Ruby Class vs Struct

Question

I have seen codebases using Structs to wrap around attributes and behavior inside a class. What is the difference between a Ruby Class and a Struct? And when should one be used

Answer 1

To add to the other answers, there are some things you can not do with a Struct, and some than you can.

For example, you can not create a Struct with no arguments:

Bar = Struct.new
=> ArgumentError: wrong number of arguments (given 0, expected 1+)

Bar = Struct.new(:bar)
bar = Bar.new(nil)
bar.class
=> Bar

However, a class will let you do that:

class Foo; end
foo = Foo.new
foo.class
=> Foo

You can not set a default value for Struct arguments:

Bar = Struct.new(bar: 'default')
=> ArgumentError: unknown keyword: bar

Bar = Struct.new(bar = 'default')
=> NameError: identifier default needs to be constant

But you can do it with a class, either passing a hash, were the arguments can be in any order or even missing:

class Bar
  attr_reader :bar, :rab
  def initialize(bar: 'default', rab:)
    @bar = bar
    @rab = rab
  end
end

bar = Bar.new(rab: 'mandatory')
bar.rab
=> 'mandatory'
bar.bar
=> 'default'

bar = Bar.new(rab: 'mandatory', bar: 'custom_value')
bar.rab
=> 'mandatory'
bar.bar
=> 'custom_value'

or passing the values directly, were the arguments should be given in the same order, with the defaulted ones always at the end:

class Bar
  attr_reader :rab, :bar
  def initialize(rab, bar = 'default')
    @rab = rab
    @bar = bar
  end
end

bar = Bar.new('mandatory')
bar.rab
=> 'mandatory'
bar.bar
=> 'default'

bar = Bar.new('mandatory', 'custom_value')
bar.rab
=> 'mandatory'
bar.bar
=> 'custom_value'

You can not do any of that with Structs, unless you set default values for your arguments in this super verbose way:

A = Struct.new(:a, :b, :c) do
  def initialize(a:, b: 2, c: 3)
    super(a, b, c)
  end
end

(example taken from this answer)

You can define methods in a Struct:

Foo = Struct.new(:foo) do
  def method(argument)
    # do something with argument
    end
  end
end

Structs can be useful to create data objects, like the point example mentioned in one of the answers.

I sometimes use them to create fakes and mocks in tests in a simple way. Sometimes RSpec allow(foo).to receive(:blah) etc. can get a bit too verbose and using a Struct is much simple.

Answer 2

Struct is a Ruby shorthand for creating Classes. Using Struct where applicable simplifies your code. There is a good discussion of this at https://www.rubytapas.com/2012/11/07/episode-020-struct/

Answer 3

From the Struct docs:

A Struct is a convenient way to bundle a number of attributes together, using accessor methods, without having to write an explicit class.

The Struct class generates new subclasses that hold a set of members and their values. For each member a reader and writer method is created similar to Module#attr_accessor.

So, if I want a Person class that I can access a name attribute (read and write), I either do it by declaring a class:

class Person
  attr_accessor :name

  def initalize(name)
    @name = name
  end
end

or using Struct:

Person = Struct.new(:name)

In both cases I can run the following code:

 person = Person.new
 person.name = "Name"
 #or Person.new("Name")
 puts person.name

When use it?

As the description states we use Structs when we need a group of accessible attributes without having to write an explicit class.

For example I want a point variable to hold X and Y values:

point = Struct.new(:x, :y).new(20,30)
point.x #=> 20

Some more examples:

• http://blog.steveklabnik.com/posts/2012-09-01-random-ruby-tricks--struct-new
• "When to use Struct instead of Hash in Ruby?" also has some very good points (comparing to the use of hash).

Answer 4

I'd like to to @sam_forgot suggested benchmark. The comparison is not very fair. Both class and struct, these days, support keyword arguments. Using keyword arguments on each has opposite effects, as you can see from my example struct's with keyword arguments performance is not that dramatically different from the class.

require 'benchmark'

REP=1000000

SUser = Struct.new(:name, :age)
SUserK = Struct.new(:name, :age, keyword_init: true)

DATA = { name: "Harry", age: 75 }
DATA2 = DATA.values

class CUser
  attr_accessor :name, :age
  def initialize(name, age)
    @name = name
    @age = age
  end
end

class CUserK
  attr_accessor :name, :age
  def initialize(name:, age:)
    @name = name
    @age = age
  end
end

Benchmark.bmbm do |x|
  x.report 'Struct create and access, without keyword arguments' do
    REP.times do
      user = SUser.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end

  x.report 'Struct create and access, with keyword arguments' do
    REP.times do
      user = SUserK.new(**DATA)
      "#{user.name} - #{user.age}"
    end
  end

  x.report 'Class create and access, without keyword arguments' do
    REP.times do
      user = CUser.new(*DATA2)
      "#{user.name} - #{user.age}"
    end
  end 

  x.report 'Class create and access, with keyword arguments' do
    REP.times do
      user = CUserK.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
end

Rehearsal ---------------------------------------------------------------------------------------
Struct create and access, without keyword arguments   3.484609   0.011974   3.496583 (  3.564523)
Struct create and access, with keyword arguments      0.965959   0.005543   0.971502 (  1.007738)
Class create and access, without keyword arguments    0.624603   0.003999   0.628602 (  0.660931)
Class create and access, with keyword arguments       0.901494   0.004926   0.906420 (  0.952149)
------------------------------------------------------------------------------ total: 6.003107sec

                                                          user     system      total        real
Struct create and access, without keyword arguments   3.300488   0.010372   3.310860 (  3.339511)
Struct create and access, with keyword arguments      0.876742   0.004354   0.881096 (  0.903551)
Class create and access, without keyword arguments    0.553393   0.003962   0.557355 (  0.568985)
Class create and access, with keyword arguments       0.831672   0.004811   0.836483 (  0.850224)

Answer 5

There is quite a big practical performance difference, example behavior in ruby 2.6.3p62:

                          user       system     total     real
Struct create and access  3.052825   0.005204   3.058029  (3.066316)
Class create and access   0.738605   0.001467   0.740072  (0.743738)

Example code:

require 'benchmark'

REP=1000000
SUser = Struct.new(:name, :age)
DATA = { name: "Harry", age: 75 }

class User
  attr_accessor :name, :age
  def initialize(name:, age:)
    @name = name
    @age = age
  end
end

Benchmark.bm 20 do |x|
  x.report 'Struct create and access' do
    REP.times do
      user = SUser.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
  x.report 'Class create and access' do
    REP.times do
      user = User.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
end