devirt.cpp

#include <utility>
#include <typeinfo>

struct Base {
	virtual int f() = 0;
};

struct Derived1 : public Base {
	virtual int f() final override {
		return 42;
	}
};

struct Derived2 : public Base {
	virtual int f() final override {
		return 13;
	}
};

template <typename T>
struct pmf_traits;

template <typename T, typename R, typename... Ps>
struct pmf_traits<R (T::*)(Ps...)> {
	using self_type = R (T::*)(Ps...);
	
	using class_type = T;
	using result_type = R;
	
	using func_type = R (Ps...);
};

// ^^ repeat for cv-qualifiers

template <typename TPMF, TPMF pmf, typename... Ds>
struct devirt_caller;

template <typename TPMF, TPMF pmf>
struct devirt_caller<TPMF, pmf> {
	using T = typename pmf_traits<TPMF>::class_type;
	using R = typename pmf_traits<TPMF>::result_type;
	
	template <typename... Ps>
	static R call(T& o, Ps&& ...params) {
		return (o.*pmf)(std::forward<Ps>(params)...);
	}
};

template <typename TPMF, TPMF pmf, typename D, typename... Ds>
struct devirt_caller<TPMF, pmf, D, Ds...> {
	using T = typename pmf_traits<TPMF>::class_type;
	using R = typename pmf_traits<TPMF>::result_type;
	
	template <typename... Ps>
	static R call(T& o, Ps&& ...params) {
		if (typeid(o) == typeid(D)) {
			auto ptr = static_cast<D*>(&o);
			const auto rpmf = static_cast<R (D::*)(Ps...)>(pmf);
			
			return (ptr->*rpmf)(std::forward<Ps>(params)...);
		}
		
		return devirt_caller<TPMF, pmf, Ds...>::call(o, std::forward<Ps>(params)...);
	}
};

template <typename TPMF, TPMF pmf>
struct devirt {
	template <typename... Ds>
	using with = devirt_caller<TPMF, pmf, Ds...>;
};

#define make_devirt(a) devirt<decltype(a), a>

int test(Base& b) {
	return make_devirt(&Base::f)::with<Derived1, Derived2>::call(b);
}

// Above should statically unfold to the following (but doesn't)
int test2(Base& b) {
	if (typeid(b) == typeid(Derived1))
		return static_cast<Derived1*>(&b)->f();
	
	if (typeid(b) == typeid(Derived2))
		return static_cast<Derived2*>(&b)->f();
	
	return b.f();
}