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crystal-lang

How are instance variables of a Class implemented in Crystal?

If I have an instance of A, are the instance variables implemented as pointers? In other words, are instance variables accessed by reference, even when using Structs?

class A
  @title = "the title"
  @my_val = MyStruct.new
end
Solution

  • @my_val is a reference to the instance of MyStruct on the stack. Checkout this example and notice the difference:

    struct MyStruct
  property x

  def initialize(@x : Int32)
  end
end

class A
  getter my_val = MyStruct.new(10)
end


# mutates a struct (passed by reference)
def method(a : A)
  a.my_val.x = 20
end

# does not mutate a struct (passed by value)
def method(s : MyStruct)
  s.x = 30
end

a = A.new
p a.my_val.x       #=> 10

a.method(a)
p a.my_val.x       #=> 20

a.method(a.my_val)
p a.my_val.x       #=> 20 (not 30)