What are some of the disadvantages of using a reference instead of a pointer?

The immediate limitations are that:

• You cannot alter a reference's value. You can alter the A it refers to, but you cannot reallocate or reassign a during B's lifetime.
• a must never be 0.

Thus:

• The object is not assignable.
• B should not be copy constructible, unless you teach A and its subtypes to clone properly.
• B will not be a good candidate as an element of collections types if stored as value. A vector of Bs would likely be implemented most easily as std::vector<B*>, which may introduce further complications (or simplifications, depending on your design).

These may be good things, depending on your needs.

Caveats:

• slicing is another problem to be aware of if a is assignable and assignment is reachable within B.

You can't change the object referred to by a after the fact, e.g. on assignment. Also, it makes your type non-POD (the type given would be non-POD anyway due to the private data member anyway, but in some cases it might matter).

But the main disadvantage is probably it might confuse readers of your code.