Create a class at run time within a module/namespace

Question

Creating a class at runtime is done as follows:

klass = Class.new superclass, &block
Object.const_set class_name, klass

Example:

class Person
  def name
    "Jon"
  end
end

klass = Class.new Person do
  def name
    "#{super} Doe"
  end
end

Object.const_set "Employee", klass

puts Employee.new.name # prints "Jon Doe"

Now, let's say that you have a module called Company:

module Company
end

How do you create the Employee class at runtime within the Company module/namespace such that the following yields the same result?

puts Company::Employee.new.name # prints "Jon Doe"

Sergio Tulentsev · Accepted Answer · 2012-06-24T11:11:08.610

15

Easier than you think :)

Company.const_set "Employee", klass

When you set something on Object, it becomes global because, well, everything is an Object. But you can do const_set to every class/module. And remember, modules/classes are just constants. So, Company::Employee is a constant Employee in a constant Company. It's simple :)

Full code:

class Person
  def name
    "Jon"
  end
end

klass = Class.new Person do
  def name
    "#{super} Doe"
  end
end

module Company
end

Company.const_set "Employee", klass

Company::Employee.new.name # => "Jon Doe"

edited Jun 24 '12 at 11:11

answered Jun 24 '12 at 11:05

Sergio Tulentsev

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Beat you by 2 secs literally :p – Michael Kohl Jun 24 '12 at 11:06
@MichaelKohl: Yes, I spent them on cutting/pasting that line :) – Sergio Tulentsev Jun 24 '12 at 11:07

score 2 · Answer 2 · answered Jun 24 '12 at 11:05

You already had all the necessary pieces:

class Person
  def name
    "Jon"
  end
end

klass = Class.new Person do
  def name
    "#{super} Doe"
  end
end

module Company
end

Company.const_set "Employee", klass

puts Company::Employee.new.name # prints "Jon Doe"

Company.constants.grep(/Emp/)
#=> [:Employee]

Create a class at run time within a module/namespace

2 Answers2