pointerof

The pointerof expression returns a Pointer that points to the contents of a variable or instance variable.

An example with a variable:

a = 1
ptr = pointerof(a)
ptr.value = 2
a # => 2

An example with an instance variable:

class Point
  def initialize(@x : Int32, @y : Int32)
  end
  def x
    @x
  end
  def x_ptr
    pointerof(@x)
  end
end
point = Point.new 1, 2
ptr = point.x_ptr
ptr.value = 10
point.x # => 10

Because pointerof involves pointers, it is considered unsafe.