Ruby accessing constants from inner-class

问题

I have a nested class like so:

class Mammal
    H = "Mammal"

    class Human
        H = "Human"
    end

end

And I want to make an Human object and after access the Human's constant, like so:

human = Mammal::Human.new # makes an object successfully

puts human::H             # does not work **
puts Mammal::Human::H     # works ["Human"]
puts Mammal::H            # works ["Mammal"]

**.. but it won't work ("..is not a class/module [TypeError]"). What am i doing wrong?

回答1:

What am I doing wrong?

You're trying to refer a constant from a wrong context. Constants are defined in class objects, not in instances. This works:

human = Mammal::Human.new
human.class.const_get(:H) # => "Human"

回答2:

Constants belong to classes, therefore constant resolution via the :: operator only works with class objects, not with instances of a class.

With that said, you can do this:

human.class::H

Object#class returns the object's class, relative to which you can resolve constants.

来源：https://stackoverflow.com/questions/14260931/ruby-accessing-constants-from-inner-class