How do I convert a string into a reference to a variable in Ruby?

问题

I need to be able to pass the name of a variable into an expression (in cucumber) and would like to be able to convert that string into a reference (i.e. not a copy) of the variable.

e.g.

Given /^I have set an initial value to @my_var$/ do
  @my_var = 10
end

# and now I want to change the value of that variable in a different step
Then /^I update "([^"]*)"$/ do |var_name_string|
  # I know I can get the value of @my_var by doing:
  eval "@my_var_copy = @#{var_name_string}"

  # But then once I update @my_var_copy I have to finish by updating the original
  eval "@#{var_name_string} = @my_var_copy"

  # How do instead I create a reference to the @my_var object?
end

Since Ruby is such a reflective language I'm sure what I'm trying to do is possible but I haven't yet cracked it.

回答1:

  class Reference
    def initialize(var_name, vars)
      @getter = eval "lambda { #{var_name} }", vars
      @setter = eval "lambda { |v| #{var_name} = v }", vars
    end
    def value
      @getter.call
    end
    def value=(new_value)
      @setter.call(new_value)
    end
  end

Got this from http://onestepback.org/index.cgi/Tech/Ruby/RubyBindings.rdoc. Good luck!

回答2:

A solution might be to wrap things into an array. Which can easily be passed around by reference.

irb(main):001:0> my_var = [10]
=> [10]
irb(main):002:0> my_var_copy = my_var
=> [10]
irb(main):003:0> my_var[0] = 55
=> 55
irb(main):004:0> my_var_copy
=> [55]

See here - http://www.rubyfleebie.com/understanding-fixnums/

And (slightly off topic, but gave me the initial idea for a solution) here - http://ruby.about.com/od/advancedruby/a/deepcopy.htm

回答3:

Could obj.instance_variable_get and obj.instance_variable_set help you?

来源：https://stackoverflow.com/questions/5210411/how-do-i-convert-a-string-into-a-reference-to-a-variable-in-ruby

标签

ruby

cucumber