Demonstrating various differences between class and instance methods in Ruby

ex.rb

#!/usr/bin/env ruby

class SomeClass
    def self.add_two_numbers(one, two)
        one + two
    end

    def add_two_numbers(one, two)
        self.class.add_two_numbers(one, two)
    end
end

obj = SomeClass.new
puts obj.add_two_numbers(1, 2)

__END__

$ ./ex.rb
3
$

ex2.rb

#!/usr/bin/env ruby

class SomeClass
    def self.call_private_method(obj)
        obj.send(:set_value, 5)
    end

    def self.retrieve_value(obj)
        obj.instance_variable_get(:@value)
    end

    #######
    private
    #######

    def set_value(val)
        @value = val
    end
end

myinstance = SomeClass.new
SomeClass.call_private_method(myinstance)
puts SomeClass.retrieve_value(myinstance)

__END__

$ ./ex2.rb
5
$

ex3.rb

#!/usr/bin/env ruby

class SomeClass
    def test_setup!
        @a = 1
        @b = 2
        @c = 'and now for something completely different'
    end

    def debug_vars
        instance_variables.sort.each do |var|
            puts "#{var}: #{instance_variable_get(var).inspect}"
        end
    end
end

obj = SomeClass.new
obj.test_setup!
obj.debug_vars

__END__

$ ./ex3.rb
@a: 1
@b: 2
@c: "and now for something completely different"
$