Ruby: shorter way to define instance variables

Question

I'm looking for the shorter way to define instance variables inside the initialize method:

class MyClass
  attr_accessor :foo, :bar, :baz, :qux
  # Typing same stuff all the time is boring
  def initialize(foo, bar, baz, qux)
    @foo, @bar, @baz, @qux = foo, bar, baz, qux
  end
end

Does Ruby have any in-build feature which lets to avoid this kind of monkey job?

# e. g.
class MyClass
  attr_accessor :foo, :bar, :baz, :qux
  # Typing same stuff all the time is boring
  def initialize(foo, bar, baz, qux)
    # Leveraging built-in language feature
    # instance variables are defined automatically
  end
end

Sergio Tulentsev · Accepted Answer · 2016-03-04T15:18:21.877

10

Meet Struct, a tool made just for this!

MyClass = Struct.new(:foo, :bar, :baz, :qux) do
  # Define your other methods here. 
  # Initializer/accessors will be generated for you.
end

mc = MyClass.new(1)
mc.foo # => 1
mc.bar # => nil

I often see people use Struct like this:

class MyStruct < Struct.new(:foo, :bar, :baz, :qux)
end

But this results in one additional unused class object. Why create garbage when you don't need to?

edited Mar 04 '16 at 15:18

answered Mar 04 '16 at 09:53

Sergio Tulentsev

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What is the advantage of defining class with this way? – Ganesh Kunwar Mar 04 '16 at 09:58
I mean the way that you are using in your answer. – Ganesh Kunwar Mar 04 '16 at 09:59
There are two ways in my answer. – Sergio Tulentsev Mar 04 '16 at 09:59
First way in your answer. – Ganesh Kunwar Mar 04 '16 at 10:00
@GaneshKunwar: how about reading the linked docs? And the rest of the answer? – Sergio Tulentsev Mar 04 '16 at 10:01
@SergioTulentsev, what if I don't need attr readers, but just need instance vars to bee defined? – DreamWalker Mar 04 '16 at 14:14
@DreamWalker: but you do need the accessors, as shown in your snippets in the question. If, in another case, you don't, then you could employ a meta-programming trick, similar to one in Lukas' answer. In my experience, if you have an ivar, you need at least the reader for it. Working with naked ivars is bad practice. – Sergio Tulentsev Mar 04 '16 at 14:23
Your answer would be better *without* the subclass example :-) – Stefan Mar 04 '16 at 14:48
1

@Stefan: it's a common mis-usage, so I thought I'd warn not to do it :) – Sergio Tulentsev Mar 04 '16 at 15:16

Ruby: shorter way to define instance variables

1 Answers1