Extending initialize

You define the method initialize to call old_initialize, but then you alias_method it to old_initialize, so old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize will call old_initialize…

I wrote a rather lengthy article about the "right" way to call an old version of an overwritten method here:

The best way to do this is to simply not do it at all. Don't overwrite methods, override them. Ruby has inheritance, use it:

class Module
  def custom_attr_accessor(attr)
    attr_reader attr
    prepend(Module.new do
      define_method(:initialize) do |*args|
        super(*args)
        instance_variable_set(:"@#{attr}", yield)
      end
    end)
  end
end

class Foo
  custom_attr_accessor :foo do 'foo' end
  custom_attr_accessor :bar do 'bar' end
end

# It works:
Foo.new
# => #<Foo:0xdeadbeef081542 @foo='foo', @bar='bar'>

# How it works:
Foo.ancestors
# => [#<Module:0xdeadbeef081523>, 
#     #<Module:0xdeadbeef081524>, 
#     Foo, 
#     Object, 
#     Kernel, 
#     BasicObject]

We can make this a little nicer by assigning the mixins to constants so they get proper names, and amending the API so that one can create multiple accessors in one go:

class Module
  def custom_attr_accessor(attr=(no_attr = true), **attr_specs, &blk)
    attr_specs[attr] = blk unless no_attr
    attr_specs.each do |attr, blk|
      attr_reader attr
      prepend CustomAttrAccessor.(attr, &blk)
    end
  end
end

module CustomAttrAccessor
  def self.call(attr)
    m = Module.new do
      define_method(:initialize) do |*args|
        super(*args)
        instance_variable_set(:"@#{attr}", yield)
      end
    end
    const_set(:"CustomAttrAccessor_#{attr}_#{m.object_id}", m)
  end
end

class Foo
  custom_attr_accessor :foo do 'foo' end
  custom_attr_accessor :bar do 'bar' end
end

# It works:
Foo.new
# => #<Foo:0xdeadbeef081542 @foo='foo', @bar='bar'>

# How it works:
Foo.ancestors
# => [CustomAttrAccessor::CustomAttrAccessor_bar_48151623420020, 
#     CustomAttrAccessor::CustomAttrAccessor_foo_48151623420010, 
#     Foo, 
#     Object, 
#     Kernel, 
#     BasicObject]

class Bar
  custom_attr_accessor :foo do 'FOO' end
  custom_attr_accessor :bar do 'BAR' end
  custom_attr_accessor baz: -> { 'BAZ' }, qux: -> { 'QUX' }
end

# It works:
Bar.new
# => #<Bar:0xdeadbeef081542 @foo='FOO', @bar='BAR' @baz='BAZ', @qux='QUX'>

# How it works:
Bar.ancestors
# => [CustomAttrAccessor::CustomAttrAccessor_qux_48151623420060, 
#     CustomAttrAccessor::CustomAttrAccessor_baz_48151623420050, 
#     CustomAttrAccessor::CustomAttrAccessor_bar_48151623420040, 
#     CustomAttrAccessor::CustomAttrAccessor_foo_48151623420030, 
#     Bar, 
#     Object, 
#     Kernel, 
#     BasicObject]

Do you look for something like this:

class Object
  def custom_attr_accessor(attr)
    define_method "#{attr}=".to_sym do |val|
      instance_variable_set("@#{attr}", val)
    end
    define_method attr do
      instance_variable_get("@#{attr}") || yield 
    end
  end
end

class Foo
  custom_attr_accessor :foo do
    "foo"
  end

  custom_attr_accessor :bar do
    "bar"
  end
end


f = Foo.new
puts f.foo #=> foo
f.foo = 1
puts f.foo #=> 1

puts f.bar #=> bar
f.bar = 2
puts f.bar #=> 2