[boost] [serialization] Big gotcha with BOOST_CLASS_EXPORT

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-01-21 09:42:00

I've just spend about an hour with a colleague looking for a very subtle 
problem with serialization and BOOST_CLASS_EXPORT.

What we were trying to do is to have some code which reads pointers to base 
class and does something with it. On the saving side, we've got several 
functions which store various derived classes. 

Now the example:

    class B { ... } ;
    class D { ... } ;
    BOOST_CLASS_EXPORT(D);

    D* d = new D;
    oa << d;

Looks innocent, but it does not store the name of derived class that is 
stored. So, after loading with

    B* b;
    is >> b;

you get object of type B, not D. I attach the program which reproduces this
problem.

The gotcha is that if you write

    B* d = new D;
    oa << d;

everything works. But it's nonintuitive, and the mistake is very easy to make 
and very hard to notice! In my case, one function was intended for saving 
specific derived type, so it naturally declared a pointer of that type, and 
it took an hour to debug the problem.

Is it possible to always save proper identification when saving a pointer to 
polymorphic objects?

- Volodya