Re: [Boost-bugs] [Boost C++ Libraries] #1418: polymorphic archive performance improvement

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #1418: polymorphic archive performance improvement
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-11-26 20:23:03

#1418: polymorphic archive performance improvement
-------------------------------------------+--------------------------------
  Reporter:  Kim Barrett <kab@…>           |       Owner:  ramey        
      Type:  Patches                       |      Status:  assigned     
 Milestone:  To Be Determined              |   Component:  serialization
   Version:  Boost 1.34.1                  |    Severity:  Optimization 
Resolution:                                |    Keywords:               
-------------------------------------------+--------------------------------

Comment (by Kim Barrett <kab@…>):

 The following discussion is with respect to boost 1.43.  I haven't
 checked whether there are any relevant changes in more recent versions
 of boost.

 The proposed implementation of archive_cast involves some relatively
 intrusive changes to base_iarchive and base_oarchive to record the
 cached archive caster and cached archive pointer.  The reason for this
 is that there is no common base class between polymorphic_[i,o]archive
 and the non-polymorphic-archive implementation archives.

 polymorphic_iarchive_impl derives only from
 detail::interface_iarchive<polymorphic_iarchive>.

 polymorphic_iarchive derives only from polymorphic_iarchive_impl.

 binary_iarchive derives from binary_iarchive_impl<binary_iarchive, ...>.

 binary_iarchive_impl<Archive, ...> derives from
 basic_binary_iprimitive<Archive, ...> and
 basic_binary_iarchive<Archive>.

 basic_binary_iarchive<Archive> derives from
 detail::common_iarchive<Archive>.

 detail::common_iarchive<Archive> derives from
 detail::basic_iarchive and from detail::interface_iarchive<Archive>.

 So there are no common base classes between polymorphic_iarchive and
 binary_iarchive.

 If there were such a common base class (which would have to be a
 virtual base class on both inheritance branches), then the
 archive_cast conversion could consist of implicit up-conversion from
 the non-polymorphic-archive implementation class to that common base
 class, and then a static (or dynamic when debugging) down-cast to the
 polymorphic-archive class.

 The obvious place to put such a common base class would be to add a
 virtual non-template base class to detail::interface_iarchive<>.  This
 would make detail::interface_iarchive polymorphic (as in
 boost::is_polymorphic<>, not to be confused with the
 polymorphic-archive classes), which it presently isn't, so there is
 some cost there.

 [Note that the detail::interface_iarchive<> non-virtual default destructor
 is
 presently public, which is probably a mistake.  And the default public
 copy
 constructor and assignment operator are also present here, which also
 seems
 like a mistake.  I think a polymorphic_iarchive can improperly be copied
 or
 assigned.]

 A different attachment point for such a common virtual base class
 would be basic_iarchive and polymorphic_iarchive_impl, both of which
 are already polymorphic.

 If it is a base class for detail::interface_iarchive<> then the
 obvious name is detail::basic_interface_iarchive, and it provides no
 functionality (beyond non-public default constructor, non-public
 destructor, and suppressed copy constructor and assignment operator).
 Not sure what the name should be if the attachment point is at the
 polymorphic_iarchive_impl and base_iarchive layer.  Also likely
 requires more documentation there.  Presently, all archives derive
 from detail::interface_iarchive<>, though perhaps that really is an
 implementation detail.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/1418#comment:7>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.