returning an abstract class from a function

You can declare the return type to be a reference or pointer to the abstract class, so that it can be assigned to references or pointers to the abstract class and used based on its interface.

However, you cannot return an actual instance of the actual abstract class because by definition you cannot instantiate it. You could, however, return instances of concrete subtypes which is good enough because by the principle of substitution, you should always be able to use a subtype instead of a supertype.

No, but a function could have a return type of a pointer (or a reference) to an abstract class. It would then return instances of a class that is derived from the abstract class.

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>(...);
}

Abstract classes cannot be instantiated and thus not returned.