Use `final` on classes sparingly

Reason

Capping a hierarchy with final classes is rarely needed for logical reasons and can be damaging to the extensibility of a hierarchy.

Example, bad

class Widget { /* ... */ };

// nobody will ever want to improve My_widget (or so you thought)
class My_widget final : public Widget { /* ... */ };

class My_improved_widget : public My_widget { /* ... */ };  // error: can't do that

Note

Not every class is meant to be a base class. Most standard-library classes are examples of that (e.g., std::vector and std::string are not designed to be derived from). This rule is about using final on classes with virtual functions meant to be interfaces for a class hierarchy.

Note

Claims of performance improvements from final should be substantiated. Too often, such claims are based on conjecture or experience with other languages.

There are examples where final can be important for both logical and performance reasons. One example is a performance-critical AST hierarchy in a compiler or language analysis tool. New derived classes are not added every year and only by library implementers. However, misuses are (or at least have been) far more common.

Enforcement

Flag uses of final on classes.