Making object methods available externally

Ruby
1

Hi all,

I’m having trouble figuring out a way of creating a method within a
class that’s available not only to the other object methods (object
instances), but also externally, as if I were to use ‘self.method’.
Below is an (admittedly, lame) example of what I’m referring to:

#############################

class CheesePhrase
def initialize(phrase)
@phrase = phrase
say_it
end

def say_it(phrase=@phrase)
puts “#{phrase}”
end
end

#############################

The above code works fine if I create an object instance, and (if I
choose to do so) call the method ‘say_it’, like so:

some_cheese_phrase = CheesePhrase.new(“I like cheese.”)
some_cheese_phrase.say_it

However, what if I’d also like to call the method directly, without
creating a new instance? For example, simply with:

CheesePhrase.say_it(“I like cheese, too.”)

I can’t do this without using ‘self.say_it’ in the method declaration.
Though, if I do that, I am no longer able to call the method in
‘initialize’ since ‘self.say_it’ is now a class method. I get the
following error:

NoMethodError: undefined method say_it' for #<CheesePhrase:0x284372f4 @phrase="I like cheese"> from (irb):4:ininitialize’
from (irb):11:in `new’

I’m sure I must be doing something wrong, or my concept of how to
implement this is totally wrong. The only other thought I’ve had is to
create duplicate methods, but this seems to violate the famed DRY
principle. Any advice would be much appreciated.

Thanks in advance!

-David

2

On Feb 4, 2009, at 1:37 PM, David S. wrote:

def initialize(phrase)

Thanks in advance!

-David

class CheesePhrase
def initialize(phrase)
@phrase = phrase
say_it
end

def self.say_it(phrase)
puts phrase
end
def say_it
self.class.say_it(@phrase)
end
end

irb> some_cheese_phrase = CheesePhrase.new(“I like cheese.”)
I like cheese.
=> #<CheesePhrase:0x629d0 @phrase=“I like cheese.”>
irb> some_cheese_phrase.say_it
I like cheese.
=> nil
irb> CheesePhrase.say_it(“I like cheese, too.”)
I like cheese, too.
=> nil

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

3

Rob B. wrote:

On Feb 4, 2009, at 1:37 PM, David S. wrote:

class CheesePhrase
def initialize(phrase)
@phrase = phrase
say_it
end

def self.say_it(phrase)
puts phrase
end
def say_it
self.class.say_it(@phrase)
end
end

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

Thanks a lot, Rob. Is this really the standard way of implementing
something like this? It just seems a little redundant, no?

Thanks!

-David

4

Hi David,
i don’t understand well what you need. Is this that you are looking for:
class CheesePhrase
def initialize(phrase)
@phrase = phrase
CheesePhrase.say_it(phrase)
end
def CheesePhrase.say_it(phrase=@phrase)
puts “#{phrase}”
end
end

a = CheesePhrase.new(‘I like cheese’)
a = CheesePhrase.say_it('I like too much cheese);

On Thu, 5 Feb 2009 03:37:30 +0900, David S.
[email protected]

5

On Feb 4, 2009, at 2:03 PM, David S. wrote:

puts phrase

Thanks a lot, Rob. Is this really the standard way of implementing
something like this? It just seems a little redundant, no?

Thanks!

-David

When you need it, yes. However, it isn’t typical that you’d do
something like this. Either the method belongs on the instance or it
belongs on the class itself. Occasionally, for convenience, you want
a class method to be called on an instance and then you’d likely do it
this way. Your example is simple (and, being an example, somewhat
contrived), but illustrates a point. You can call #say_it with no
parameter because the instance variable is there so an instance
knows what to say, but to call .say_it on the class, you need to
pass the parameter.

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

6

Put “self.” in front of the method name in it’s definition. This is
called a class method. If you need both, have both.

Blog: http://random8.zenunit.com/
Learn rails: http://sensei.zenunit.com/

7

Thanks for all the responses, guys.

Judging from everyone’s response, it sounds like what I’m trying to do
isn’t typically done. My mindset was, first, to simply create a standard
class. While creating one of the object methods for that class, I
realized that the same method code would be useful to call directly,
without having to create an object instance; so, as I said, I was
thinking there must be some way to do this, without repeating the same
(or a similar piece) of code somewhere else.

So, thanks again!

8

On Wed, Feb 4, 2009 at 3:06 PM, Rob B.
[email protected]wrote:

When you need it, yes. However, it isn’t typical that you’d do something
like this. Either the method belongs on the instance or it belongs on the
class itself. Occasionally, for convenience, you want a class method to be
called on an instance and then you’d likely do it this way. Your example is
simple (and, being an example, somewhat contrived), but illustrates a point.
You can call #say_it with no parameter because the instance variable is
there so an instance knows what to say, but to call .say_it on the class,
you need to pass the parameter.

