How do I find a instance variable through its "actual" class? - class

Question

The program I'm writing is storing elements in a hash called grid of type Position => LivingBeing | Thing. This grid is stored on a Map and I would like this Map to return the position of an element of class Apple which is a subclass of Thing.
However, when using typeof() to get the class, I get LivingBeing | Thing instead of the subclass Apple
Here is the Map class:
class Map
##grid = {} of Position => LivingBeing | Thing
def initialize()
end
# Add an entity to the grid
def add_entity(new_entity : LivingBeing | Thing)
##grid[new_entity.position] = new_entity
end
# Return the position of an object of class "something"
def self.where_is?(something : Class)
# First attempt was to get the key by the value
# ##grid.key(something)
##grid.each do |position, thing|
# Returns "thing #<Apple:0x55f1772085c0> at Position(#x=1, #y=2) is (LivingBeing | Thing)"
puts "thing #{thing} at #{position} is #{typeof(thing)}"
position if typeof(thing) == something
end
end
Here the Thing class:
abstract class Thing
getter position
#name = "Unkown object"
def initialize(#position : Position)
end
end
class Apple < Thing
#name = "Apple"
end
Here the Position struct:
struct Position
getter x, y
def initialize(#x : Int32, #y : Int32)
end
end
And here is the test I try to make it pass:
it "gives a random thing location based on its class" do
world = Map.new()
apple = Apple.new(Position.new(1, 2))
puts "Apple type : #{typeof(apple)}" # Returns "Apple type : Apple"
world.add_entity(apple)
position = Map.where_is?(Apple)
position.should eq Position.new(1, 2)
end
Is there some class method or function which could give the Apple class?
Or is it a design issue?
Thank you for your answer !

Answer 1

You can use forall to solve this:
# Return the position of an object of class "something"
def self.where_is?(something : T.class) forall T
##grid.each do |position, thing|
return position if thing.is_a?(T)
end
end
and call it using Map.where_is? Apple just like you wish.
This works because the type variable T (introduced using forall T) can be inferred to be Apple from passing in the constant Apple which matches the T.class type restriction. T is then a constant you can use with is_a?.

Answer 2

One solution I have is this for my function:
# Return the position of an object of class "something"
def self.where_is?(something)
##grid.each do |position, thing|
return position if thing.is_a?(typeof(something))
end
end
And this for the test:
it "gives a random thing location" do
world = Map.new(4)
apple = Apple.new(Position.new(1, 2))
world.add_entity(apple)
position = Map.where_is?(Apple.new(Position.new(0, 0)))
position.should eq Position.new(1, 2)
end
I will use it like this if there is no other solution. But I would prefer to be able to search directly the class Apple instead of creating an instance of Apple
I would like to be able to do position = Map.where_is?(Apple) instead of
position = Map.where_is?(Apple.new(Position.new(0, 0)))

Answer 3

As # RX14 said, it looks like you want to check the run-time "type", i.e. .class. Here's an example:
class Apple
#name = "Apple"
end
def check(obj : Object)
obj.class == Apple
end
a=Apple.new
p check(a)