I think you could actually do this in C++, but not with runtime polymorphism. I think you should be able to define extractBody and extractTextBody as template functions. If the signatures (return type, name and parameters) of the functions of the interface are identical - and it looks like they are - then this would work as a C++ template function. Behind the scenes, the compiler would generate two different kinds of functions for you, one for each of the types you called it with.

(Please note I'm not saying this would be better, just pointing out that it's possible.)