how to use reinterpret_cast to cast to a derived class pointer in c++

Question

Here is my test example:

struct base {
    virtual ~base(){}
    int x;
};

struct derived: public virtual base {
    base * clone() {
        return new derived;
    }
    derived(): s("a") {}
    std::string s;
};

int main () {
    derived d;
    base * b = d.clone();
    derived * t = reinterpret_cast<derived*>(b);
    std::cout << t->s << std::endl;
    return 0;
}

It crashes at the line where I print s. Since "b" is a pointer to the derived class, reinterpret_cast should just work. I wonder why it crashes. At the same time, if I replace reinterpret_cast with dynamic_cast, then it works.

Answer 1

Even if b is here dynamically of type derived, you have to use dynamic_cast. This is what dynamic_cast is for, to dynamically convert a pointer of a base class into a derived class at runtime.

reinterpret_cast takes the raw pointer and considers it as being of the derived type. However, because of the virtual inheritance, a slight adjustment must be done to the pointer to point to the correct method dispatch table, and that's precisely what dynamic_cast will do.

Answer 2

Don't reinterpret_cast it, it will cause trouble with multiple or virtual inheritance, like in your case. Wont a simply static_cast do the job here?

To know why, search for implementations of virtual inheritance. A common one is to store a pointer to the base class within the object, so the virtual base does not share the same address than its derived classes. There is a similar case when multiple inheritance is used.

In short, reinterpret_cast can't do much more than casting pointers to ints and back (if there is enough size in the int to contain a pointer).