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/Crystal 0.31

instance_sizeof

The instance_sizeof expression returns an Int32 with the instance size of a given class. For example:

class Point
  def initialize(@x, @y)
  end
end

Point.new 1, 2

# 2 x Int32 = 2 x 4 = 8
instance_sizeof(Point) #=> 12

Even though the instance has two Int32 fields, the compiler always includes an extra Int32 field for the type id of the object. That's why the instance size ends up being 12 and not 8.

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https://crystal-lang.org/docs/syntax_and_semantics/instance_sizeof.html