Methods

Procedures always use static dispatch. Methods use dynamic dispatch. For dynamic dispatch to work on an object it should be a reference type.

type
Expression = ref object of RootObj ## abstract base class for an expression
Literal = ref object of Expression
x: int
PlusExpr = ref object of Expression
a, b: Expression
method eval(e: Expression): int {.base.} =
  # override this base method
  raise newException(CatchableError, "Method without implementation override")

method eval(e: Literal): int = return e.x

method eval(e: PlusExpr): int =
  # watch out: relies on dynamic binding
  result = eval(e.a) + eval(e.b)

proc newLit(x: int): Literal =
  new(result)
  result.x = x

proc newPlus(a, b: Expression): PlusExpr =
  new(result)
  result.a = a
  result.b = b

echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4)))

In the example the constructors newLit and newPlus are procs because they should use static binding, but eval is a method because it requires dynamic binding.

As can be seen in the example, base methods have to be annotated with the base pragma. The base pragma also acts as a reminder for the programmer that a base method m is used as the foundation to determine all the effects that a call to m might cause.

Note: Compile-time execution is not (yet) supported for methods.

Note: Starting from Nim 0.20, generic methods are deprecated.

Multi-methods

Note: Starting from Nim 0.20, to use multi-methods one must explicitly pass --multimethods:on{.interpreted-text role="option"} when compiling.

In a multi-method, all parameters that have an object type are used for the dispatching:

type
  Thing = ref object of RootObj
  Unit = ref object of Thing
    x: int

method collide(a, b: Thing) {.inline.} =
  quit "to override!"

method collide(a: Thing, b: Unit) {.inline.} =
  echo "1"

method collide(a: Unit, b: Thing) {.inline.} =
  echo "2"

var a, b: Unit
new a
new b
collide(a, b) # output: 2

Inhibit dynamic method resolution via procCall

Dynamic method resolution can be inhibited via the builtin system.procCall. This is somewhat comparable to the super keyword that traditional OOP languages offer.

type
  Thing = ref object of RootObj
  Unit = ref object of Thing
    x: int

method m(a: Thing) {.base.} =
  echo "base"

method m(a: Unit) =
  # Call the base method:
  procCall m(Thing(a))
  echo "1"