Question

I’m not sure how to explain it because I don’t really know the correct terms, but here is an example:

template <typename T>
struct PointerWrapper {
    T *ptr;
};

struct Base {};

struct Derived : public Base {};

void test(PointerWrapper<Base>) {}

int main() {
    PointerWrapper<Derived> p;
    test(p);
    return 0;
}

Since “Derived” is derived from “Base” and the “PointerWrapper” struct only works with pointers to the type specified by the template, there is nothing wrong with this. But the compiler complains:

asdfg.cpp:15:11: error: could not convert ‘p’ from ‘PointerWrapper<Derived>’ to ‘PointerWrapper<Base>’

I even tried test(reinterpret_cast<PointerWrapper<Base>>(p)), but it says:

asdfg.cpp:15:51: error: invalid cast from type ‘PointerWrapper<Derived>’ to type ‘PointerWrapper<Base>’

So what should I do?

Answer 1

PointerWrapper<Base> and PointerWrapper<Derived> are utterly different types, despite the relationship between Base and Derived.

If you want to allow any PointerWrapper<> to be passed in, do:

template<typename T>
void test(PointerWrapper<T>)
{ }

If you want to allow only PointerWrapper<T> instances where T is derived from Base, do:

template<typename T>
void test(
    PointerWrapper<T>,
    typename std::enable_if<std::is_base_of<Base, T>::value, void*>::type = 0
)
{ }

If you’re using C++03, replace std:: with boost:: and get rid of ::value.