Conversion from void* to the pointer of the base class

Question

I have some hierarchy: base, derived classes and some structure storing user data as void*. That void can store both Base and Derived classes pointers. Main problem that I d

Answer 1

void*'s can only be converted back to their original type. When you store a Derived* in a void*, you can only cast back to Derived*, not Base*.

This is especially noticeable with multiple inheritance, as your derived object might not necessarily be at the same address as your base. If you really need to store things (and retrieve things) with void*, always cast to the base type first, so you have a stable way of getting the object back:

#include 

struct base { int type; };
struct intruder { int iminyourclassstealingyourbase; };
struct derived : intruder, base {};

int main()
{
    derived d; d.type = 5;

    void* good = (base*)&d;
    void* bad = &d;

    base* b1 = (base*)good;
    base* b2 = (base*)bad;

    std::cout << "good: " << b1->type << "\n";
    std::cout << "bad: " << b2->type << "\n";
}

If you then want to go back to the derived type, use a dynamic_cast (or static_cast if you're guaranteed it has to be of the derived type.)