[boost] Re:[serialization] serialization and pimpl

From: Vladimir Prus (ghost_at_[hidden])
Date: 2003-10-24 05:22:50

Robert Ramey wrote:

> I'm completely at loss how to serialize classes which use the pimpl idiom.
[...]
>>So, what's the solution?
> 
> This is a very interesting question.  It never came up with previous
> version of the library reviewed last november because that implementation
> used
> virtual functions.  In this case serialization could be kept in libraries
> as there was no "Archive" template parameter.
> 
> I moved to a templated version for a couple of reasons:
> 
> a) Its noticebly faster.
> b) I was going crazy trying to keep up with all variations of primitive
> types supported by different compilers.
> c) It started as an experiment to see what would happen.  I came to
> prefer it.
> 
> However, it created two issues.
> a) the one you describe here.
> b) related ones if one wants to create a system for "plug-ins" similar
> to Microsofts "COM"

Yes, that's the problem for plug-ins, too.

> First simple solution.
> 
> separt serialization in a separt module - presumably already done
> as part of the pimpl implementation.
> 
> Instantiate templates for all 5 currently archives.  Make sure that
> compile granularity is set properly or that the templated
> code is included in 5 separte modules.

I think this will be okay for me.

> This will work well and provide maximum perfomrance.  It can't be
> used to support a system of plug-ins whereby serialization is
> applied to all archives that meet the interface - including
> archive implementations yet to be written.
> 
> Second more involved solution.
> 
> It is possible to create another archive based on virtual functions.
> suppose we call it
> 
> virtual_archive
> 
> this would maintain the same archive interface but would
> dispatch to derived classes through the normal vtable
> dispatch mechanism.  

I was thinking about this same solution yesterday evening. It sure decreases
performance, but nothing's for free.

> Note that I personally have no plans to write such a
> virtual_archive but I would be pleased to support
> anyone who wants to.   I believe the system would
> currently support the construction of such a virtual
> archive with not changes to the current code.

I seems possible. The 'virtual_archive' will call virtual functions for
saving built-in types and serialization::save for all the rest? Is the
"vsave" family of method what you have in mind?

- Volodya