From: Brian Candler <b.candler@...>
Date: 2010-08-17T21:35:04+09:00
Subject: Re: Refactoring Global Constants

Frank Guerino wrote:
> For example, I have a variable that shows up in three different files
> and looks like:
> 
>      maxColumns = 30
> 
> I'd like to know what the most elegant way is to define it in one place
> and share it, across files.

The best way to define a constant is using a constant :-)

MAX_COLUMNS = 30

Put this in a source file (e.g. limits.rb) then require 'limits' where 
you need it.

For any code which might be re-used or shared, you'd be better keeping 
your constants in their own namespace:

module MyStuff
  MAX_COLUMNS = 30
end

This can be referenced as MyStuff::MAX_COLUMNS, but usually your code 
would also be inside the same module, so it can just use MAX_COLUMNS.

If your code consists mainly of one class, then just put the constants 
inside that class (remember that a Class is also a Module)

class Foo
  MAX_COLUMNS = 30
end

...

require 'foo/constants'
class Foo
  def initialize(n)
    raise ArgumentError, "Too many columns" if n > MAX_COLUMNS
    @cols = n
  end
end

If this value is not actually a constant, but might change at runtime 
(e.g. from a command-line option), then you could use a 
$global_variable, but I'd suggest a class instance variable.

class Foo
  @max_columns = 30  # default value
  def self.max_columns
    @max_columns
  end
  def self.max_columns=(n)
    @max_columns = n
  end
end

...

Foo.max_columns = Integer(ARGV[0]) if ARGV[0]
puts Foo.max_columns

Note that class Foo is itself an object (of class Class), and so 
@max_columns is an instance variable of the class itself.
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