So far, we've seen classical polymorphism in just a couple of places in the standard library. We just saw the classically polymorphic std::pmr::memory_resource in Chapter 8, Allocators; and polymorphism is used "behind the scenes" in the type-erased types std::any and std::function, as detailed in Chapter 5, Vocabulary Types. However, by and large, the standard library gets by without classical polymorphism.
Two places in the standard library, however, make massive use of classical polymorphism. One is the standard exception hierarchy--for convenience, all exceptions thrown by the standard library are subclasses of std::exception. (We don't cover the exception hierarchy in this book.) The other is the contents of the standard <iostream> header, which we will cover in this chapter. However, we have a lot of background to cover before we get there...
