Ruby struct creation block cannot access variable outside the block

Question

Accessing the default variable inside the Struct creation block doesn't seem to work in this case:

default = 'test'
A = Struct.new(:a, :b) do
  def initialize(*args)
    super(*args)
    self.b ||= default
  end
end

It is throwing the following error:

'initialize': undefined local variable or method `default' for #<struct A a=2, b=nil> (NameError)

Can someone explain why this is happening and whether there is a work around?

(Tested on Ruby 1.9.3 and 2.1.2)

Answer 1

It's because def keyword starts new local variables scope, so default local variable isn't visible inside of it. The workaround is to use define_method, because the block you pass into it is closure:

default = 'test'
A = Struct.new(:a, :b) do
  define_method(:initialize) do |*args|
    super(*args)
    self.b ||= default
  end
end
a = A.new
a.b
# => "test"