Re: [Boost-users] Serialization : unregistered_cast & otherquestions

From: RIVASSEAU Jean Noel (JN.RIVASSEAU_at_[hidden])
Date: 2006-02-28 05:25:21

Hello

I agree with you, and currently have the same comprehension of the
problem than you. I am currently also including BOOST_EXPORT_CLASS in
only one implementation file *per class*. This seems to work.

Robert is saying however, that even that can cause problem, but as
written on a previous message, I cannot manage to follow his advice on
including the archive headers in only one instantiation module (except
putting all serialization code into that module, which I did previously:
this is a maintenance nightmare, to the point that I ended up trying to
break my code into smaller places).

I'll chime in, as I'm having exactly the same issue, but on darwin/ 
gcc-4 with code in multiple dynamically loaded libraries... (I have  
other issues too, I get to those later)

The problem is that although export.hpp is only included once per  
generated .o file, there are multiple .o files all including the same  
headers - and each therefore generates the same template  
instantiations, thereby causing the multiple definitions during  
linking.  Also - the same headers may be included in multiple shared  
libraries, exacerbating the problem with the same code instantiated  
into multiple libraries - a serious problem if there are singletons.

I may not be understanding this correctly - here's what >I< think  
needs to happen - please correct me.

For any derived class, there is a set of code (templates) that need  
to be instantiated for each archive type (specifically, the list  
boost::archive::detail::known_archive_types::type, for the templates  
guid_initializer<T> and export_instantiate<T, archive_list>)
 From what I see, these are NOT declarations required by the other  
serialization code, but singletons that, during construction,  
register the serializers to the derived classes with the  
extended_type_info class registry.  Is that incorrect?

If the above is correct, then the BOOST_EXPORT_CLASS should only be  
in exactly one implementation file (.cpp, .C, .cc, whatever).  And  
also, THAT implementation file should ONLY be linked into one shared  
library.

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