When is static cast safe when you are using multiple inheritance?

Question

I found myself in a situation where I know what type something is. The Type is one of three (or more) levels of inheritance. I call factory which returns B* how

Answer 1

You can safely upcast if you are sure that the object is actually an instance of that class.

class Base {};
class Derived1 : public Base {};
class Derived2 : public Base {};

int main()
{
    Base* b = new Derived1;

    Derived1* d1 = static_cast(b); // OK
    Derived2* d2 = static_cast(b); // Run-time error - d isn't an instance of Derived2
}