I have another user case: template <class Final, typename Base=base_transaction_object> class deep_transaction_object : public Base { Final* final() { return static_cast<Final*>(this); } const Final* final() const { return static_cast<Final const*>(this); } public: static Final* make_cache(Final const* rhs, transaction& ) { return new Final(*rhs); } virtual base_transaction_object* make_cache(transaction& t) const { return new Final(*final()); } virtual void delete_cache() { delete this; } virtual void copy_cache(base_transaction_object const & rhs) { *final() = *smart_cast<Final const*>(&rhs); // (1) } }; deep_transaction_object is a mixin class that is be used as follows class F : public deep_transaction_object<F> {...}; At the moment we use smart_cast in deep_transaction_object (1), F is not completly defined. smart_cast is a template class that needs its class parameters to be completly defined, as is_virtual_base_of needs this. In this context I can not check if Final is_virtual_base_of base_transaction_object. As F is a deep_transaction_object<F> which is a Base, the check could be limited to see if Base is_virtual_base_of base_transaction_object. Thus I need a kind of incomplete_smart_cast which in addition has an intermidiate class used for the tests *final() = *incomplete_smart_cast<Final const *, Base const*>(&rhs); incomplete_smart_cast<Target,Intermediate, Source> works like smart_cast except that instead of testing with Target, it test with Intermediate to select the cast to apply to target incomplete_smart_cast<F*, I*>(ptr) = static_cast<F*,smart_cast<I*>(ptr)> For the case of single inheritance this seems to too much to add, but with multiple inheritance the situation we can not build it from static_cast and smart_cast, we need to check if one of the intermediate classes is_virtual_base_of the base class. For example if the mixin allows to inherit from two classes template <class Final, class Base1, class Base1> class deep_transaction_object2 : public Base1, public Base2 { ... virtual void copy_cache(base_transaction_object const & rhs) { *final() = *incomplete_smart_cast2<Final const*, Base1 const*, Base2 const*>(&rhs); // (1) } }; Please let me know if my rationale is correct and if my use case is enough common to interest the Boost community. Best _____________________ Vicente Juan Botet Escribá