If 'Self' Is Always the Implied Receiver in Ruby, Why Doesn't 'Self.Puts' Work

If `self` is always the implied receiver in Ruby, why doesn't `self.puts` work?

Private methods can't have a receiver

I think the answer is this: Ruby's way of enforcing method privacy is that it doesn't allow calling private methods with an explicit receiver.

An example:

class Baker
  def bake_cake
    make_batter
    self.use_oven # will explode: called with explicit receiver 'self'
  end

  private
  def make_batter
    puts "making batter!"
  end

  def use_oven
    puts "using oven!"
  end

end

b = Baker.new
b.bake_cake

Since there can be no explicit receiver, you certainly can't do b.use_oven. And that is how method privacy is enforced.

Why won't Ruby allow me to specify self as a receiver inside a private method?

The Problem

In Ruby, private methods can't be called directly with an explicit receiver; self doesn't get any special treatment here. By definition, when you call self.some_method you are specifying self as the explicit receiver, so Ruby says "No!"

The Solution

Ruby has rules for its method lookups. There may be a more canonical source for the rules (other than going to the Ruby source), but this blog post lays out the rules right at the top:

1) Methods defined in the object’s singleton class (i.e. the object itself)
2) Modules mixed into the singleton class in reverse order of inclusion
3) Methods defined by the object’s class
4) Modules included into the object’s class in reverse order of inclusion
5) Methods defined by the object’s superclass, i.e. inherited methods

In other words, private methods are first looked up in self without requiring (or allowing) an explicit receiver.

Global methods in Ruby

For runtime Ruby has special object the main.

It's some kind of trick that all code runs in the context of this object.

So when you're typing methods like puts, p and so on, all of them are calling in the context of self object and passing to self object.

And here's the second thing - the access control.
As you probably know, Ruby has keywords like private, protected and public - all of them manage the access of calling methods on object. Ruby is checking this access control only when you're have construction like

<name_of_object>.your_method and self.your_method

So when you're typing

self.p "something"

Ruby will decline this call because the p method is private method.

Ruby's self vs. Python's self

Regarding Python I can tell you nothing new. The self there is passed conventionally as the first parameter of a method, as pst said. From Python docs

Often, the first argument of a method is called self. This is nothing more than a convention: the name self has absolutely no special meaning to Python. Note, however, that by not following the convention your code may be less readable to other Python programmers, and it is also conceivable that a class browser program might be written that relies upon such a convention.

CRuby (or 'MRI') has something similar which happens under the hood though. Every C extension may define (module/class/singleton) methods on a Ruby class by using

• rb_define_method (instance)
• rb_define_singleton_method (singleton class)
• rb_define_module_function (class/module)

The actual implementing functions always take VALUE self as their first argument, in analogy to the Python idiom. self in these cases refers to the object instance this particular message has been sent to, i.e. if you have

 person = Person.new
 person.do_sth

and do_sth would happen to be implemented in C, then there would be a corresponding C function

VALUE
person_do_sth(VALUE self) {
    //do something
    return self;
}

Every such implementation has to return a VALUE (the C representation of a Ruby object), which relates to the fact that every method call or message sent (to stick to Smalltalk parlance) has a return value in Ruby. There is no such thing as a void function in Ruby.

Although we need to pass self back and forth in the low-level C code, you don't need to do so in Ruby code, Ruby takes care of this for you. The current value of self is stored internally in the current thread context that is executed, so the existence of self is granted, the message "self" will always evaluate to some object.

Due to the dynamic nature of Ruby, the actual value of of this object being referenced by self changes with the current scope of the code that is currently interpreted. Run this to see for yourself:

puts "#{self} - declared in global scope"  # the 'top self' aka 'main'
class << self
  puts "#{self} - 'main's singleton class" # main's singleton or 'eigenclass'
end

puts "Starting to interpret class A code"

class A
  puts "#{self} - When do I get executed!?" # self is class A

  class << self
    puts "#{self} - And me!?" # now A's  singleton class
    def a # declaring method in class's singleton class results in class method
      puts "#{self} - declared in singleton class" # it's A
    end 
  end

  def self.b
    puts "#{self} - declared in class method" # self is class A again -> class method
    class << self
      puts "#{self} - declared in Class A's singleton class" # now it's Class A's singleton class
    end
  end

  def c
    puts "#{self} - declared in instance method" # self is instance of A
    class << self
      puts "#{self} - declared in instance's singleton class" # now it's the A instance's singleton class
    end
  end

end  

puts "All so far has happened simply by interpreting A's code" 

a = A.new
A.a
A.b
a.c

If you want to call a method/send a message from any context to self, you could do this either explicitly (e.g. self.method) or you omit self as the receiver - then, by convention, the implicit receiver of the message will be self.

An interesting side note to this is Ruby's interpretation of private methods, which differs e.g. from the Java notion of private. Ruby's private methods are only callable by sending a message using self as an implicit receiver, i.e.

class A

  def a
    b
  end

  private

  def b
    puts "I'm private"
  end
end

a = A.new
a.a # => I'm private

works, whereas replacing method a by

def a
  self.b
end

would raise an exception. This implies that something very common in Java

class A {
    private boolean compareUs(A a1, A a2) { ... }

    public boolean equals(A a1, A a2) {
        return (a1.compareUs() == a2.compareUs());
    }
}

won't work in Ruby. Silly example, but just to illustrate the point: In Java we can access private methods of other instances of the same class, this would not be possible in Ruby because we can only access private methods of the current self.

Finally, to complicate things a bit further, the instance_eval and class_eval functions will also alter the value of self during execution.

Why does Ruby have both private and protected methods?

protected methods can be called by any instance of the defining class or its subclasses.

private methods can be called only from within the calling object. You cannot access another instance's private methods directly.

Here is a quick practical example:

def compare_to(x)
 self.some_method <=> x.some_method
end

some_method cannot be private here. It must be protected because you need it to support explicit receivers. Your typical internal helper methods can usually be private since they never need to be called like this.

It is important to note that this is different from the way Java or C++ works. private in Ruby is similar to protected in Java/C++ in that subclasses have access to the method. In Ruby, there is no way to restrict access to a method from its subclasses like you can with private in Java.

Visibility in Ruby is largely a "recommendation" anyways since you can always gain access to a method using send:

irb(main):001:0> class A
irb(main):002:1>   private
irb(main):003:1>   def not_so_private_method
irb(main):004:2>     puts "Hello World"
irb(main):005:2>   end
irb(main):006:1> end
=> nil

irb(main):007:0> foo = A.new
=> #<A:0x31688f>

irb(main):009:0> foo.send :not_so_private_method
Hello World
=> nil

What is main in Ruby?

Everything in Ruby occurs in the context of some object. The object at the top level is called "main". It's basically an instance of Object with the special property that any methods defined there are added as instance methods of Object (so they're available everywhere).

So we can make a script consisting entirely of:

puts object_id
@a = 'Look, I have instance variables!'
puts @a

and it will print "105640" and "Look, I have instance variables!".

It's not something you generally need to concern yourself with, but it is there.

Dot syntax vs param passing syntax

Yes, you can add methods to the String class to achieve your desired effect; the variable "self" refers to the object which receives the method call.

class String
  def namechanger
    "This is the name to change: #{self}"
  end
end
"Steve".namechanger # => This is the name to change: Steve

This practice is known as monkey patching and should be used carefully.

---

Related Topics