dynamic_cast from “void *”

Question

According to this, void* has no RTTI information, therefore casting from void* is not legal and it make sense.

If I remember correctly,

Answer 1

To add to Tony's nice answer, this little code snippet helps me for some reason. First, we establish a simple hierarchy. Then, we see if dynamic_cast can "survive" a static_cast. Before this experiment I thought "the run time type information is there, dynamic cast should figure it out." Now I realize "dynamic_cast must have to look up its information based on some tables the compiler is aware of, so it can't have some magical power."

#include 
#include 

using namespace std;

class A {
  protected:
  virtual void foo() { cout << "A" << endl; }
};

class B1 : public A {
  private:
  virtual void foo() override { cout << "B1" << endl; }
};

class B2 : public A {
  public:
  virtual void foo() override { cout << "B2" << endl; }
};

int main(int argc, char **argv) {
  B1 b1;
  // undefined behavior even though dynamic_cast didn't return null
  dynamic_cast(
      static_cast(
        static_cast(&b1)))->foo();
  // dynamic_cast returns null here though
  assert (!dynamic_cast
          (static_cast
           (static_cast
            (static_cast(&b1)))));
}