How can I create a polymorphic object on the stack?

How do I allocate a polymorphic object on the stack? I'm trying to do something similar to (trying to avoid heap allocation with new)?:

A* a = NULL;

switch (some_var)
{
case 1:
    a = A();
    break;
case 2:
    a = B(); // B is derived from A
    break;
default:
    a = C(); // C is derived from A
    break;
}