Re: [Boost-users] [serialization] Skipping unexpected elements in XMLarchive

From: Robert Ramey (ramey_at_[hidden])
Date: 2006-07-22 17:34:40

Sounds to me that others might find this useful.  How about making a little 
write up, and maybe a small test/demo, zipping it up and uploading to the 
serialization section of the boost vault?

Robert Ramey.

Todd Greer wrote:
> Hi,
>
> I'm using the Serialization library, but need to skip unrecognized
> data when reading in an archive. This is for forward compatability
> purposes; I need for older versions of my software to consume files
> produced by newer ones, which may have added elements.
>
> As this functionality is not currently in the library, I have created
> a new archive type (as I think Robert Ramey suggested at some point).
> It inherits from xml_iarchive_impl and overrides load_override to
> identify when it has just read an opening tag with an unexpected
> name. It then skips whatever XML it finds within the unexpected
> element, reads the closing tag, and returns to normal processing. I
> use it with the unaltered xml_oarchive, and the pair satisfy my
> requirements.
>
> I'm including it below in the hopes that someone will find it useful.
> I've tested it on MSVC 7.1 (Visual Studio .NET 2003) with a reasonably
> complex XML file with both nested and non-nested unexpected elements.
> If you'd like, I can post some tests as well (I just need to strip
> out some project-specific details).
>
> Please let me know if you see any places where it could be improved,
> or where I've completely missed something; this is my first use of the
> serialization library, and my understanding is far from comprehensive.
>
> Many thanks to all who've contributed to the Serialization library,
> Todd Greer    <tgreer <at> affinegy dot com>
> Senior Software Developer, Affinegy LLC
>
>
> #include <boost/archive/xml_iarchive.hpp>
> #include <boost/range/end.hpp>
> #include <algorithm>
>
> const char whitespace[] = " \t\n\r";
>
> //Skip whitespace, then return true if we're looking at '</'.
> //Leave the stream right before the '<'.
> template<class IStream>
> bool at_close_tag(IStream& is)
> {
>  char ch;
>  using boost::end;
>  do
>  {
>    ch = is.get();
>  } while(std::find(whitespace, end(whitespace), ch) !=
> end(whitespace));
>
>  assert(ch == '<');
>  ch = is.get();
>  bool const end_tag = ch == '/';
>  is.putback(ch);
>  is.putback('<');
>  return end_tag;
> }
>
> template<class Archive>
> class skip_xml_iarchive_impl: public
> boost::archive::xml_iarchive_impl<Archive>
> {
> #if BOOST_WORKAROUND(BOOST_MSVC, <= 1300) //see xml_iarchive_impl
> public:
> #elif defined(BOOST_MSVC)
>  friend boost::archive::detail::interface_iarchive<Archive>;
> protected:
> #else
>  friend class boost::archive::detail::interface_iarchive<Archive>;
> protected:
> #endif
>
>  skip_xml_iarchive_impl(std::istream & is, unsigned int flags = 0):
> xml_iarchive_impl<Archive>(is, flags)
>  {}
>
>  void skip_all_elements()
>  {
>    std::string ignore;
>    if(this->This()->gimpl->parse_string(this->This()->get_is(),
>      ignore) && ignore.npos != ignore.find_first_not_of(whitespace))
>      return;
>
>    //We're either looking at a start tag or an end tag.
>    while(!at_close_tag(this->This()->get_is()))
>    {//It's a start tag.
>      if(!this->This()->gimpl->parse_start_tag(this->This()->get_is()))
>
> boost::throw_exception(archive_exception(archive_exception::stream_error
> ));
>
>      ++depth;
>      std::string const name = this->This()->gimpl->rv.object_name;
>      skip_all_elements();
>      load_end(name.c_str());
>    }
>  }
>
>  template<class T>
>    void load_override(
> #ifndef BOOST_NO_FUNCTION_TEMPLATE_ORDERING
>    const
> #endif
>    boost::serialization::nvp<T> & t,
>    int
>    ){
>      if(!t.name())
>        return xml_iarchive_impl<Archive>::load_override(t, 0);
>      load_start(t.name());
>      // don't check start tag at highest level
>      const std::string name_found =
> this->This()->gimpl->rv.object_name;
>      bool const skip =
>        depth > 1
>        && 0 != (this->get_flags() & no_xml_tag_checking)
>        && name_found != t.name();
>      if(skip)
>        skip_all_elements();
>      else
>        boost::archive::load(* this->This(), t.value());
>      load_end(skip ? name_found.c_str() : t.name());
>      if(skip) //We still haven't loaded t...
>        load_override(t, 0); //...so load t.
>    }
>
>    template<class T>
>    void load_override(T & t, BOOST_PFTO int i)
>    { xml_iarchive_impl<Archive>::load_override(t, i); }
> };
>
> class skip_xml_iarchive: public
> skip_xml_iarchive_impl<skip_xml_iarchive>
> {
> public:
>    skip_xml_iarchive(std::istream & is, unsigned int flags = 0) :
>        skip_xml_iarchive_impl<skip_xml_iarchive>(is, flags)
>    {}
> };