////////////////////////////////////////////////////////
//
// class Ptr
//
////////////////////////////////////////////////////////////////////////
template <class T> class Ptr
{
public:
  explicit Ptr(T *p = nullptr);
  Ptr(const Ptr<T> &);
  Ptr(Ptr<T> &&);
  ~Ptr();

  void    clear();
  Ptr<T> &operator=(const Ptr<T> &);
  Ptr<T> &operator=(Ptr<T> &&);
  Ptr<T> &operator=(T *);

  T *
  operator->() const
  {
    return (m_ptr);
  }
  T &
  operator*() const
  {
    return (*m_ptr);
  }

  // Making this explicit avoids unwanted conversions.  See https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool .
  explicit
  operator bool() const
  {
    return m_ptr != nullptr;
  }

  bool
  operator==(const T *p)
  {
    return (m_ptr == p);
  }

  bool
  operator==(const Ptr<T> &p) const
  {
    return (m_ptr == p.m_ptr);
  }

  bool
  operator!=(const T *p)
  {
    return (m_ptr != p);
  }

  bool
  operator!=(const Ptr<T> &p)
  {
    return (m_ptr != p.m_ptr);
  }

  // Return the raw pointer.
  T *
  get() const
  {
    return m_ptr;
  }

  // Return the raw pointer as a RefCount object. Typically
  // this is for keeping a collection of ogenous objects.
  RefCountObj *
  object() const
  {
    return static_cast<RefCountObj *>(m_ptr);
  }

  // Return the stored pointer, storing NULL instead. Do not increment
  // the refcount; the caller is now responsible for owning the RefCountObj.
  T *
  detach()
  {
    T *tmp = m_ptr;
    m_ptr  = nullptr;
    return tmp;
  }

  // XXX Clearly this is not safe. This is used in HdrHeap::unmarshal() to swizzle
  // the refcount of the managed heap pointers. That code needs to be cleaned up
  // so that this can be removed. Do not use this in new code.
  void
  swizzle(RefCountObj *ptr)
  {
    m_ptr = ptr;
  }

private:
  T *m_ptr;
};

template <typename T>
Ptr<T>
make_ptr(T *p)
{
  return Ptr<T>(p);
}

////////////////////////////////////////////////////////////////////////
//
// inline functions definitions
//
////////////////////////////////////////////////////////////////////////
template <class T> inline Ptr<T>::Ptr(T *ptr /* = 0 */) : m_ptr(ptr)
{
  if (m_ptr) {
    m_ptr->refcount_inc();
  }
}

template <class T> inline Ptr<T>::Ptr(const Ptr<T> &src) : m_ptr(src.m_ptr)
{
  if (m_ptr) {
    m_ptr->refcount_inc();
  }
}

template <class T> inline Ptr<T>::Ptr(Ptr<T> &&src) : m_ptr(src.m_ptr)
{
  src.m_ptr = nullptr;
}

template <class T> inline Ptr<T>::~Ptr()
{
  if (m_ptr && m_ptr->refcount_dec() == 0) {
    m_ptr->free();
  }
}

template <class T>
inline Ptr<T> &
Ptr<T>::operator=(T *p)
{
  T *temp_ptr = m_ptr;

  if (m_ptr == p) {
    return (*this);
  }

  m_ptr = p;

  if (m_ptr) {
    m_ptr->refcount_inc();
  }

  if (temp_ptr && temp_ptr->refcount_dec() == 0) {
    temp_ptr->free();
  }

  return (*this);
}

template <class T>
inline void
Ptr<T>::clear()
{
  if (m_ptr) {
    if (!m_ptr->refcount_dec()) {
      m_ptr->free();
    }
    m_ptr = nullptr;
  }
}

template <class T>
inline Ptr<T> &
Ptr<T>::operator=(const Ptr<T> &src)
{
  return (operator=(src.m_ptr));
}

template <class T>
inline Ptr<T> &
Ptr<T>::operator=(Ptr<T> &&src)
{
  if (this != &src) {
    this->~Ptr();
    m_ptr     = src.m_ptr;
    src.m_ptr = nullptr;
  }
  return *this;
}

// Bit of subtly here for the flipped version of equality checks
// With only the template versions, the compiler will try to substitute @c nullptr_t
// for @c T and fail, because that's not the type and no operator will be found.
// Therefore there needs to be specific overrides for @c nullptr_t.

template <typename T>
inline bool
operator==(std::nullptr_t, Ptr<T> const &rhs)
{
  return rhs.get() == nullptr;
}

template <typename T>
inline bool
operator!=(std::nullptr_t, Ptr<T> const &rhs)
{
  return rhs.get() != nullptr;
}

template <typename T>
inline bool
operator==(T const *lhs, Ptr<T> const &rhs)
{
  return rhs.get() == lhs;
}

template <typename T>
inline bool
operator!=(T const *lhs, Ptr<T> const &rhs)
{
  return rhs.get() != lhs;
}