When is it better to use a Struct rather than a Hash in Ruby?

Question

A Ruby Struct allows an instance to be generated with a set of accessors:

# Create a structure named by its constant Customer = Struct.new(:name, :address)     #=> Customer Customer.new("Dave", "123 Main")           #=> #<Customer name="Dave", address="123 Main"> 

This looks convenient and powerful, however, a Hash does something pretty similar:

Customer = {:name => "Dave", :address => "123 Main"} 

What are the real-world situations where I should prefer a Struct (and why), and what are the caveats or pitfalls in choosing one over the other?

Answer 1

Personally I use a struct in cases when I want to make a piece of data act like a collection of data instead of loosely coupled under a Hash.

For instance I've made a script that downloads videos from Youtube and in there I've a struct to represent a Video and to test whether all data is in place:

 Video = Struct.new(:title, :video_id, :id) do   def to_s     "http://youtube.com/get_video.php?t=#{id}&video_id=#{video_id}&fmt=18"   end    def empty?     @title.nil? and @video_id.nil? and @id.nil?   end end 

Later on in my code I've a loop that goes through all rows in the videos source HTML-page until empty? doesn't return true.

Another example I've seen is James Edward Gray IIs configuration class which uses OpenStruct to easily add configuration variables loaded from an external file:

#!/usr/bin/env ruby -wKU  require "ostruct"  module Config   module_function    def load_config_file(path)     eval <<-END_CONFIG     config = OpenStruct.new     #{File.read(path)}     config     END_CONFIG   end end  # configuration_file.rb config.db = File.join(ENV['HOME'], '.cool-program.db') config.user = ENV['USER']  # Usage: Config = Config.load_config('configuration_file.rb') Config.db   # => /home/ba/.cool-program.db Config.user # => ba Config.non_existant # => Nil 

The difference between Struct and OpenStruct is that Struct only responds to the attributes that you've set, OpenStruct responds to any attribute set - but those with no value set will return Nil

Answer 2

A Struct has the feature that you can get at its elements by index as well as by name:

irb(main):004:0> Person = Struct.new(:name, :age) => Person irb(main):005:0> p = Person.new("fred", 26) => # irb(main):006:0> p[0] => "fred" irb(main):007:0> p[1] => 26 irb(main):008:0> p.name => "fred" irb(main):009:0> p.age => 26

which sometimes is useful.