Re: [boost] [intrusive_ptr] atomic ref counting?

From: Eric Niebler (eric_at_[hidden])
Date: 2006-04-17 13:01:09

Andreas Huber wrote:
> Eric Niebler wrote:
> [snipped]
> 
> FWIW, I rolled my own counted_base class for statechart. It can be found 
> here:
> http://cvs.sourceforge.net/viewcvs.py/boost/boost/boost/statechart/detail/counted_base.hpp?view=markup
> 
> I'd also be interested in intrusive_ptr providing a class along the 
> lines of counted_base...

I've added something similar to your counted_base to xpressive, and it 
works well. I think there is a clear need for this to be a standard part 
of intrusive_ptr. A quick grep through the archives turns up several 
requests for this functionality, including a recent request from Ulrich 
Eckhardt.

The implementation is quite trivial (see below). Peter, can this (or 
something like it) just be added to intrusive_ptr.hpp, or should we go 
through a fast-track review? If you want to just add it, I can see what 
I can do about tests and docs.

Open questions:
1) What about the name (counted_base)? Can we do better?
2) Should there be a way to get the actual ref-count, or is bool 
unique() enough (like shared_ptr)?

<<< Begin Code >>

//////////////////////////////////////////////////////////////////////////////
// (c) Copyright Andreas Huber Doenni 2002-2005, Eric Niebler 2006
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//////////////////////////////////////////////////////////////////////////////

#ifndef BOOST_XPRESSIVE_DETAIL_UTILITY_COUNTED_BASE_HPP_EAN_04_16_2006
#define BOOST_XPRESSIVE_DETAIL_UTILITY_COUNTED_BASE_HPP_EAN_04_16_2006

#include <boost/assert.hpp>
#include <boost/noncopyable.hpp>
#include <boost/checked_delete.hpp>
#include <boost/detail/atomic_count.hpp>

namespace boost { namespace xpressive { namespace detail
{
     template<typename Derived>
     struct counted_base_access;

     ////////////////////////////////////////////////////////////////////
     // counted_base
     template<typename Derived>
     struct counted_base
       : private noncopyable
     {
         bool unique() const
         {
             return 1 == this->count_;
         }

     protected:
         counted_base()
           : count_(0)
         {
         }

     private:
         friend struct counted_base_access<Derived>;
         mutable boost::detail::atomic_count count_;
     };

     ////////////////////////////////////////////////////////////////////
     // counted_base_access
     template<typename Derived>
     struct counted_base_access
     {
         static void add_ref(counted_base<Derived> const *that)
         {
             ++that->count_;
         }

         static void release(counted_base<Derived> const *that)
         {
             BOOST_ASSERT(0 < that->count_);
             if(0 == --that->count_)
             {
                 boost::checked_delete(
                     static_cast<Derived const *>(that));
             }
         }
     };

     template<typename Derived>
     inline void intrusive_ptr_add_ref(counted_base<Derived> const *that)
     {
         counted_base_access<Derived>::add_ref(that);
     }

     template<typename Derived>
     inline void intrusive_ptr_release(counted_base<Derived> const *that)
     {
         counted_base_access<Derived>::release(that);
     }

}}} // namespace boost::xpressive::detail

#endif

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com