weak_ref

// vi:set ft=cpp: -*- Mode: C++ -*-
/*
 * Copyright (C) 2015, 2017 Kernkonzept GmbH.
 * Author(s): Sarah Hoffmann <sarah.hoffmann@kernkonzept.com>
 *            Alwexander Warg <alexander.warg@kernkonzept.com>
 *
 * This file is distributed under the terms of the GNU General Public
 * License, version 2.  Please see the COPYING-GPL-2 file for details.
 */
#pragma once
 
#include "hlist"
 
namespace cxx {
 
class Weak_ref_base : public H_list_item_t<Weak_ref_base>
{
protected:
  Weak_ref_base(void const *ptr = nullptr) : _obj(ptr) {}
  void reset_hard() { _obj = nullptr; }
  void const *_obj;
 
public:
  struct List : H_list_t<Weak_ref_base>
  {
    void reset()
    {
      while (!empty())
        pop_front()->reset_hard();
    }
 
    ~List()
    { reset(); }
  };
 
  explicit operator bool () const
  { return _obj ? true : false; }
};
 
 
template <typename T>
class Weak_ref : public Weak_ref_base
{
public:
  T *get() const
  { return reinterpret_cast<T*>(const_cast<void *>(_obj)); }
 
  T *reset(T *n)
  {
    T *r = get();
    if (r)
      r->remove_weak_ref(this);
 
    _obj = n;
    if (n)
      n->add_weak_ref(this);
 
    return r;
  }
 
  Weak_ref(T *s = nullptr) : Weak_ref_base(s)
  {
    if (s)
      s->add_weak_ref(this);
  }
 
  ~Weak_ref() { reset(0); }
 
  void operator = (T *n)
  { reset(n); }
 
  Weak_ref(Weak_ref const &o) : Weak_ref_base(o._obj)
  {
    if (T *x = get())
      x->add_weak_ref(this);
  }
 
  Weak_ref &operator = (Weak_ref const &o)
  {
    if (&o == this)
      return *this;
 
    reset(o.get());
    return *this;
  }
 
  T &operator * () const { return get(); }
  T *operator -> () const { return get(); }
};
 
class Weak_ref_obj
{
protected:
  template <typename T> friend class Weak_ref;
  mutable Weak_ref_base::List weak_references;
 
  void add_weak_ref(Weak_ref_base *ref) const
  { weak_references.push_front(ref); }
 
  void remove_weak_ref(Weak_ref_base *ref) const
  { weak_references.remove(ref); }
};
 
}