Useless interfaces

Question

Why would you ever use an interface if you are only going to have one implementation of it?

Answer 1

Yes, especially with Delphi.

If you have an interface reference to a object then you get reference counting automatically. So it is really common to have an interface with only one implementation when you want the object to be cleaned up automatically. Reference counting is better than garbage collecting since the object's destructor is called as soon as the last reference either goes out of scope or is no longer referencing it.