When a variable's type combines different types under the same class hierarchy, its type becomes a virtual type. This applies to every class and struct except for
Float. An example:
class Animal end class Dog < Animal def talk "Woof!" end end class Cat < Animal def talk "Miau" end end class Person getter pet def initialize(@name : String, @pet : Animal) end end john = Person.new "John", Dog.new peter = Person.new "Peter", Cat.new
If you compile the above program with the
tool hierarchy command you will see this for
- class Object | +- class Reference | +- class Person @name : String @pet : Animal+
You can see that
+ means it's a virtual type, meaning "any class that inherits from
The compiler will always resolve a type union to a virtual type if they are under the same hierarchy:
if some_condition pet = Dog.new else pet = Cat.new end # pet : Animal+
The compiler will always do this for classes and structs under the same hierarchy: it will find the first superclass from which all types inherit from (excluding
Float). If it can't find one, the type union remains.
The real reason the compiler does this is to be able to compile programs faster by not creating all kinds of different similar unions, also making the generated code smaller in size. But, on the other hand, it makes sense: classes under the same hierarchy should behave in a similar way.
Lets make John's pet talk:
john.pet.talk # Error: undefined method 'talk' for Animal
We get an error because the compiler now treats
@pet as an
Animal+, which includes
Animal. And since it can't find a
talk method on it, it errors.
What the compiler doesn't know is that for us,
Animal will never be instantiated as it doesn't make sense to instantiate one. We have a way to tell the compiler so by marking the class as
abstract class Animal end
Now the code compiles:
john.pet.talk #=> "Woof!"
Marking a class as abstract will also prevent us from creating an instance of it:
Animal.new # Error: can't instantiate abstract class Animal
To make it more explicit that an
Animal must define a
talk method, we can add it to
Animal as an abstract method:
abstract class Animal # Makes this animal talk abstract def talk end
By marking a method as
abstract the compiler will check that all subclasses implement this method, even if a program doesn't use them.
Abstract methods can also be defined in modules, and the compiler will check that including types implement them.