rubycoding-styleeigenclass

Why is `class << self` more common than `class << Foo` for opening a class object's eigenclass?

Ruby programmers typically use class << self inside a class body to open up the class object's eigenclass, like so:

class Foo
  class << self
    # ...
  end
end

However, I seldom see this equivalent form (assume that Foo is already defined to be a class):

class << Foo
  # ...
end

Is there a reason for preferring the first style to the second?

Solution

  • When using class << Foo, or when defining explicitly def Foo.some_method(args) you are repeating the name of the class.

    Using class << self is DRYer, and makes refactoring easier (changing the class name is done in one place, and does not have to be repeated in the code), as well as copy+paste to other classes/projects.

    class Foo1
  # ..
end

class << Foo # <- :(
  #..
end