By Ankit Soni


2011-07-12 18:56:24 8 Comments

from what I understand of the self keyword, it simply refers to the current instance of the class. Isn't this the default behaviour at all times anyways? For example, isn't

self.var_one = method(args) equivalent to just var_one = method(args) ?

If so then what is the use of self?

4 comments

@Andrew 2011-07-12 21:21:52

There are several important uses, most of which are basically to disambiguate between instance methods, class methods, and variables.

First, this is the best way to define class methods. IE:

class Foo
  def self.bar
    "class method bar"
  end

  def bar
    "instance method bar"
  end
end

Foo.bar  #returns "class method bar"

foo = Foo.new
foo.bar #returns "instance method bar"

Also, within instance methods self refers to the instance, within class methods it refers to the class, and it can always be used to distinguish from local variables.

class Bar
  def self.foo
    "foo!"
  end

  def baz
    "baz!"
  end

  def self.success
    foo #looks for variable foo, doesn't find one, looks for class method foo, finds it, returns "foo!"
  end

  def self.fail
    baz #looks for variable baz, doesn't find one, looks for class method baz, doesn't find one, raises exception
  end

  def instance_success
    baz #looks for variable baz, doesn't find one, looks for instance method baz, finds it, returns "baz!"
  end

  def instance_fail
    foo #looks for variable foo, doesn't find one, looks for instance method foo, doesn't find one, raises exception
  end

  def local_variable
    baz = "is my favorite method"
    baz #looks for variable baz, finds it, returns "is my favorite method"
  end

  def disambiguate
    baz = " is my favorite method"
    self.baz + baz #looks for instance method baz, finds it, looks for local variable baz, finds it, returns "baz! is my favorite method"
  end
end

So, in the end, you can avoid using self in many cases, but it's often helpful to go ahead and use it to make sure that you don't inadvertently create naming conflicts later on. Sometimes those can create bugs that are very hard to find. In the end it's often a matter of personal style.


Update: As noted in the comments, one more really important thing:

In a class, if you have a method like this:

def bar=(string)
  ...
end

And in another method you call:

def other_method
  bar = "abcd"
end

It isn't going to call your bar= method, it's going to create a local variable bar. So, in this case you use self to tell ruby not to create a local variable, like so:

def other_method
  self.bar = "abcd"
end

The same thing applies if you want to take an argument with the name of a method, like so:

def example
  ...
end

def other_thing(example)
  self.example(example)
end

If you left off self it would assume you meant the local variable with the same name.

So, in general, self in method names is used to distinguish between class and instance variables, and everywhere else you use it when Ruby needs help distinguishing between method calls and local variables or local variable assignment.

I hope that makes sense!

@mltsy 2013-03-11 19:54:39

This is a more thorough answer, except that it doesn't include the assignment caveat. There is also a good blog article on the usages: jimmycuadra.com/posts/self-in-ruby which includes, specifically, assignment. You can't call an assignment method without using self (as noted in the first answer).

@Fernando 2013-06-21 14:56:19

Very helpful for Java guys =)

@fengd 2013-06-25 05:23:37

probably, it's better to outline the principle that "you can only access class variable/method inside a class method"

@Engr. Hasanuzzaman Sumon 2015-04-27 08:52:53

@fengd Of-course you can call class variable/method from instance method using self.class.class_method " class Person def self.class_method 'golbal variable' end def show self.class.class_method end end person = Person.new puts person.show

@brayne 2011-07-12 20:11:01

One other use of self is to declare class methods (similar to static methods in Java).

class foo
 def self.bar
  #do class related stuff here
 end
end

That being said, you could also have used def foo.bar instead for the method signature.

@Sohan 2011-07-12 19:14:51

here's an example use:

def run miles
  self.miles = miles
end

In this case self will help. in most cases self is redundant.

@sepp2k 2011-07-12 18:59:44

In most cases self.foo is indeed redundant because you can just write foo for the same effect, but in this case it is not and the self is required.

var_one = method(args) will create a local variable called var_one, it will not call any method or do anything else to self.

self.var_one = method(args) will call the method var_one= on self with the argument method(args).

Another case where the use of self is non-optional would be if you want to pass it as an argument to a method, i.e. some_method(self) - you can't do that without the self keyword.

@Ankit Soni 2011-07-12 19:52:00

I'm sorry I'm confused. How can var_one= be a method? I meant for var_one to be a variable, and assign the result of the method named method to that variable.

@sepp2k 2011-07-12 20:30:35

@Ankit: It can be a method by being a method. Sorry if that's a bit of a non-answer, but I don't really know how else to answer that. If you want var_one to be a local variable, putting self. in front of it is wrong and will not do what you want.

@cam 2011-07-12 20:30:49

@Ankit Soni: If you send any message to self (e.g., self.foo) it is calling the foo method of your class. var_one= is just a setter method. If you want var_one to be a variable, you must not use self (which is only useful for calling methods). If you're looking for instance variables then you simply prefix an identifier with @ (e.g., @var_one)

@Ankit Soni 2011-07-12 20:41:36

OK I understand how var_one= is the setter method. Now ,even in this case, wouldnt var_one= also call the same setter method of that same instance of the object as self.var_one=?

@Ankit Soni 2011-07-12 20:46:49

Wait are you saying that var = "bla" exists only as a setter method in ruby? i.e there is no assignment operator, and this writing a = b is only a convenient way of writing a.set(b)?

@sepp2k 2011-07-12 21:10:46

@Ankit: No. If you write var = "bla" this will always create a local variable and never invoke a setter method. If you write foo.var = "bla", it will call the setter method var= on foo. So to invoke the var= setter method on self, you need to write self.var = "bla". Writing var = "bla" will not suffice as it will simply create a local variable.

@Ankit Soni 2011-07-13 12:54:41

@sepp2k So writing var = "bla" will make a local variable var and set it to "bla". Isn't that what my setter method would do for me anyways? Sorry I'm asking so many questions about this by the way, I'm just trying to figure this out and be done with it.

@sepp2k 2011-07-13 13:32:52

@Ankit: Your setter method will do what ever you define it to do. However it will almost certainly not create a local variable named var, as that variable, being local, would only live inside that setter method and thus have no effect. If you defined the setter method using attr_accessor, what it will do is create an instance variable called @var. So in that specific case @var = "foo" and self.var = "foo" would do the same thing. However var = "foo" would not.

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