Use cases for final classes

Question

I was reading comments on Herb Sutter\'s Guru of the Week redux about virtual functions, and finally saw him mentioning this:

[...] “uses

Answer 1

One interesting unusual use case I have found I described here. In short, by preventing inheritance from your int-like class, you buy yourself a possibility to replace it with a built-in type in the future releases of your library, without the risk of breaking your user's code.

But a more common example is devirtualization. If you mark your class as final, compiler can apply certain run-time optimizations. For instance,

struct Object {
  virtual void run() = 0;
  virtual ~Object() {}
};

struct Impl final : Object
{
  void run() override {}
};

void fun(Impl & i)
{
  i.run(); // inlined!
}

The call to i.run() can be now inlined due to final specifier. The compiler knows that vtable look-up will not be needed.