Question

Where is the attribute @color defined here? Presumably the assignment in the constructor? If this is the case, what if a type has multiple constructors, one of which does not assign a value to @color?

class Baz
  def initialize(color)
    @color = color
  end

  def color
    @color
  end

  def color=(value)
    @color = value
  end
end

Answer 1

Instance variables in ruby are not “defined” per se. They pop into existence when they are used.

The first time you assign something to an instance variable, that is the closest thing to a “definition” (but really it is more of an “initialization”). If you reference an instance variable that has not been assigned a value yet, its value will be nil.

So if your constructor does not assign a value to @color, then @color will simply remain uninitialized (and thus return nil anywhere else in the class that references it…unless it is assigned a value elsewhere outside the constructor).

See here for more information: http://www.rubyist.net/~slagell/ruby/instancevars.html

A relevant quote from the article:

Instance variables do not need to be declared. This indicates a
flexible object structure; in fact, each instance variable is
dynamically appended to an object when it is first assigned.

And:

As with globals, instance variables have the nil value until they are
initialized.