How do I list all objects created from a class in Ruby?

Question

Is there any way in Ruby for a class to know how many instances of it exist and can it list them?

Here is a sample class:

class Project

  attr_accessor :name, :tasks

  def initialize(options)
    @name = options[:name]
    @tasks = options[:tasks]
  end

  def self.all
    # return listing of project objects
  end

    def self.count
          # return a count of existing projects
    end


end

Now I create project objects of this class:

options1 = {
  name: 'Building house',
  priority: 2,
  tasks: []
}

options2 = {
  name: 'Getting a loan from the Bank',
  priority: 3,
  tasks: []
}

@project1 = Project.new(options1)
@project2 = Project.new(options2)

What I would like is to have class methods like Project.all and Project.count to return a listing and count of current projects.

How do I do this?

Andrew Haines · Accepted Answer · 2013-01-14T13:35:30.917

51

You can use the ObjectSpace module to do this, specifically the each_object method.

ObjectSpace.each_object(Project).count

For completeness, here's how you would use that in your class (hat tip to sawa)

class Project
  # ...

  def self.all
    ObjectSpace.each_object(self).to_a
  end

  def self.count
    all.count
  end
end

edited Jan 14 '13 at 13:35

answered Jan 14 '13 at 12:50

Andrew Haines

6,574
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34

Do you need to `include ObjectSpace` in the class for this to work? – onebree Oct 20 '15 at 14:24
2

@HunterStevens no, we're not mixing the module into our class, just calling a method on it – Andrew Haines Oct 20 '15 at 14:34
4

**WARNING**: this solution can make it easy to shoot oneself in the foot. If you don't keep a reference to your objects (for example if you do `Project.new` without assigning the result to something), they will be garbage collected at some point and `ObjectSpace.each_object` will obviously stop reporting them. Using `@@instances = []` instead like in rohit89's answer solves this problem by keeping a reference to those objects. – vmarquet Jan 29 '17 at 16:28

score 7 · Answer 2 · edited Jan 14 '13 at 13:32

7

One way to do is to keep track of it as and when you create new instances.

class Project

    @@count = 0
    @@instances = []

    def initialize(options)
           @@count += 1
           @@instances << self
    end

    def self.all
        @@instances.inspect
    end

    def self.count
        @@count
    end

end

If you want to use ObjectSpace, then its

def self.count
    ObjectSpace.each_object(self).count
end

def self.all
    ObjectSpace.each_object(self).to_a
end

edited Jan 14 '13 at 13:32

Andrew Haines

6,574
21
34

answered Jan 14 '13 at 12:24

rohit89

5,745
2
25
42

This is what I'd do. It'll work for sure in all Ruby implementations, and can be extended for different purposes if needed. – Teemu Leisti Oct 21 '14 at 13:28

score 4 · Answer 3 · answered Jan 14 '13 at 12:50

4

class Project
    def self.all; ObjectSpace.each_object(self).to_a end
    def self.count; all.length end
end

answered Jan 14 '13 at 12:50

sawa

165,429
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381

score 3 · Answer 4 · answered Jan 14 '13 at 12:29

3

Maybe this will work:

class Project
  class << self; attr_accessor :instances; end

  attr_accessor :name, :tasks

  def initialize(options)
    @name = options[:name]
    @tasks = options[:tasks]

    self.class.instances ||= Array.new
    self.class.instances << self
  end

  def self.all
    # return listing of project objects
    instances ? instances.dup : []
  end

  def self.count
    # return a count of existing projects
    instances ? instances.count : 0 
  end

  def destroy
    self.class.instances.delete(self)
  end
end

But you will have to manually destroy these objects. Maybe other solution can be build based on ObjectSpace module.

answered Jan 14 '13 at 12:29

yattering

436
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7

I like this but can there should be some built in reflection - Doesn't that exist in ruby? I have no clue how to use the ObjectSpace module. Example would really help – Amit Erandole Jan 14 '13 at 12:34
1

Well. ObjectSpace lets you interact with garbage collector. This is stuff that I try not to do in my code. You can experiment with `ObjectSpace.each_object(Project).to_a` but I can't help you any more with this. – yattering Jan 14 '13 at 12:52
any particular reason why this should be avoided? – Amit Erandole Jan 14 '13 at 12:52
For example in [doc](http://www.ruby-doc.org/core-1.9.3/ObjectSpace.html) is written that method `count_objects` is "not expected to work except C Ruby". So you have to be careful. – yattering Jan 14 '13 at 13:01
`count_objects` is only expected to work for MRI (aka C Ruby). `each_object` works in MRI, Rubinius and JRuby (tested on the 1.9.3 variants). – Andrew Haines Jan 14 '13 at 13:38

How do I list all objects created from a class in Ruby?

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