alignof

The alignof expression returns an Int32 with the ABI alignment in bytes of a given type. For example:

alignof(Int32) # => 4
alignof(Int64) # => 8

struct Foo
  def initialize(@x : Int8, @y : Int16)
  end
end

@[Extern]
@[Packed]
struct Bar
  def initialize(@x : Int8, @y : Int16)
  end
end

alignof(Foo) # => 2
alignof(Bar) # => 1

For Reference types, the alignment is the same as the alignment of a pointer:

# On a 64 bits machine
alignof(Pointer(Int32)) # => 8
alignof(String)         # => 8

This is because a Reference's memory is allocated on the heap and a pointer to it is passed around. To get the effective alignment of a class, use instance_alignof.

The argument to alignof is a type and is often combined with typeof:

a = 1
alignof(typeof(a)) # => 4