Is it valid to cast from A* to B* via dynamic_cast when A and B haven't a common ancestor?

Question

Both clang 3.5.0 and g++ 4.9.0 compile the following code fine (with -std=c++11 -Wall -Wextra -pedantic-errors) and the program outputs true:

#include <iostream>

struct A
{
    virtual ~A() = default;
};

struct B
{
    virtual ~B() = default;
};

struct C : A, B
{
    virtual ~C() = default;
};

int main()
{
    C c;
    A* ap = &c;
    B* bp = dynamic_cast<B*>(ap);

    std::cout << std::boolalpha << (bp != nullptr) << std::endl;
}

Answer 1

Yes. That's sometimes known as cross-casting, and will succeed if they are both base sub-objects of the same derived object, as they are here.

dynamic_cast is necessary, since the conversion needs the run-time information that both are part of a C object. To statically cast, you'd have to explicitly convert to C* first.