get instance variable name from itself in ruby

Question

I have an instance variable @foo and I want to write some code so that I get string 'foo'

any hint?

Answer 1

If all you have is a reference to the object, you can't really do it cleanly.

def foo
  bar @something
end

def bar(value)
  value # no clean way to know this is @something
end

---

The only hack I can think of is to loop through ALL instance variables on self, looking for matches. But its a very messy approach that's likely to be slow.

def bar(value)
  instance_variables.each do |ivar_name|
    if instance_variable_get(ivar_name) == value
      return ivar_name.to_s.sub(/^@/, '') # change '@something' to 'something'
    end
  end

  # return nil if no match was found
  nil 
end

@something = 'abc123'
bar @something # returns 'something'

# But passing the same value, will return a value it's equal to as well
bar 'abc123' # returns 'something'

This works because instance_variables returns an array of symbols that are the names of instance variables.

instance_variables
#=> [:@something, :@whatever] 

And instance_variable_get allows you to fetch the value by it's name.

instance_variable_get :@something # note the @
#=> 'abc123'

Combine the two methods and you can get close to what you want.

---

Just use it wisely. Before using a solution based on this, see if you can refactor things a way so that it's not necessary. Meta-programming is like a martial art. You should know how it works, but have the discipline to avoid using it whenever possible.

Answer 2

In Ruby, you can only manipulate objects. Variables (including instance variables) aren't objects.

Besides, what do you want your magic method to return in this case:

foo = Object.new
bar = foo
@baz = bar
@qux = bar

magic_method(foo) # what should the return value be and how would it know?

Answer 3

You can call the method instance_variables to get the name of all instance variables of an object. Caution though that instance variables are only included in that list after they have been initialized.

>> class A; attr_accessor :foo; end
=> nil
>> a = A.new
=> #<A:0x103b310b0>
>> a.instance_variables
=> []
>> a.foo = 42
=> 42
>> a.instance_variables
=> ["@foo"]