Hey all,
I am a little confused about a tutorial I read. Here's an example below
taken from the tutorial:
Object.instance_eval do
def has_attribute( *attrs )
attrs.each do | attr |
self.class_eval %Q{
def #{attr}=(val)
instance_variable_set("@#{attr}", val)
end
def #{attr}
instance_variable_get("@#{attr}")
end
}
end
end
end
class A
has_attribute :my_attribute, :another_attribute
end
a = A.new
puts a.methods - Object.methods
# => ["my_attribute", my_attribute=", "another_attribute",
"another_attribute="]
a.my_attribute = 1
a.my_attribute
# => 1
a.another_attribute = "A String"
a.another_attribute
# => "A String"
THe guy says:
"The first instance_eval is used to add class method has_attribute into
Object so that we can call it in all the inherited class."
I'm a little thrown off by this statement. First, Object itself is an
instance of class Class. Someone here even said before "Object is an
instance of Class". So "class Object end "implies that Object is an
instance of class, equivalent to: Object = Class.new. Hence, we can
invoke instance_eval on Object, since Object is an instance. What
confuses me here is has_attribute appears to be an instance method (e.g.
def a end) not class method (e.g. def self.a end). So why does guy say
we add class method has_attribute?
In fact, the difference between Object class and Class class is that
when you add methods to class Class, those methods become available to
any class that is created as well (since all classes created using class
construct inherit from Class) and you can use those methods within any
class. However, that's not what we do above. We extend Object above.
When you instantiate a class, you inherently create an object instance,
so that object instance should have available the methods defined in
Object. So then how can we access that method within a class, such as
that being done in class A above?
Thanks for response
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