[Boost-users] multi_index with base class member

please give me some advices. I get a compile error,when to index on base class member. the codes are listed below: struct Foo { int a; }; struct Bar : public Foo { int b; }; struct idx0 {}; typedef multi_index_container< Bar, indexed_by< ordered_unique< tag < idx0 > ,member < Bar,int,&Bar::a>

test_type;

Error 1 error C2440: 'specialization' : cannot convert from 'int Foo::* ' to 'int Bar::* ' and the codes are compiled in boost 1.40, vs 2008 sp1