How is dynamic_cast implemented

Question

Consider this simple hierarchy:

class Base { public: virtual ~Base() { } }; class Derived : public Base { }; 

Trying to downcast Base* p to Derived* is possible using dynamic_cast<Derived*>(p). I used to think dynamic_cast works by comparing the vtable pointer in p to the one in a Derived object.

But what if we derive another class from Derived? We now have:

class Derived2 : public Derived { }; 

In this case:

Base* base = new Derived2; Derived* derived = dynamic_cast<Derived*>(base); 

We still get a successful downcast, even though the vtable pointer in Derived2 has nothing to do with a vtable pointer in Derived.

How does it actually work? How can the dynamic_cast know whether Derived2 was derived from Derived (what if Derived was declared in a different library)?

I am looking for specific details about how this actually works (preferably in GCC, but others are fine too). This question is not a duplicate of this question (which doesn't specify how it actually works).

Answer 1

How can the dynamic_cast know whether Derived2 was derived from Derived (what if Derived was declared in a different library)?

The answer to that is surprisingly simple: dynamic_cast can know this by keeping this knowledge around.

When the compiler generates code it keeps around the data about the class hierarchies in some sort of table that dynamic_cast can look up later. That table can be attached to the vtable pointer for easy lookup by the dynamic_cast implementation. The data neeeded for typeid for those classes can also be stored along with those.

If libraries are involved, this sort of thing usually requires these type information structures to be exposed in the libraries, just like with functions. It is possible, for example, to get a linker error that looks like "Undefined reference to 'vtable for XXX'" (and boy, are those annoying!), again, just like with functions.