How to create an object in Ruby without using new

Question

It's possible to create a Complex number in Ruby using

c = Complex.new(1,2)

but, it can be shortened to

c = Complex(1,2)

Is it possible to achieve the same functionality without having to define a function outside the class, like in the example below?

class Bits
  def initialize(bits)
    @bits = bits
  end
end

def Bits(list) # I would like to define this function inside the class
  Bits.new list
end

b = Bits([0,1])

I think Ruby should allow at least one of the proposed constructors below

class Bits
  def initialize(bits)
    @bits = bits
  end

  def self.Bits(list) # version 1
    new list
  end

  def Bits(list)      # version 2
    new list
  end

  def Bits.Bits(list) # version 3
    new list
  end
end

The code you’ve given works (except adding your own functions to the top level is a little questionable). What’s wrong with it? — matt
– matt, Commented Jun 22, 2014 at 12:46
I think it is better to keep the constructors inside the class. This is what most other OO languages allows. — Kri-ban
– Kri-ban, Commented Jun 22, 2014 at 12:50
Ah, ok I think I see. Then you can’t do this. In the case of Complex there is both the Complex class and a separate Complex method defined on Kernel (so it’s available at the top level), but they are separate things that just share the name. The best you can do is probably put their definitions in the same file. — matt
– matt, Commented Jun 22, 2014 at 12:54
self.Bits vs Bits.Bits -> they are almost the same. The disadvantage of Bits.Bits is that when you change your class's name, you have to change that code too. As for you 3 constructors, they are meant to be aliases of 1 method so why not use alias or alias_method -like construct? Something that looks like attr_* :var. I think it is more safe and less repetitive. — Darek Nędza
– Darek Nędza, Commented Jun 22, 2014 at 19:27

Saturn · Accepted Answer · 2014-06-22 22:56:40Z

7

Have this snippet:

def make_light_constructor(klass)
    eval("def #{klass}(*args) #{klass}.new(*args) end")
end

Now you can do this:

class Test
    make_light_constructor(Test)
    def initialize(x,y)
        print x + y
    end 
end

t = Test(5,3)

Yes, I know you're still defining a function outside a class - but it is only one function, and now any class you want can make use of its implementation rather than making one function per class.

answered Jun 22, 2014 at 22:56

Saturn

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Comments

steenslag · Accepted Answer · 2014-06-22 12:41:41Z

5

c = Complex(1,2)

is actually calling a method on Kernel

answered Jun 22, 2014 at 12:41

steenslag

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2 Comments

David Unric Over a year ago

Right, however can't be applied generally. Those Kernel methods are implemented internally, beyond language's scope.

matt Over a year ago

@DavidUnric you can use this if you want. Just create a top level method (which ends up as a private method on Object which has the same effect). You can even open up Kernel and add a method there if you specifically want it there.

Martin · Accepted Answer · 2014-06-22 12:39:20Z

2

Basically you can't - the () operator cannot be overriden in Ruby (Complex class is written in C).

You could achieve something similar using []:

class Bits
  def self.[](list)
    Bits.new list
  end
end

Which would allow something like:

b = Bits[[1,2]]

answered Jun 22, 2014 at 12:39

Martin

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2 Comments

David Unric Over a year ago

Or simply use arguments list by def self.[](*list) and then call with b = Bits[1,2].

Darek Nędza Over a year ago

I think it is better to link it to something like this: github.com/sorah/ruby/blob/master/complex.c

Community · Accepted Answer · 2017-05-23 12:19:23Z

1

If you pack your classes into some module you can use 2 methods:

self.included - called when you include Mod
self.extend - called when you extend Mod

I have created very basic method using self.included.

Cons: It is hard to write. You can say it is complex; It may not contain all features.
Pros: It looks exactly like Complex(2,3) (it uses () instead of [] as in https://stackoverflow.com/a/24351316/2597260 answer); You create just initialize, self.included create the rest.

module M1
  # some random classes
  class A; end 
  class B
    def initialize list
        @list = list
    end
    attr_accessor :list
  end
  class C
    def initialize var1
        @var1 = var1
    end
    attr_accessor :var1
  end
  Answer = 42
  # called on `include module_name`
  def self.included mod
    # classes are constants (in normal cases)
    constants.each do |cons| 
      class_eval do
        # I don't like hard-coded `::M1`
        klass = ::M1.const_get cons
        if klass.class==Class 
            define_method cons do |*args, &block|
              klass.new *args, &block 
            end
        end
      end
    end
  end
end

include M1


p A()

b = B([1,2,3])
p b.list

c = C 42
p c.var1

puts Answer() 
# NoMethodError: undefined method `Answer' for main:Object
# thats good, because Answer is not a class!

edited May 23, 2017 at 12:19

CommunityBot

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answered Jun 22, 2014 at 22:14

Darek Nędza

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4 Comments

Kri-ban Over a year ago

I'm accepting your answer as it solves my problem. But, it's too much code for replacing a simple def outside the class. This has to be a part of Ruby to be useful.

Darek Nędza Over a year ago

@Kri-ban I guess it is called over-engineering (it does something more than you required) but you may find using something like this, or self.included/self.extend somewhere in your code. What you mean "part of Ruby" ? All above methods are part of the Ruby (I am using Ruby 1.9.3).

Kri-ban Over a year ago

I just meant the Ruby definition should be extended to support my original request. Probably this is the wrong forum for this discussion.

Darek Nędza Over a year ago

@Kri-ban I guess you can look for Ruby's mailing list or you can check github.

jcsahnwaldt Reinstate Monica · Accepted Answer · 2020-06-14 16:40:51Z

Here's another hack that you could (but shouldn't) use, inspired by this blog post:

def method_missing(sym, *args, **kwargs, &blk)
    Object.const_get(sym).new(*args, **kwargs, &blk)
end

This simply expects any unknown method name to be the name of a class and calls :new on the class.

With rudimentary error handling:

alias sys_method_missing method_missing

def method_missing(sym, *args, **kwargs, &blk)
    cls = Object.const_get(sym) if Object.constants.include? sym
    if cls.is_a?(Class) then cls.new(*args, **kwargs, &blk)
    else sys_method_missing(sym, *args, **kwargs, &blk) end
end

If an unknown method name is the name of a class, this calls :new on the class. Otherwise, it delegates the call to the original implementation of method_missing().

Usage:

class Foo
end

foo = Foo()
p foo

Result:

#<Foo:0x00007f8fe0877180>

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