Question

I am just wondering if the following C++ code guaranteed to work:

struct B1 {
    virtual void f() {};
};

struct B2 {
    virtual void f2() {};
};

struct D:public B1,public B2 {
};

int main() {
    D d;
    B1 *b1=&d;
    if (dynamic_cast<B2*>(b1)) {
      B2* b2 = reinterpret_cast<B2*>(b1); //is this conversion valid?
    };
    return 1;
};

Of course, you would why do i need this?
Because i want to replace this:

C::C(B1* b): member(dynamic_cast<B2*>(b)?dynamic_cast<B2*>(b)->m():b) {};

with better construction (by performance, to not check type safety twice):

C::C(B1* b): member(dynamic_cast<B2*>(b)?reinterpret_cast<B2*>(b)->m():b) {};

Thanks in advance!

Answer 1

No, that’s definitely not valid. All you can do safely with reinterpret_cast is cast it back to the original type; anything else is implementation defined.