Boost C++ Libraries: Ticket #9799: polymorphic_downcast does not support virtual inheritance

attachment set

Thu, 20 Mar 2014 16:34:54 GMT

attachment → cast.diff

status changed; resolution set

Steven Watanabe — Thu, 20 Mar 2014 17:34:20 GMT

status new → closed
resolution → wontfix

The real question is whether should this be supported.

From the documentation: "... or when efficiency is not important, polymorphic_cast is preferred." Since we have to do a dynamic_cast anyway, efficiency really doesn't matter.

It's also clear that the behavior is consistent with the documentation:

template <class Derived, class Base>
inline Derived polymorphic_downcast(Base* x);
// Effects: assert( dynamic_cast<Derived>(x) == x );
// Returns: static_cast<Derived>(x)

Also, your patch doesn't compile.

Thu, 20 Mar 2014 23:29:11 GMT

IMHO, in the case of virtual inheritance, dynamic_cast being the only viable way makes it the most efficient.

Still, this function not handling virtual inheritance should be documented : the current documentation does not tell anything about this case. Moreover, I don't think that static_cast's not handling this case is well-known enough.

Sorry for the patch, copy-paste errors come from a workaround used elsewhere. Here comes a corrected version of the patch (which does compile).

attachment set

Thu, 20 Mar 2014 23:30:19 GMT

attachment → cast.2.diff

Corrected patch