External Polymorphism - Ways of getting the same extensibility and composability of types you get with 'normal' inheritance based polymorphism but without virtual functions or thunks and their associated costs. Without changing the types that you're extending and composing or imposing heavy requirements on them.
The idea has been around for at least thirty years and there have been lots of C++ implementations over the years. Mostly they started out recreating vtables outside the target classes in an effort to implement type erasure. These days type erasure is common place but, useful as it is, it is only one form of external polymorphism.
I've recently been porting a std::function alternative library magic_func which uses type erasure over function pointers and also looking at Ilya Korolev's Contracts library which puts what could maybe be external polymorphism back into the types being adapted. That gives you access to all the internals of the target type and uses the encapsulation of the target type to encapsulate the contract machinery as well.
Can we do some of what Contracts is doing without modifying the target classes and at the same time do type safe external polymorphism rather than fully generic type erasure.?
