The best explanation on the subject that I’ve ever read is an article by Luca Cardelli, a renown type theorist. The article is named On Understanding Types, Data Abstraction, and Polymorphism. Types of Polymorphism Cardelli defines several types of polymorphism in this article: Universal parametric inclusion Ad-hoc overloading coercion The kind of polymorphism related to … Read more
The correct way to think about this is to imagine that every class requires its objects to have a certain number of “slots”; those slots are filled with methods. The question “what method actually gets called?” requires you to figure out two things: What are the contents of each slot? Which slot is called? Let’s … Read more
There are some resources on the Anti-IF Campaign site, such as this article. I believe it’s a matter of degree. Conditionals aren’t always bad, but they can be (and frequently are) abused. Additional thoughts (one day later) Refactoring: Improving the Design of Existing Code is a good reference on this subject (and many others). It … Read more
It is useful when you handle lists… A short example: List<CoolingMachines> coolingMachines = … // a list of CoolingMachines for (CoolingMachine current : coolingMachines) { current.start(); } Or when you want to allow a method to work with any subclass of CoolingMachines
As per the TAPL, §23.2: Parametric polymorphism (…), allows a single piece of code to be typed “generically,” using variables in place of actual types, and then instantiated with particular types as needed. Parametric definitions are uniform: all of their instances behave the same. (…) Ad-hoc polymorphism, by contrast, allows a polymorphic value to exhibit … Read more
What you are seeing is Object Slicing. You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as … Read more
Because in the general case, it’s impossible at compile time to determine what type it will be at run time. Your example can be resolved at compile time (see answer by @Quentin), but cases can be constructed that can’t, such as: Base *bp; if (rand() % 10 < 5) bp = new Derived; else bp … Read more
Is polymorphism lost by allowing empty list handling? If so, how so, and why? Are there particular cases which would make this obvious? The free theorem for head states that f . head = head . $map f Applying this theorem to [] implies that f (head []) = head (map f []) = head … Read more
“Surely so long as you allocate memory on the heap” – where the memory is allocated has nothing to do with it. It’s all about the semantics. Take, for instance: Derived d; Base* b = &d; d is on the stack (automatic memory), but polymorphism will still work on b. If you don’t have a … Read more
Vec<Animal> is not legal, but the compiler can’t tell you that because the type mismatch somehow hides it. If we remove the calls to push, the compiler gives us the following error: <anon>:22:9: 22:40 error: instantiating a type parameter with an incompatible type `Animal`, which does not fulfill `Sized` [E0144] <anon>:22 let mut v: Vec<Animal> … Read more