Object Oriented Programming

Object oriented programming is supported through inheritance and methods, which are virtual procedures. Methods work in the same way as procs, including UFCS, but the runtime type of the object that a method is called with is used to determine which version to call.

Inheritance is created with the of keyword in the type declaration of an object. A method is overridden by creating a new method with parameter types of the subtype.

type Animal = ref object of RootObj
  name: string
  age: int
method vocalize(this: Animal): string {.base.} = "..."
method ageHumanYrs(this: Animal): int {.base.} = this.age

type Dog = ref object of Animal
method vocalize(this: Dog): string = "woof"
method ageHumanYrs(this: Dog): int = this.age * 7

type Cat = ref object of Animal
method vocalize(this: Cat): string = "meow"


var animals: seq[Animal] = @[]
animals.add(Dog(name: "Sparky", age: 10))
animals.add(Cat(name: "Mitten", age: 10))

for a in animals:
  echo a.vocalize()
  echo a.ageHumanYrs()

$ nim c -r oop.nim
woof
70
meow
10

Testing subtypes

It is also possible to check if an object is of a given subtype with the of keyword. For example,

echo(animals[0] of Dog)
echo(animals[0] of Cat)
echo(animals[0] of Animal)

$ nim c -r of_op.nim
true
false
true