How to downcast properly in C++

Question

For any reason, I have an object created by a static method which calls the private constructor. (It isn't a singleton)

I want to make a new object derives from the first one, which have more members and functions.
But it's problematic, becuase the static method returns a firstObject* object, so a creation with downcasting of the secondObject* will make a memory overflow.

What should I do? I have an access to the first object's code, but it is impossible to change its constructor (If I change it, I will have to change a huge written code).

EDIT:

Thank to all responders. I can change the constructor to be protected.

Answer 1

Make sure your constructor is at least protected so that child classes can use it.

Not sure what you fear about memory overflow but this:

class Base {
public:
  static Base* getInstance();
  virtual ~Base() {};
protected:
  Base() {};
};

class Derived : public Base {};

// Implementation
Base* Base::getInstance() { return new Derived(); }

int main() {
Base::getInstance();
};

Works perfectly.

Now I would advise you against returning a raw pointer in that situation (std::unique_ptr would be way better) but that's probably off-topic.