From: Eric Hodel <drbrain@...7.net>
Date: 2004-12-04T03:55:59+09:00
Subject: Re: how to create an object of a class you don't know yet

On 03 Dec 2004, at 10:44, Bradley, Todd wrote:

> Hi, I'm just now getting into Ruby's OO-ness, and could use some 
> advice.
> I'm trying to create an object, but which specific class needs to be
> determined at runtime.  I figured out how to do this by creating a
> string and executing it using the "eval" command, but I know there must
> be an easier way.  My first guess was to do something like 
> #{answer}.new
> but that didn't work.
>
>
> Here's my code:
>
>
> class Foo
> 	def method1
> 	end
> end
>
> class Bar
> 	def method2
> 	end
> end
>
> # Pretend this was determined at runtime
> answer = "Foo"
>
>
> # There must be a better way of doing this:
>
> myobj = Object.new # Needs to exist in this scope
> mystring = "myobj = #{answer}.new"
>
> eval mystring
>
> puts "I just created a #{myobj.class} object."

klass = answer.split('::').inject(Object) { |klass,const| 
klass.const_get const }

myobj = klass.new

In longer terms:

answer.split('::') # for Foo::Bar::Baz nested classes/modules

answer.split('::').inject(Object) do |klass, const| # namespaces start 
from Object
   klass.const_get const # #inject passes the value of this expression in
						# as the first arg to the block, so use that namespace
						# to find the next part of the namespace
end

klass = answer.split [...] # #inject returns the last result, which will
						   # be a class, provided answer references a class

myobj = klass.new # instantiate an instance of the class

You can also do things like this:

KLASSES = { 'html' => HTMLWriter, 'pdf' => PDFWriter, 'plain-text' => 
TextWriter }

output = ARGV.shift

raise "invalid output type" unless KLASSES.include? output

writer = KLASSES[output].new