Effect of std::move

Question

struct Base
{
    Base(int a_) : a(a_)
    {
    }
    int a;
};
class Derived: public Base
{
public:
    Derived(int i): Base(i)
    {
    }
    Derived(const Derived && rhs)
        : Base(std::move(rhs))
    {
    }
};

int main()
{
    Derived d1(2);
    Derived d2 = std::move(d1);
    std::cout << d1.a << '\n';
}

Why the thing from which we just moved, that is, the thing that we just pilfered, is still accessible on subsequent lines of code? I mean d1.a.