When creating a new object in ruby, what method does it call first?

Question

When i create the object as soon above, is the method initliase called first? In PHP, we have something called the constructor with run first whenever an object is created, what if there is more than 1 method in the class, which method is called first in ruby?

Thanks.

Answer 1

Class#new is just a normal method like any other method. It looks a bit like this, although in most implementations it is actually not written in Ruby:

class Class
  def new(*args, &block)
    new_obj = allocate

    new_obj.initialize(*args, &block)
    # actually, `initialize` is private, so it's more like this instead:
    # new_obj.__send__(:initialize, *args, &block)

    return new_obj
  end
end

The documentation also says it clearly:

new(args, …) → obj

Calls allocate to create a new object of class’s class, then invokes that object’s initialize method, passing it args. This is the method that ends up getting called whenever an object is constructed using .new.

Here's the source code for Class#new in the various implementations:

• Rubinius: machine/builtin/class.cpp
• JRuby: core/src/main/java/org/jruby/RubyClass.java
• Topaz: topaz/objects/classobject.py
• MagLev: src/kernel/bootstrap/Class.rb
• YARV: object.c (BTW, you will notice that this method is defined in object.c, not in class.c. This is one of those reasons I prefer not to look at YARV for implementations details, and instead look at Rubinius, JRuby, IronRuby, or Topaz. They are just much better organized.)

Answer 2

the initialize method is called whenever you call new.

Any other methods declared in that class should be called in your code.

For example:

class Example
  def initialize
    #some initialization code here
    puts "initialize method has just been called"
  end

  def foo
    #some foo code
    puts "this is the foo method"
  end 
end 

then, in your code:

my_obj = Example.new #initialize method will be called here

my_obj.foo #now the foo method will be called

That's about it, good luck!