When to use self

suggest change

Most Ruby code utilizes the implicit receiver, so programmers who are new to Ruby are often confused about when to use self. The practical answer is that self is used in two major ways:

1. To change the receiver.

Ordinarily the behavior of def inside a class or module is to create instance methods. Self can be used to define methods on the class instead.

class Foo
  def bar
    1
  end

  def self.bar
    2
  end
end

Foo.new.bar #=> 1
Foo.bar #=> 2

2. To disambiguate the receiver

When local variables may have the same name as a method an explicit receiver may be required to disambiguate.

Examples:

class Example
  def foo
    1
  end

  def bar
    foo + 1
  end

  def baz(foo)
    self.foo + foo # self.foo is the method, foo is the local variable
  end

  def qux
    bar = 2
    self.bar + bar # self.bar is the method, bar is the local variable
  end 
end

Example.new.foo    #=> 1
Example.new.bar    #=> 2
Example.new.baz(2) #=> 3
Example.new.qux    #=> 4

The other common case requiring disambiguation involves methods that end in the equals sign. For instance:

class Example
  def foo=(input)
    @foo = input
  end

  def get_foo
    @foo
  end

  def bar(input)
    foo = input # will create a local variable
  end

  def baz(input)
    self.foo = input # will call the method
  end
end

e = Example.new
e.get_foo #=> nil
e.foo = 1
e.get_foo #=> 1
e.bar(2)
e.get_foo #=> 1
e.baz(2)
e.get_foo #=> 2

Found a mistake? Have a question or improvement idea? Let me know.

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