Question

the ruby book I’m reading has confused me a bit. If I do the following, I understand completely why the code throws an error;

class Person
  def show_name
    puts @name
  end
end

person = Person.new
person.show_name
Person.show_name   #(note the capital P) this line falls over

It throws an error stating that the Person class does not have a method called show_name, because it is an instance method. I understand this completely. The book then throws in this example;

class Class
  def add_accessor(accessor_name)
    self.class_eval %Q{attr_accessor :#{accessor_name}}
  end
end

class Person
end

person = Person.new
Person.add_accessor :name    #note the capital P
Person.add_accessor :age     #capital P here too

person.name = "Mikey"
person.age = 30

puts person.name

and goes on to state how cool it is that you can add methods to classes dynamically. What I don’t understand is why I am suddenly allowed to call the “add_accessor” method as a class method (with a capital P) when the method itself isn’t defined as such? I thought all class methods had to be declared like this?

class Math
  def self.PI
    3.141
  end
end

puts Math.PI

Is anyone able to enlighten me?

Answer 1

Look at this:

person.instance_of?(Person) #=> true
Person.instance_of?(Class)  #=> true

You have defined an instance method add_accessor for all the instances of the class Class, so Person class can use that method because it is an instance of Class.

I recommend yout to take a look at metaprogramming and Ruby Object Model.

Hoep this helps