Allocate a polymorphic class member at a specific location?

I have a class with a few class members and a polymorphic member:

class Container{

    Container::Container(){ p = new Derived();}

    Poly* p;
    A a;
    B b;
};

I would like to allocate the memory for the actual polymorphic object Container::p points to, contiguously after Container::b, rather than via the default new() and therefore malloc().

What is the best way to achieve this?