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The instance_alignof expression returns an Int32 with the instance alignment of a given class.

Unlike alignof which would return the alignment of the reference (pointer) to the allocated object, instance_alignof returns the alignment of the allocated object itself.

For example:

class Foo

class Bar
  def initialize(@x : Int64)

instance_alignof(Foo) # => 4
instance_alignof(Bar) # => 8

Even though Foo has no instance variables, the compiler always includes an extra Int32 field for the type id of the object. That's why the instance alignment ends up being 4 and not 1.