Question

If I wish to use dynamic_cast with a shared_ptr I can use dynamic_pointer_cast. What do I use if I wish to cast an auto_ptr? I’m assuming something that would look like below.

struct B : A { };
...
auto_ptr<A> base( ... );
auto_ptr<B> derive = dynamic_pointer_cast<B>(base);

I’m using boost for shared_ptr’s

Answer 1

auto_ptr<A> base( ... );
if(B* query = dynamic_cast<B>(base.get())) {
    // take ownership
    base.release();
    auto_ptr<B> derived(query);

    // use and/or pass derived around here
    // ...
} else {
    // no dice: leave base as owner
}

---

Since you’re already doing that, then refactor it, slap it in the namespace of your choice and call it a day:

// Postcondition:
// where r is the returned pointer and p == from.get()
// EITHER r == 0 AND from.get() == p
// OR r.get() == dynamic_cast<To>(p) AND from == 0
template<typename To, typename From>
std::auto_ptr<To>
dynamic_pointer_cast(std::auto_ptr<From>& from)
{
    To* query = dynamic_cast<To>(from.get());
    return query ? from.release(), query : 0;
}