The Structure extension provides facilities for declaring properties that a Hash can have. This provides for the creation of structures that still behave like hashes but do not allow setting non-allowed keys.

@example

  class RestrictedHash < Hash
    include Hashie::Extensions::MergeInitializer
    include Hashie::Extensions::Structure

    key :first
    key :second, :default => 'foo'
  end

  h = RestrictedHash.new(:first => 1)
  h[:first]  # => 1
  h[:second] # => 'foo'
  h[:third]  # => ArgumentError