Ruby does have a method Module#module_function which can be used in
cases
similar to this. For a rather silly example:

module Test
def foo
puts “Foo”
end

module_function :foo
end

Test.foo

begin
foo
rescue Exception => ex
puts “oops! #{ex}”
end

self.extend Test
foo

when run produces:

Foo
oops! undefined local variable or method `foo’ for main:Object
Foo

What module_function does is to copy one or more instance methods and
make
them singleton methods of the module. It only works for modules not
classes.

The canonical use case is in the Math module so that you can either
write:

Math.sin(.2)

or

class X
include Math

def compute(val)
2 + sin(val)
end
end

Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale

9

2009/2/5 David S. [email protected]:

Judging from everyone’s response, it sounds like what I’m trying to do
isn’t typically done.

Agreed.

My mindset was, first, to simply create a standard
class. While creating one of the object methods for that class, I
realized that the same method code would be useful to call directly,
without having to create an object instance; so, as I said, I was
thinking there must be some way to do this, without repeating the same
(or a similar piece) of code somewhere else.

In that case I would have the method definition only at class level
since apparently it is independent of instance state. I wouldn’t even
have it as instance method at all. That avoids confusion. If you
need to invoke the method from instance methods the
self.class.a_method() idiom works well IMHO.

Kind regards

robert

10

Rick Denatale wrote:

Ruby does have a method Module#module_function which can be used in
cases similar to this. For a rather silly example:

module Test
def foo
puts “Foo”
end

module_function :foo
end

To make a minor point, I see this use of module_function often, even in
the standard library. I don’t like it.

In addition to the module_function method, there’s the module_function
scoping state, like public or private or protected, which is likewise
enabled by calling module_function without arguments. Therefore we can
make a D.R.Y. equivalent:

module Test
module_function

def foo
puts “Foo”
end
end

11

David S. wrote:

Hi all,

I’m having trouble figuring out a way of creating a method within a
class that’s available not only to the other object methods (object
instances), but also externally, as if I were to use ‘self.method’.

you could do something like
module X
def shared_method
end
end
class Y
include X
extend X
end
?
-=r

12

On Fri, Feb 6, 2009 at 6:58 AM, Robert K.
[email protected] wrote:

without having to create an object instance; so, as I said, I was
thinking there must be some way to do this, without repeating the same
(or a similar piece) of code somewhere else.

In that case I would have the method definition only at class level
since apparently it is independent of instance state. I wouldn’t even
have it as instance method at all. That avoids confusion. If you
need to invoke the method from instance methods the
self.class.a_method() idiom works well IMHO.

I think that depends on why the method is useful at both places; one
thing I’ve seen occasionally is the use case where it is useful at the
class level because there are lots of times you’d like to create an
instance, call the method, and then be done with the instance, so it
makes sense to have something like:

class Foo
def initialize(init_arg1, …, init_argn)
# setup
end

def action(action_arg1, …, action_argn)
# do the action
end

def self.action(init_arg1, …, init_argn, action_arg1, …,
action_argn)
new(init_arg1, …, init_argn).action(action_arg1, …, action_argn)
end
end

If that’s the reason, using the CheesePhrase example:

class CheesePhrase

all the stuff in the original example

def self.say_it(phrase)
new(phrase).say_it
end
end

13 
 puts phrase

end
def say_it
self.class.say_it(@phrase)
end
end

Thanks a lot, Rob. Is this really the standard way of implementing
something like this? It just seems a little redundant, no?

Where do you see the redundancy? Nothing is repeated.

mfg, simon … l

14

On Thu, Feb 5, 2009 at 5:31 PM, David S. [email protected]
wrote:

Thanks for all the responses, guys.

Judging from everyone’s response, it sounds like what I’m trying to do
isn’t typically done. My mindset was, first, to simply create a standard
class. While creating one of the object methods for that class, I
realized that the same method code would be useful to call directly,
without having to create an object instance; so, as I said, I was
thinking there must be some way to do this, without repeating the same
(or a similar piece) of code somewhere else.

Probably s bit extreme for one method, but say you had 10 you wanted
to do this with:

module Foo
def self.included(base)
base.extend(self)
end

def method1; end
def method2; end
#…
end

class A
include Foo # no need for manual extend call
end

Not tested or necessarily recommended. :slight_smile:

-greg