Is there a way to make class and object constants in Ruby?

ruby-talk
1

I am reading up in other areas, and it occurs to me much of the items I have as class and object variables really should be constants, but ideally still in the class or object context. Is there a way to do this?

xc

···

--
"It's the preponderance, stupid!" - Professor Stephen Schneider, IPCC member

2

Variables that begin with capital letters are constants

initial_constants = Module.constants
class MyClass
  MY_FAVOURITE_NUMBER = 12
  MyFavouriteNumber = 12
end
Module.constants - initial_constants # => [:MyClass]
MyClass.constants # => [:MY_FAVOURITE_NUMBER, :MyFavouriteNumber]

So you can see that classes are already constants, and an example of how to
create constants within a class.

In Ruby, this really means that the reference may not be changed (or at
least you will be warned if you try to do so), not that the object being
referenced may not be mutated.

class MyClass
  MyFavouriteNumber = 12
  MyFavouriteNumber = 13 # !> already initialized constant MyFavouriteNumber
  MyFavouriteNumber # => 13
end

If you change your constant, you will know, it will warn you (ie, it won't
happen by accident). If your code breaks because of this... well... you'll
have a pretty good idea where to look. While Ruby is dynamic enough to make
such a change, it is not something you should do. If you need to change your
constants, then they shouldn't be constants.

If you really need your data to not change, to the point where you don't
even want to allow yourself the opportunity to do so, you can freeze it, and
it will throw an exception

class MyClass
  MyFavouriteWord = "ambivalent"
  MyFavouriteWord << 'l'
  MyFavouriteWord # => "ambivalentl"
  MyFavouriteWord.freeze
  MyFavouriteWord << 'y'
  MyFavouriteWord # =>
end
# ~> -:6:in `<class:MyClass>': can't modify frozen string (RuntimeError)
# ~> from -:1:in `<main>'

But as far as I am aware, there is no way to recursively free any references
it has (probably a good thing, you could recursively freeze your entire
program)

class MyClass

  SomeArray = [ 'abc' , 'def' ].freeze
  SomeArray # => ["abc", "def"]

  begin
    SomeArray << 'ghi'
  rescue => e
    e # => #<RuntimeError: can't modify frozen array>
  end

  SomeArray.last # => "def"
  SomeArray.last << 'ghi'
  SomeArray.last # => "defghi"

end

Generally, constants are used for data that should be hard coded in, you use
a constant so that you can easily change it later, like using a macro in C.
You can then drop it into your code, and it is replaced with the actual
value (you can redefine macros too). And they are also used for keeping
important references, like the constant STDIN. We can change the global we
use as stdin, but we don't want to lose track the actual stdin, so we keep
it in the constant.

STDIN # => #<IO:<STDIN>>
$stdin # => #<IO:<STDIN>>

$stdin = DATA # => #<File:untitled>
gets # => "this is the data\n"

$stdin = STDIN # => #<IO:<STDIN>>

__END__
this is the data

···

On Wed, Apr 14, 2010 at 8:00 PM, Xeno Campanoli <xeno.campanoli@gmail.com>wrote:

I am reading up in other areas, and it occurs to me much of the items I
have as class and object variables really should be constants, but ideally
still in the class or object context. Is there a way to do this?

xc
--
"It's the preponderance, stupid!" - Professor Stephen Schneider, IPCC
member

3

In class context you could do:

class MyClass

  MYCONSTANT=6

  def initialize
    puts MYCONSTANT
  end

end

But I wonder, whether it makes sense at all to have constants at object
level. This sounds weird to me.

Thomas

···

Am Wed, 14 Apr 2010 20:00:54 -0500 schrieb Xeno Campanoli:

I am reading up in other areas, and it occurs to me much of the items I
have as class and object variables really should be constants, but
ideally still in the class or object context. Is there a way to do
this?

xc

4

I do for things that don't (or shouldn't) change. For example, I'm working
on a Rails project where the admin can assign a level of trust to a user
such that they can post images directly to the main page from Twitter. The
database doesn't inherently support enumerated types, so the attribute is an
integer, then I have class level constants that map those values to their
meanings, and a set of methods that allow you to get booleans back when
asking if that user is able to do that authorized thing, and they are
embedded into named scopes so that I can pull all users who have this
authorization level, or that authorization level, and so forth. So I just
store a single integer in the database, but my application interprets it the
same across every instance.

···

On Wed, Apr 14, 2010 at 8:40 PM, Thomas Volkmar Worm <tvw@s4r.de> wrote:

Am Wed, 14 Apr 2010 20:00:54 -0500 schrieb Xeno Campanoli:

> I am reading up in other areas, and it occurs to me much of the items I
> have as class and object variables really should be constants, but
> ideally still in the class or object context. Is there a way to do
> this?
>
> xc

In class context you could do:

class MyClass

MYCONSTANT=6

def initialize
   puts MYCONSTANT
end

end

But I wonder, whether it makes sense at all to have constants at object
level. This sounds weird to me.

Thomas

5

Thank you Josh and Thomas. The source of this question is my study of Erlang, where all 'Variables' do not vary after the initial assignment. I realized when reading up on the language that there are many areas in my classes where the thing I use, especially with Object Variables, are effectively constants, so I would do best to have them as actual constants. It turns out there is a lot of value in knowing that something cannot change, even if it can change but will have at least an obvious error. I don't expect to Erlangize my programs, but I have already identified things I can do to make my stuff safer and clearer just by making class and object variables into constants. Too bad there is not a special prefix for these as there is for the variables. That would be nice for referencing things. Still, no big deal.

xc

Josh Cheek wrote:

···

On Wed, Apr 14, 2010 at 8:40 PM, Thomas Volkmar Worm <tvw@s4r.de> wrote:

Am Wed, 14 Apr 2010 20:00:54 -0500 schrieb Xeno Campanoli:

I am reading up in other areas, and it occurs to me much of the items I
have as class and object variables really should be constants, but
ideally still in the class or object context. Is there a way to do
this?

xc

In class context you could do:

class MyClass

MYCONSTANT=6

def initialize
   puts MYCONSTANT
end

end

But I wonder, whether it makes sense at all to have constants at object
level. This sounds weird to me.

Thomas

I do for things that don't (or shouldn't) change. For example, I'm working
on a Rails project where the admin can assign a level of trust to a user
such that they can post images directly to the main page from Twitter. The
database doesn't inherently support enumerated types, so the attribute is an
integer, then I have class level constants that map those values to their
meanings, and a set of methods that allow you to get booleans back when
asking if that user is able to do that authorized thing, and they are
embedded into named scopes so that I can pull all users who have this
authorization level, or that authorization level, and so forth. So I just
store a single integer in the database, but my application interprets it the
same across every instance.

--
"It's the preponderance, stupid!" - Professor Stephen Schneider, IPCC member

6

To make objects immutable you can 'freeze' them. After being frozen
any attempt
to change will result in an error:

x = 'test'

=>
"test"

x.freeze

=>
"test"

x[0] = 'w'

RuntimeError: can't modify frozen string