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Re: simple subclass question

Ball, Donald A Jr (Library)

5/2/2007 5:10:00 PM

Okay, so maybe this turns out to be a not-so-simple subclass question
after all. I appreciate all the thoughtful answers and links, I'm pretty
close to wrapping my head around it all. I'm still having trouble with
class instance variable initialization, and have come up with code
sample which should illustrate the point:

class Breakfast
def self.add_food(*args)
@foods ||= []
args.each do |arg|
@foods << arg
end
end

def self.foods
if superclass.respond_to?(:foods)
superclass.foods + @foods
else
@foods
end
end

add_food :eggs
end

class RubyBreakfast < Breakfast
add_food :grapefruit, :chunky_bacon
end

class BoringBreakfast < Breakfast
add_food :dry_toast, :gruel
end

Everybody loves breakfast, right? This code works great, except... maybe
I don't want Breakfast to have :eggs by default, but if I remove the
add_food :eggs from Breakfast, then Breakfast.foods returns nil instead
of [] since @foods isn't initialized until add_food is called, and
RubyBreakfast.foods throws an exception trying to its delicious foods
array to nil. But I'll be damned if I can figure out how to properly
initialize @foods in Breakfast. Can someone point me in the proper
direction?

- donald
1 Answer

Robert Dober

5/2/2007 6:49:00 PM

0

On 5/2/07, Ball, Donald A Jr (Library) <donald.ball@nashville.gov> wrote:
> Okay, so maybe this turns out to be a not-so-simple subclass question
> after all. I appreciate all the thoughtful answers and links, I'm pretty
> close to wrapping my head around it all. I'm still having trouble with
> class instance variable initialization, and have come up with code
> sample which should illustrate the point:
>
> class Breakfast
> def self.add_food(*args)
> @foods ||= []
just initialize @foods in the class

class Breakfast
@foods = [] # class instance variable, I did this in my original example
# because of (1)
def self.add_food( *args)
@foods += args
end

......
<snip>
> Everybody loves breakfast, right? This code works great, except... maybe
> I don't want Breakfast to have :eggs by default, but if I remove the
> add_food :eggs from Breakfast, then Breakfast.foods returns nil instead
> of [] since @foods isn't initialized until add_food is called, and
> RubyBreakfast.foods throws an exception trying to its delicious foods
> array to nil. But I'll be damned if I can figure out how to properly
> initialize @foods in Breakfast. Can someone point me in the proper
> direction?
Hopefully I did, if not continue asking I am a bad teacher, I know :(.

(1)

class A
@cl_inst_var = 42
end

is the same (unless class A has been defined before) as

A = Class.new { @cl_inst_var = 42 }

Cheers
Robert


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