How to generate initializer in Ruby?

Question

It's time to make it shorter:

class Foo
  attr_accessor :a, :b, :c, :d, :e

  def initialize(a, b, c, d, e)
    @a = a
    @b = b
    @c = c
    @d = d
    @e = e
  end
end

We have 'attr_accessor' to generate getters and setters.

Do we have anything to generate initializers by attributes?

possible duplicate of [Idiomatic object creation in ruby](http://stackoverflow.com/questions/1778638/idiomatic-object-creation-in-ruby) — Andrew Grimm, Apr 30 '12 at 22:48

score 15 · Answer 1 · answered Apr 30 '12 at 14:08

15

Easiest:

Foo = Struct.new( :a, :b, :c )

Generates both accessors and initializer. You can further customize your class with:

Foo = Struct.new( … ) do
  def some_method
    …
  end
end

answered Apr 30 '12 at 14:08

Phrogz

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1

This is neat. I'd never thought about using a Struct for that purpose before. Of course, it comes with limitations, like the inability to descend from some other class too. – d11wtq Apr 30 '12 at 14:25
5

You can also do `class Foo < Struct.new(:a, :b, :c)`. – Andrew Marshall Apr 30 '12 at 14:30
@d11wtq You can inherit from or to a Struct. – steenslag Apr 30 '12 at 18:01
@Andrew Nice, I had never thought to do that before. – Phrogz Apr 30 '12 at 18:53

Casper · Answer 2 · 2012-04-30T19:36:27.083

2

We can create something like def_initializer like this:

# Create a new Module-level method "def_initializer"
class Module
  def def_initializer(*args)
    self.class_eval <<END
      def initialize(#{args.join(", ")})
        #{args.map { |arg| "@#{arg} = #{arg}" }.join("\n")}
      end
END
  end
end

# Use it like this
class Foo
  attr_accessor   :a, :b, :c, :d
  def_initializer :a, :b, :c, :d

  def test
    puts a, b, c, d
  end
end

# Testing
Foo.new(1, 2, 3, 4).test

# Outputs:
# 1
# 2
# 3
# 4

edited Apr 30 '12 at 19:36

answered Apr 30 '12 at 14:20

Casper

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1

+1 for using the string form of `eval` to require the proper arity. You should probably place this method on `Module` or the singleton class of `Class`, as there's not a lot of sense in using this on object instances other than classes. – Phrogz Apr 30 '12 at 18:56

score 1 · Answer 3 · answered Apr 30 '12 at 13:56

You can use a gem like constructor. From the description:

Declarative means to define object properties by passing a hash to the constructor, which will set the corresponding ivars.

It is easily used:

Class Foo
  constructor :a, :b, :c, :d, :e, :accessors => true
end

foo = Foo.new(:a => 'hello world', :b => 'b',:c => 'c', :d => 'd', :e => 'e')
puts foo.a # 'hello world'

If you don't want the ivars generated with accessors, you can leave off the :accessors => true

Hope this helps /Salernost

score 1 · Answer 4 · edited Apr 30 '12 at 14:07

1

class Module
  def initialize_with( *names )
    define_method :initialize do |*args|
      names.zip(args).each do |name,val|
        instance_variable_set :"@#{name}", val
      end
    end
  end
end

edited Apr 30 '12 at 14:07

fl00r

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answered Apr 30 '12 at 14:06

Phrogz

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112
651
745

1

This is pretty nice, except it will skip the normal `initialize` argument checking: `initialize_with :a, :b` + `Foo.new(1,2,3,4,5)` will produce no error. – Casper Apr 30 '12 at 14:48

score -1 · Answer 5 · answered Apr 30 '12 at 13:58

-1

class Foo
  class InvalidAttrbute < StandardError; end

  ACCESSORS = [:a, :b, :c, :d, :e]
  ACCESSORS.each{ |atr| attr_accessor atr }

  def initialize(args)
    args.each do |atr, val|
      raise InvalidAttrbute, "Invalid attribute for Foo class: #{atr}" unless ACCESSORS.include? atr
      instance_variable_set("@#{atr}", val)
    end
  end
end

foo = Foo.new(a: 1)
puts foo.a
#=> 1

foo = Foo.new(invalid: 1)
#=> Exception

answered Apr 30 '12 at 13:58

fl00r

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I think the opening sentence was something along the lines of "It's time to make it shorter" ;) – d11wtq Apr 30 '12 at 14:20
Yeap. But Struct do pretty the same underneath :) – fl00r Apr 30 '12 at 14:51

How to generate initializer in Ruby?

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