C++ Pointer to virtual function

If you have a struct like this one

struct A {
    void func();
};

and a reference like this one

A& a;

you can get a pointer to its func method like this:

someMethod(&A::func);

Now what if that method is virtual and you don't know what it is at run-time? Why can't you get a pointer like this?

someMethod(&a.func);

Is it possible to get a pointer to that method?