Can dynamic_cast<Derived*>(static_cast<Base*>(voidPtr)) ever go wrong if voidPtr is null pointer or pointing to object that is inherited from Base?

Question

class Base {
    // ...
};

class Derived : public Base {
    // ...
};

class OtherDerived : public Base {
    // ...
};

If I know for a fact that voidPtr of type void* is a null pointer or pointing to an object of Derived or OtherDerived. Can dynamic_cast<Derived*>(static_cast<Base*>(voidPtr)) return non-null pointer if voidPtr is pointing to an object of type OtherDerived

void someFunction(void* voidPtr) {
  auto derived = dynamic_cast<Derived*>(static_cast<Base*>(voidPtr));
  if (derived == nullptr) {
    return;
  }

  // do something with derived
}

I came across with this problem when I was using Qt and its void *QVariant::data() function.

Answer 1

Can dynamic_cast<Derived*>(static_cast<Base*>(voidPtr)) ever go wrong if voidPtr is null pointer or pointing to object that is inherited from Base?

No, it will return nullptr or a pointer to a Derived. Only if voidPtr is not nullptr and is not actually pointing at a Base will there be problems (undefined behavior). Therefore, you must cast the Derived* to Base* in the call to someFunction.

Undefined behavior example:

#include <iostream>

class Base {
public:
    virtual ~Base() = default;
};

class Derived : public Base {};
class OtherDerived : public Base {};

void someFunction(void* voidPtr) {
    // Undefined behavior:
    auto derived = dynamic_cast<Derived*>(static_cast<Base*>(voidPtr));

    if (derived == nullptr) {
        std::cout << "fail\n";
        return;
    }
    // do something with derived
}

int main() {
    auto d = new Derived;
    someFunction(d); // should be `someFunction(static_cast<Base*>(d));`
    delete d;
}

Can dynamic_cast<Derived*>(static_cast<Base*>(voidPtr)) return non-null pointer if voidPtr is pointing to an object of type OtherDerived

No, not with the inheritance as you've defined it.