test.rb

class Test
    class << self
        def x
            return "X"
        end
    end

    def y
        return "Y"
    end

    class_eval <<-END
        def self.a
            puts "#{self.class}.a"
            puts "#{self.x}"
        end
    END
    # Test.a => Class.a, 'X'
    # add puts "#{self.y}" inside self.a => NoMethodError: undefined method `y' for Test:Class
    # Test.new.a => undefined method `a' for #<Test:0x8cba20c>

    instance_eval <<-END
        def b
            puts "#{self.class}.b"
            puts "#{self.x}"
        end
    END
    # Test.b => Class.b, 'X'
    # add puts "#{self.y}" inside self.b => NoMethodError: undefined method `y' for Test:Class
    # Test.new.b => undefined method `b' for #<Test:0x8ca2e40>

    instance_eval do
        define_method(:c) do
            puts "#{self.class}.c"
            puts "#{self.y}"
        end
    end
    # Test.c => NoMethodError: undefined method `c' for Test:Class
    # Test.new.c => Test.c, 'Y'
    # add puts "#{self.x}" inside self.c, and call Test.new.c
    #   => NoMethodError: undefined method `x' for #<Test:0x9f34518>

    class << self
        define_method(:d) do
            puts "#{self.class}.d"
            puts "#{self.x}"
            #puts "#{self.y}"
        end
    end
    # Test.d => Class.d, 'X'
    # add puts "#{self.y}" inside self.c, and call Test.c
    #   => NoMethodError: undefined method `y' for Test:Class
    # Test.new.d => undefined method `d' for #<Test:0x8cdce4c>
end