Get the value of an instance variable given its name

metaprogramming, ruby

Solution

The most idiomatic way to achieve this is:

some_object.instance_variable_get("@#{name}")

There is no need to use `+` or `intern`; Ruby will handle this just fine. However, if you find yourself reaching into another object and pulling out its ivar, there's a reasonably good chance that you have broken encapsulation.

If you explicitly want to access an ivar, the right thing to do is to make it an accessor. Consider the following:

class Computer
  def new(cpus)
    @cpus = cpus
  end
end

In this case, if you did `Computer.new`, you would be forced to use `instance_variable_get` to get at `@cpus`. But if you're doing this, you probably mean for `@cpus` to be public. What you should do is:

class Computer
  attr_reader :cpus
end

Now you can do `Computer.new(4).cpus`.

Note that you can reopen any existing class and make a private ivar into a reader. Since an accessor is just a method, you can do `Computer.new(4).send(var_that_evaluates_to_cpus)`

Problem

In general, how can I get a reference to an object whose name I have in a string? More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly). Each parameter is an object that also has an `from_s` method. I want to do something like the following (which of course doesn't work...): ``` define_method(:from_s) do | arg | @ordered_parameter_names.each do | param | instance_eval "field_ref = @#{param}" field_ref.from_s(param) end end ```

Original source