Why is 'self' required to call a Ruby method from within its constructor?

Question

If I have a class like this:

class Person

  def initialize(options={})
    self.name = options[:name]
  end

  def name=(name)
    @name = name
  end

end

... then the 'name=' method gets called and I get what I expect. But if I alter the example slightly to drop the 'self' from the call to 'name=' then the method is never called:

class Person

  def initialize(options={})
    name = options[:name]
  end

  def name=(name)
    @name = name
  end

end

Why is this? Why must 'self' be there? I was under the impression that the implicit 'self' would be set as expected in the 'initialize' constructor and therefor would behave as the first example.

BTW, self is required to call an accessor not just in the constructor but in every other instance method too — Thong Kuah, Jun 29 '13 at 05:00
This is certainly *related* to the question it is marked a duplicate of, but by the time someone knows the question asked in the marked duplicate, this question is already answered. This question is about the appearance of needing `self.` for all instance methods called in `initialize`. The fact that `self.` is needed for accessors is the *answer* to this question. — Darshan Rivka Whittle, Jun 29 '13 at 09:24

score 7 · Accepted Answer · answered Jun 29 '13 at 04:55

7

Ruby doesn't know if you want to make a local variable named "name" or use the member variable.

self.name tells it you want to use the member variable.

There isn't any syntax for saying you want to create a local variable, so that has to be the default.

(Also, member variables aren't real, but you know what I mean)

answered Jun 29 '13 at 04:55

xaxxon

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note that this is not only the case during initialization, but also during any instance method. – Justin L. Jun 29 '13 at 04:57
2

@xaxxon, the Ruby term for member variable would be `instance method` – Thong Kuah Jun 29 '13 at 04:59
@ThongKuah yeah, I hate that. because they serve two different logical purposes. – xaxxon Jun 29 '13 at 05:15
@ThongKuah: Instance methods and instance variables are very different things. – mu is too short Jun 29 '13 at 05:20
@muistooshort yup, I'm however, referring to `self.name=` which is calling an instance method :) – Thong Kuah Jun 29 '13 at 06:08

Darshan Rivka Whittle · Answer 2 · 2013-06-29T08:16:56.690

Why is 'self' required to call a Ruby method from within its constructor?

In general, it's not:

class Person
  def initialize(options={})
    set_name(options[:name]) # Calls `set_name` method; no `self.` required
  end

  def set_name(name)
    @name = name
  end
end

The need for self. in your code has nothing to do with being in the constructor: you can call any normal instance method from within initialize or elsewhere in the class without using self..

What's going on is that you're trying to call a setter method (an instance method whose name ends with =, e.g. name=), which are handled as a special case in Ruby.

According to The Ruby Programming Language:

The rule is that assignment expressions will only invoke a setter method when invoked through an object. If you want to use a setter from within the class that defines it, invoke it explicitly through self.

So again, it's not that Ruby doesn't know what self refers to during execution of the constructor -- it does -- it's that in this case you're trying to call a setter, which requires invocation through an object.

In your code, name = options[:name] creates and sets a local variable called name.

Why is 'self' required to call a Ruby method from within its constructor?

2 Answers2