Get Instance Name

Question

Is there a way to get the name of the instance we created of a specific class in a class method?

This is what i'm trying to do:

module MyObjectStore
  values = {}
  temp=  {}
  define_method :add_check_attribute do |method,*args|
    if method.to_s =~ /=$/
      temp[method[0..-2]] = args[0]
    else
      instance_variable_get("@#{method}")
    end
  end
  define_method :method_missing do |method,*args|
    add_check_attribute(method,*args)
  end
  define_method :save do
    temp.each {|key,value| instance_variable_set("@#{key}",value)}
    values[self] = temp
    end
end

class C
  include MyObjectStore
end

a = C.new
a.id = 1
a.name = 'gaurav'
a.save

On the third-to-last line of the module, I'm trying to store the values of the object into the new hash having the instance name as the key.

Is anything like that possible?

I'm using self now, but it gives the whole object, not the instance name.

score 6 · Accepted Answer · answered Dec 20 '11 at 14:59

Ruby objects have no name besides a memory address. Each object, though, has an object_id which is always unique. Using ObjectSpace._id2ref, instances and IDs are interchangeable. Here's an example:

id="Hello, World".object_id    => 82609910
ObjectSpace._id2ref(id)        => "Hello, World"
"Hello, Ruby".object_id        => 82721540

It is worth mentioning that the object id, although constant for an object across its lifetime, it is almost never constant between runs of a program, and different objects that have the exact same data will have different IDs:

"Hi".object_id                 => 82719050
"Hi".object_id                 => 82715090

Object IDs are also run dependent and OS dependent, etc. Never hard code object ids into your source code, only use ids that you got through object_id.

score 2 · Answer 2 · edited Nov 08 '12 at 23:57

2

What should the instance name be? Unless you are talking about a specific attribute (like name), an instance doesn't really have a name. It can be referenced by variables that have a name, but the two are distinct.

So:

An object doesn't have a "name", except for some memory handle or similar mechanism used by the interpreter.
Variables have names, but can refer to different instances during program execution.
Different variables may refer to the same instance.

Given the last point, how should an instance decide on a "name"?

One way to solve this problem would be to use either the memory address or some kind of hashing function as the object name. A hashing function is a little tricky, since it will change as the object changes.

You can also decide to have an id or name field on all your instances and use that. Keep it unique. Have fun guaranteeing that. Go insane. Come back enlightened. Teach us.

edited Nov 08 '12 at 23:57

the Tin Man

158,662
42
215
303

answered Dec 20 '11 at 10:35

Daren Thomas

67,947
40
154
200

Like in the above code, i have **a** as an instance of class C. I want to use this value "a" as the key for the hash. – Gaurav Manchanda Dec 20 '11 at 10:37
@Gaurav Manchanda `a` is not the instance it just _references_ the instance of class `C`. – Howard Dec 20 '11 at 10:39
@GauravManchanda, are you trying to do hsh.merge({ a.name => a }), then you'll have a pointer to the object a by the name attribute of that object! – Syed Aslam Dec 20 '11 at 10:44
@Syed Aslam Exactly. Can i do that?? I want to use this new hash to search object attributes like name and id, and if found, it'll give me the instance name "a" – Gaurav Manchanda Dec 20 '11 at 11:27

Get Instance Name

2 Answers2

Linked