returning an abstract class from a function

You can return an abstract class pointer – assuming B is a concrete class derived from abstract class A:

A * f() {
    return new B;
}

or a reference:

A & f() {
    static B b;
    return b;
}

or a smart pointer:

std::unique_ptr<A> f() {
    return std::make_unique<B>(...);
}