Question

I encountered a strange question about override initialize message of BigDecimal.

class Test1 < String
  def initialize(a, b)
    puts a
    puts b
  end
end

require 'bigdecimal'
class Test2 < BigDecimal
  def initialize(a, b)
    puts a
    puts b
  end
end

>> Test1.new('a', 'b')
a
b
>> Test2.new('a', 'b')
TypeError: wrong argument type String (expected Fixnum)
    from (irb):17:in `new'
    from (irb):17

Why I can override the initialize message of String, but not of BigDecimal?

Answer 1

When you look into sources of Ruby classes, you’ll see that the String class defines method String#initialize, which is called after String#new (inherited from Object) for allocating a new instance. You don’t call String#initialize (or #super) in your new instance so you got "" when you inspect the newly created object.

BigDecimal defines method Bigdecimal#new, which allocates its own object. Object creation consists of two parts – allocating space for new object and initializing it. You only defined initializing new object, so you stay with default allocating space for object. If you want to override it, you should define #new in your new class and call BigDecimal‘s #new with proper arguments.

Hope that clarifies a little what happen in your example.