Re: [Boost-users] Trouble with deriving from boost::archive

From: Robert Ramey (ramey_at_[hidden])
Date: 2005-10-08 23:57:49

Try deriving form text_iarchive_impl.  Note that a comment in text_iarchive.hpp explicitly states that it shouldn't be derived from.  For an example on how to derive from an existing archive look at demo_fast_archive and review the section of the manual which discusses this topic.

Robert Ramey
  "Haskins, Gregory" <ghaskins_at_[hidden]> wrote in message news:5CFCA4B6F572E74384A26FBA61CAE837018A5D34_at_MAANDMBX1.ets.enterasys.com...
  I posted this earlier, but I don't think it went through all the way.  I apologize if this is a duplicate:

  --------------------------------------------------------------------------------------------

  Hi All,
    Boost newbie here:  I am having problems with getting serialization to work
  with my app.  I have a top-level "environment" context that objects in my app
  need to be associated with.  I thought about various ways to accomplish this 
  and I thought the best way might be to derive an Archive class that maintains a
  reference to the context and use custom save_construct_data/load_construct_data
  overrides.

  However, I am hitting a snag and I don't quite understand what is happening.  I
  derive an input and output archive from text_iarchive/text_oarchive 
  respectively and I use this archive context.  However I get the following 
  error in compilation:

  db.hh: In function `void boost::serialization::load_construct_data(Archive&, 
     FlatFileDb::Db*, unsigned int) [with Archive = 
     boost::archive::text_iarchive]':
  /ccase/cca/vendor/boost/boost/serialization/serialization.hpp:170:  
  instantiated from `void boost::serialization::load_construct_data_adl(Archive&,
   T*, unsigned int) 
  [with Archive = boost::archive::text_iarchive, T = FlatFileDb::Db]'
  /ccase/cca/vendor/boost/boost/archive/detail/iserializer.hpp:303:  instantiated
  from `void boost::archive::detail::pointer_iserializer<T, 
  Archive>::load_object_ptr(boost::archive::detail::basic_iarchive&, void*&,
  unsigned int) const [with T = FlatFileDb::Db, Archive =
  boost::archive::text_iarchive]'
  /ccase/cca/vendor/boost/boost/archive/detail/iserializer.hpp:209:   
  instantiated from here
  db.hh:84: error: 'class boost::archive::text_iarchive' has no member named '
     m_env'

  Somehow the system is assigning <class Archive> to 'class
  boost::archive::text_iarchive' even though I am creating the archive as
  something derived from text_iarchive.  The weird thing is that this happens 
  even if I comment out the code the invokes the actual archiving.  Its almost 
  like there is some default value of Archive that gets assigned once I include 
  the text_iarchive.hpp file.  Any ideas on how to stop this from happening?  
  Is there a better way to accomplish my goal?

  Thanks in advance!
  -Greg

  PS: Here are some code snippets of what I am doing.

  I have a header file called "archive.hh" with

  -----------------------------------------------------------

  #include <boost/archive/text_iarchive.hpp>
  #include <boost/archive/text_oarchive.hpp>

  namespace FlatFileDb
  {
      class Env;

      class OArchive : public boost::archive::text_oarchive_impl<OArchive>
      {
      public:
          
          OArchive(Env &env, std::ostream & os, unsigned int flags = 0) :
              boost::archive::text_oarchive_impl<OArchive>(os, flags), m_env(env)
              {}
          ~OArchive(){} 
          
          Env &m_env;
      };  

      class IArchive : public boost::archive::text_iarchive_impl<IArchive>
      {
      public:
          
          IArchive(Env &env, std::istream & is, unsigned int flags = 0) :
              boost::archive::text_iarchive_impl<IArchive>(is, flags), m_env(env)
              {}
          ~IArchive(){} 
          
          Env &m_env;
      };
  };

  --------------------------------------------------

  and then I have another header file (db.hh) with:

  --------------------------------------------------

  <.../snip...>

  namespace boost { namespace serialization {
  template<class Archive>
  inline void save_construct_data(
      Archive & ar, const FlatFileDb::Db *t, const unsigned int file_version
  ){
      // save data required to construct instance
      ar << t->m_name;
  }

  template<class Archive>
  inline void load_construct_data(
      Archive & ar, FlatFileDb::Db* t, const unsigned int file_version
  ){
      // retrieve data from archive required to construct new instance
      std::string name;
      ar >> name;
      // invoke inplace constructor to initialize instance of my_class
      ::new(t)FlatFileDb::Db(ar.m_env, name);  <-- !!! blows up here !!!y
  }
  }} // namespace ...

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