In Nim, objects are like structs from C family languages and define a grouping of fields. They are by default traced by the garbage collector, so there is no need to explicitly free them when allocated.

  Animal* = object
    name*, species*: string
    age: int

proc sleep*(a: var Animal) =
  a.age += 1

proc dead*(a: Animal): bool =
  result = a.age > 20

var carl: Animal
carl = Animal(name : "Carl",
              species : "L. glama",
              age : 12)

let joe = Animal(name : "Joe",
                 species : "H. sapiens",
                 age : 23)

assert(not carl.dead)
for i in 0..10:
assert carl.dead

Object types are declared in a type section, as usual. They can be exported, and individual fields can also be exported. Fields can be safely exported without violating encapsulation because call syntax is equivalent between them.

Initially, carl is created on the stack and initialized to zeros (or "" in case of fields of type string), so its value is [name = "", species = "", age = 0]. It is mutable, so that means that the contents of carl can be changed. This also means it can be passed to functions that require a variable parameter, like sleep(), which can modify its value.

joe is also created on the stack, but it’s contents are immutable and can not be changed. Attempting to do so, say through joe.age = 57, will fail with an error at compile time.

let mittens: ref Animal = new(Animal)

mittens.name = "Mittens"
mittens.species = "P. leo"
mittens.age = 6

mittens is a reference to an object allocated on the heap. The value of mittens cannot be changed, so mittens can never point to anything else, but the value that mittens is pointing at can and is changed from the default initialization value of zeros.

You might ask whether there is a more concise way of initializing reference types, and there is if you give the reference type a name:

  AnimalRef* = ref Animal

let spot = AnimalRef(name: "Spot",
                   species: "C. lupus",
                   age: 1)

In many cases it is only wanted to have the object be a reference type, which is possible by declaring it as a ref object.

  Thing* = ref object
    positionX*, positionY*: int