[boost] [serialization] BOOST_CLASS_EXPORT worries

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-07-07 01:29:15

>> I recall that I did suggested one approach. BOOST_CLASS_EXPORT can
>> register the class with all previously included archives and,
>> unconditionally, with polymorphic archive. During saving, you can
>> check if the saved class is registered with specific archive type.
>> If not, you wrap archive in polymorphic archive and save. That would
>> be slower, but in most situation extra virtual function call won't
>> be a practical problem.
> 
>> In the rare case where it will be a problem, user can:
>> 1. Include necessary archive headers
>> 2. Invoke BOOST_CLASS_EXPORT *again* in his module, after including
>> another archive header. If BOOST_CLASS_EXPORT tolerates multiple
>> invocations, this will instantiate the code for the needed archive
>> type, and make saving to that archive type go without polymorphic
>> arhive.
> 
> export.hpp is already quite complicated.  This would seem to add a huge
> amount of effort at compiler and at rntime.  

Why? From my naive point of view, that would be a simple comparison when 
saving. As for double registering -- I suppose that if registration data is 
kept in some std::map, then there are problems at all.

> It would also add dead code 
> to
> almos t every executable using the library.  

IMO, this is clearly better than instantiate all of serialization library once 
for each archive type.

> It would seem a huge price to 
> pay just to permit one to say.
> 
> #include <boost/serialization/export.hpp
> #include <boost/archive/text_iarchive.hpp>
> 
> rather than
> 
> #include <boost/archive/text_iarchive.hpp>
> #include <boost/serialization/export.hpp

No, there's a different concern -- the current approach does not scale. Say I 
have my_class.hpp and my_class.cpp. Somewhere in those files I have 
BOOST_CLASS_EXPORT, which instantiates serialization code for a number of 
archive types. Now I write a new archive type, in my_cool_archive.cpp. 
Suddenly, serialization of MyClass does not work. Now I need to add another 
include to my_class.cpp and too all other files that have BOOST_CLASS_EXPORT 
in it. That's no good.

Now consider that a class is defined in a plugin. IIRC, that was one of the 
motivating examples for BOOST_CLASS_EXPORT. The situation becomes worse -- if 
a plugin is built with N archive types, and is packaged in binary form, but 
application defines it's own archive type, then serializing a class from 
plugin using that new archive type is not possible.

Now assume that we don't have plugins and applications written by the same 
developer, but just a shared library. Say, I want to write a library for 
manipulating EBNF grammar and want to add serialization. Client applications 
are not written by me at all. Now if that application defines a new archive 
type, nothing will work. Not to mention that I'm not happy with bloating my 
library with code for 5 archive types (binary, text, xml + wide variants of 
the latter two).

- Volodya