downcast example

downcast.cpp

#include <type_traits>

#define DOWNCAST_METHOD(name_, Base_, Derived_)                             \
    template <typename Base_>                                               \
    static Derived_ *from(Base_ *base_) {                                   \
        static_assert(std::is_base_of<Base_, Derived_>::value,              \
                      "method from() requires base object as argument");    \
        return static_cast<Derived_*>(base_);                               \
    }


class A {
};

class B : public A {
public:
    DOWNCAST_METHOD(from, A, B)
};

class C {
};

class D : public C {
public:
    DOWNCAST_METHOD(from, C, D)
};

int main()
{
    B b;
    A *a = &b;
    B *x = B::from(a);

    // error
    //C *x = B::from(a);

    // error
    //C *c = D::from(a);

    // error
    //C *c = D::from(&b);

    return 0;
}