[boost] [serialization] test_void_cast problem diagnosed

From: Vladimir Prus (ghost_at_[hidden])
Date: 2003-11-19 02:56:50

Robert, I think I've finally figured out why test_void_cast was failing with 
gcc. Consider the following code:

 BOOST_CHECK(vpa == boost::serialization::void_upcast( 
        boost::serialization::extended_type_info_typeid<MostDerived>(),  
        boost::serialization::extended_type_info_typeid<Base1>(),  
        vpd 
    )); 

    BOOST_CHECK(vpb == boost::serialization::void_upcast( 
        boost::serialization::extended_type_info_typeid<MostDerived>(),  
        boost::serialization::extended_type_info_typeid<Base2>(),  
        vpd 
    )); 

The first call fails to find direct cast from MostDerived to Base1. So it 
finds a chain of casts and registers the final found cast: i.e. it creates a 
new instance of void_caster. That instances stores both from_type and 
to_type.

But... it stores them by reference. In this case, the reference points to 
temporary objects you've created in function call. So, after the first call 
is done, you have void_caster from MostDerived to Base1, but the references 
in that void_caster are invalid -- they refer to temporary objects that were 
destroyed after the call.

The second call creates a fresh pair of temporary objects, *at the same memory 
address*. So, the void_caster instance suddenly becomes caster from 
MostDerived to Base2. Of course, it does not work quite nice. If I use the 
'get_instance' method of extended_type_info_typeid, the simplified version of 
test_void_cast.cpp I was using starts to work.

Is there any chance you'll make the fix for all the calls to void_cast in 
test_void_cast.cpp and send me the updated version so that I could test 
further?

- Volodya