How to avoid instance variable initializing ugliness

Question

I see this popping up all the time in my code

class Foo
  def initialize(foo)
    @foo = foo
  end
  #...
end

This isn't too bad, but it gets worse:

class Foo
  def initialize(foo,baz,bar,a,b,c,d)
    @foo = foo
    @baz = baz
    @bar = bar
    #etc...

You can sortof get around this by doing something like

@foo, @baz, @bar = foo, baz, bar

But even that feels wrong and is annoying to type. Is there a better way to define instance variables according to arguments?

Edit: There seem to be 2 distinct solutions to this problem. See:

stackoverflow.com/questions/2680523

Arie Xiao
– Arie Xiao

2015-02-15 05:35:40 +00:00
Commented Feb 15, 2015 at 5:35 — Arie Xiao
– Arie Xiao, Commented Feb 15, 2015 at 5:35

spickermann · Accepted Answer · 2015-02-18 09:04:26Z

7

You might want to consider using a Struct:

class Foo < Struct.new(foo,baz,bar,a,b,c,d)
end

foo = Foo.new(1,2,3,4,5,6,7)
foo.bar #=> 2

No need to define an extra initialize method at all...

edited Feb 18, 2015 at 9:04

answered Feb 15, 2015 at 5:58

spickermann

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Comments

Tobias Cohen · Accepted Answer · 2015-02-15 06:10:38Z

3

Yes, that's the preferred way to initialize instance variables in Ruby. It can be annoying to type, but it's a well understood pattern. As always in Ruby, using metaprogramming to automate it away is possible, but will make your code harder to follow.

I'd also argue that it's probably a good thing for a class to look ugly when it's taking more than two or three arguments. If your class depends on six different things to function, it's a strong candidate for refactoring.

answered Feb 15, 2015 at 6:10

Tobias Cohen

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Comments

Antarr Byrd · Accepted Answer · 2015-02-15 13:29:11Z

3

def initialize args
  @foo, @baz, @bar = *args
end

edited Feb 15, 2015 at 13:29

answered Feb 15, 2015 at 5:37

Antarr Byrd

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media-slave24 · Accepted Answer · 2015-02-15 12:49:18Z

0

I think there are 3 ways to make initialization of instance variables shorter:

Use Struct or OpenStruct.
Use ruby's parallel assignment.
Use metaprogramming to make a macro like this.

answered Feb 15, 2015 at 12:49

media-slave24

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DGM · Accepted Answer · 2015-02-15 21:02:34Z

0

The fattr gem was recently endorsed on Ruby Tapas to help solve this problem. Another consideration though, is whether there are too many things being passed into the initializer. It could be that this class is doing too much and needs to be broken into smaller pieces.

answered Feb 15, 2015 at 21:02

DGM

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Comments

ka8725 · Accepted Answer · 2025-06-12 21:12:40Z

I prefer to follow the following pattern: Dry::Initializer + Memoist/MemoWise (or any similar implementation). It allows me to get rid of the instance variables mentioned in the code completely. That makes the code more robust and readable. For example, this is a real class from one of my projects:

class TransposeKey
  extend Dry::Initializer
  extend Memoist

  option :song_key
  option :capo

  TRANSPOSED_KEYS = [['A'], ['G#', 'Ab'], ['G'], ['Gb', 'F#'], ['F'], ['E'], ['Eb', 'D#'], ['D'], ['Db', 'C#'], ['C'], ['B'], ['A#', 'Bb']]

  SHARP_KEYS = ['A#', 'C#', 'D#', 'F#', 'G#']
  OTHER_KEYS = Parser::Transposer::KEYS.keys - SHARP_KEYS

  def perform
    return capo if song_key == 'numbers'
    transposed_key_index = TRANSPOSED_KEYS.index(TRANSPOSED_KEYS.detect { |i| i.include?(song_key) })
    index = (transposed_key_index + capo) % TRANSPOSED_KEYS.length
    (TRANSPOSED_KEYS[index] & related_keys).join
  end

  private

  memoize def related_keys
    SHARP_KEYS.include?(song_key) ? SHARP_KEYS : OTHER_KEYS
  end
end

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