davidflanagan.com > July 2007

Ruby Structs for Immutable Types

Ruby's Struct class is a great way to define simple classes. For example, to define a simple Point class with getter and setter methods for x and y attributes, along with working ==, hash, and to_s methods, we can just write:

Point = Struct.new(:x,:y)

Struct was designed with mutable types in mind, but that doesn't mean that we are restricted to mutable types. Here's how we change the just-defined Point class to be immutable:

Point = Struct.new(:x, :y)  # Define mutable class
class Point                 # Open the class
  undef x=,y=,[]=           # Undefine mutator methods
end

Why do this for all of your Struct-based classes, however? Let's add a new factory method to Struct itself. If you want an immutable structure, use Struct.immutable instead of Struct.new:

def Struct.immutable(*args)  # Factory method for immutable classes
  Struct.new(*args).class_eval do # Define struct, then modify it
    undef []=                     # Undefine general mutator
    args.each do |sym|            # For each field of the struct
      mutator = :"#{sym.to_s}="   # Symbol for setter method
      remove_method mutator       # Undefine that method
    end
    self                          # Return the class
  end
end

Warning: I haven't actually tested this carefully, but it seems to work. Suggestions, corrections, etc., are welcome in the comments.