“Chris Pine” nemo@hellotree.com wrote in message news:02b701c31c8d$c6b0fc40$6401a8c0@MELONBALLER…
I know it’s wrong to ask the mailing list for help on your homework, but
this isn’t really homework…
My talk is “Understanding Ruby’s Object Model”, but I’d like to start (and
maybe end) with some really cool examples about the cool things you can do
in Ruby relating to classes and modules being objects.
Not sure if this counts as cool, but it was useful to me: I’ve
programmed an object-relational mapping framework in Ruby where fields
are stored as data structures, to be retrieved at runtime. They’re
class methods, so let’s say you have a User class:
class User < DomainObject
def User.fields
# return an array of Field objects
end
end
… so then you can have code like
class ObjectCreator
def verify (valueHash, aClass)
# for each field defined for this domain object class, give it a
chance to
# throw an exception if there’s an invalid value
aClass.fields.each { |field|
field.verify (valueHash[field.name])
}
end
end
“aClass” in the code above is a metaclass: You don’t pass in an
instance, but the class itself. (You’d call “verify (valueHash,
User)”, not “verify (valueHash, johnDoe)”.)
The neat thing about this is that you can attach class-specific data
and use it without knowing at runtime what class you’re dealing with.
You can’t do this in Java without jumping through some really really
nasty hoops: It binds class methods at compile-time. (Java class
methods aren’t really class methods at all; they’re more like global
methods that are bound to a particular namespace.)
Also, a question: what’s the difference between a class’s singleton class
and a metaclass? If I’m not mistaken, these are the same in Ruby, but I
mean conceptually: what is a metaclass?
Traditionally a singleton is any class which is guaranteed to have
only one instance. It’s useful but also easy to trip over. (See
http://c2.com/cgi/wiki?SingletonPattern for a big discussion of this.)
Ruby also uses the word “singleton” to describe any instance whose
methods have been explicitly set to vary from the standard class
definition. For example:
myStrangeArray = [ 0, 1, 2 ]
def myStrangeArray.length
999
end
(I don’t know where this usage comes from or if it’s unique to Ruby;
it tripped me up when I first came across it.)
Francis
