From: Brian Candler <lists@...>
Date: 2012-11-11T18:31:25+09:00
Subject: Re: Instance name of a class/object?

Carolyn Grant wrote in post #1083906:
> basic_config = MySettings.new # Picks up & processes basic_config.yml
> adv_config = MySettings.new   # Picks up & processes adv_config.yml

In general it doesn't make any sense for an object to know its "name", 
because an object may have zero or many names at any time. You can 
assign the object reference to local variables, to global variables, to 
instance variables of other objects, to constants, as elements of an 
Array or Hash, etc.

In the above code, the object created by MySettings.new has no name at 
the time it is created. Only after the #new function has returned the 
object reference, it is assigned to the local variable.

So for the code you are writing above, I would strongly advise passing 
the filename (or filename stem) as an argument:

basic_config = MySettings.new("basic_config.yaml")
adv_config = MySettings.new("adv_config.yaml")

Of course, as with many things in Ruby, there is an exception. The 
exception is to do with class objects; that is, objects of class Class. 
Normally classes are held in constants, and the first time a class is 
assigned to a constant, it learns its name, and the name becomes an 
attribute of the class itself.

A class definition like "class Foo" both creates the class and assigns 
it to a constant, but you can see the behaviour if you first create an 
anonymous class, and then later assign it to a constant.

>> foo = Class.new
=> #<Class:0x106c91650>
>> my_instance = foo.new
=> #<#<Class:0x106c91650>:0x106c8e6f8>
>> Bar = foo
=> Bar
>> foo
=> Bar
>> my_instance
=> #<Bar:0x106c8e6f8>
>> Baz = foo
=> Bar
>> foo
=> Bar
>> my_instance
=> #<Bar:0x106c8e6f8>

> I tried my_class.name; that returns a "Method not defined" error" (I'm
> using 1.9.3p194)

You'll have to show the code. I use ruby 1.8, but it works fine for me. 
Notice that "name" is a method of the class, not the instance, and is 
only for Class objects because of the special-case behaviour described 
above.

>> foo.name
=> "Bar"
>> my_instance.class.name
=> "Bar"
>> my_instance.name
NoMethodError: undefined method `name' for #<Bar:0x106c8e6f8>
        from (irb):11

> What I'd really like to avoid is:
>
> basic_config = MySettings.new(basic_config).

Why? It seems you want to create "magic" which will be very difficult 
for another programmer to follow. When you create an object, you should 
pass all the data which will be used to create state in that object, and 
you should allow the caller to decide what to do with the returned 
object.

The closest I could recommend is if you have a single config object 
which holds all the sub-configs e.g. a hash. Then a single method 
argument can say which file to load *and* which hash member to put it 
in. For example:

require 'yaml'
class Configs
  def initialize
    @configs = {}
  end
  def load(stem)
    @configs[stem] = File.open("#{stem}.yaml") do |f|
      YAML.load(f)
    end
  end
  def [](stem)
    @configs[stem]
  end
end

config = Configs.new
config.load "basic_config"
config.load "adv_config"
puts config["basic_config"].inspect

Of course, if you want the config to be globally accessible throughout 
your application, then you can assign it to a constant instead of a 
local variable.

Config = Configs.new
Config.load "basic_config"
Config.load "adv_config"
puts Config["basic_config"].inspect

Or you could use a global variable, but a constant is generally 
preferred. Constants can be put into namespaces, e.g. MyApp::Config, to 
avoid clashes with other uses of Config.

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