Re: Persistence Library Needed in Boost? (Was: [boost] A tip from the Dr.)

From: quendezus (quendez_at_[hidden])
Date: 2002-02-15 05:05:46

--- In boost_at_y..., Matthias Troyer <troyer_at_i...> wrote:
> 
> On Wednesday, February 13, 2002, at 06:19  PM, quendezus wrote:
> >>
> >> No, I need at least three different types of streams at runtime.
> >> That's why I think I need some type of polymorphism (templates or
> >> virtual functions) on the stream.
> >>
> >> Matthias
> >
> > Ok, I think you are right: the person who calls the serialization
> > function should be able to transmit some info to the serialization
> > code (in your case, the stream to format the output). I realize 
this
> > is necessary.
> >
> > Therefore the type of the stream (or the type of a context class)
> > should be a template parameter :
> >
> > template < class T, class Context >
> > void serialize( std::streambuf& dest, const T& t, Context& t )
> >
> > or:
> >
> > template < class T, class Stream >
> > void serialize( Stream& dest, const T& t )
> >
> > I have already tried a solution like that and it seems to work 
but it
> > is complicated. I will think about how to simplify it.
> 
> Jen's library already does this in a very elegant way using
> templates. My concern was that it seems in some cases I need
> runtime-polymorphism using virtual functions.
> 
> Matthias

Jen's design is elegant but does not deal with types unknown at 
compile time. When dealing with types only known at runtime, it is 
necessary, I think, to pass the Reader and Writer template parameters 
during the registration process. It adds complexity.

Can you explain some more why you need runtime-polymorphism? For 
example, using Jen's code, why is the following not sufficient in 
your case?

  std::map<std::string, person> database;
  database.insert(std::make_pair(me.name, me));
  database.insert(std::make_pair(boss.name, boss));

  // Using one type of stream
  boost::save_file<boost::shift_writer>(database, "data.txt");

  // Using another type of stream
  boost::save_file<boost::binary_writer>(database, "data.txt");

Sylvain