This places restrictions on the sorts of relationships that subclassing can represent.

With new-style classes, objects and types were unified, allowing the subclassing of types.

Object-oriented programming languages offer subtyping polymorphism using subclassing (also known as inheritance).

Subtyping, Subclassing, and Trouble with OOP, an essay discussing a related problem, whether sets should inherit from bags.

Whether through completely new custom controls or through clever subclassing, the Finder needs to implement its views with an eye towards efficiently handling thousands of files and folders with ease and speed.