# Dumb reflection problem - create object from String **URL:** \ **Category:** Ruby\ **Created:** [April 9, 2006, 8:14pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325 "2006-04-09T20:14:03Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 8:14pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/1 "2006-04-09T20:14:03Z")
Hello - I’m bashing my head against the wall - I’m sure this is a simple question, but damned if I can figure it out. If I have a String which corresponds to a class name, how do I create an instance of this class? Example: class MyExampleClass … end s = ‘MyExampleClass’ # We want to instantiate a new instance of the class myExampleClassInstance = ??? I want the behaviour of ‘MyExampleClass.new’ but only using the String value of s. I’m sure it’s a one-liner. Any help appreciated. Thanks. parki… ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 8:26pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/2 "2006-04-09T20:26:42Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Brian P. wrote: > end > > s = ‘MyExampleClass’ > > # We want to instantiate a new instance of the class > > myExampleClassInstance = ??? > > I want the behaviour of ‘MyExampleClass.new’ but only using the String > value of s. Have a look at Module.const\_get Module.const\_get(s).new HTH, Sven -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (FreeBSD) Comment: Using GnuPG with Mozilla - [http://enigmail.mozdev.org](http://enigmail.mozdev.org) iD8DBQFEOVItevlgTHEIT4YRAl3wAJ4j4zJTlcMR9A8ZJKixHXvig0eGywCfUPGN W26rak8DMs/szMC+niiVvds= =8+ZL -----END PGP SIGNATURE----- ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 8:29pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/3 "2006-04-09T20:29:43Z")
On 4/10/06, Brian P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > end $ irb irb(main):001:0\> class K irb(main):002:1\> end =\> nil irb(main):003:0\> k1 = K.new =\> #\ irb(main):004:0\> k2 = eval(‘K’).new =\> #\ irb(main):005:0\> k3 = Object.const\_get(‘K’).new =\> #\ ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 8:30pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/4 "2006-04-09T20:30:17Z")
Sven K. wrote: [deletia] > Module.const\_get(s).new Thanks! I knew it would be a one-liner. 🙂 parki… ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 9:06pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/5 "2006-04-09T21:06:31Z")
DÅ?a Nedeľa 9. Apríl 2006 20:26 Gyoung-Yoon N. napísal: > > ``` > > ... > > > > ``` > > > > I’m sure it’s a one-liner. > > – > [http://nohmad.sub-port.net](http://nohmad.sub-port.net) I would, naively hoping to see YARV released Sometime Soon ™, rather use , since it probably is / will be easier for YARV to work with , as far as optimizations are concerned. It also makes the intent of the code clearer. David V. ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 9:25pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/6 "2006-04-09T21:25:02Z")
On 4/9/06, Brian P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > parki… I am afraid that the only one-liner is eval ☹ suppose s contains the name of your class Module.const\_get s does not work unless your class is defined on top level. In oder to have the _same_ semantics as eval(s).new we have to write m = Module s.split(%r{::|.}).each do |name| m = m.const\_get(name) end m.new I suggest to look at ObjectSpace for another alternative, hardly more efficent though ☹ Robert – Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui concerne l’univers, je n’en ai pas acquis la certitude absolue. - Albert Einstein ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 9:43pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/7 "2006-04-09T21:43:41Z")
On Apr 9, 2006, at 3:21 PM, Robert D. wrote: > > I knew it would be a one-liner. 🙂 > > - Albert Einstein > s.split(/::/).inject(Object) { |p, c| p.const\_get© }.new There, a one liner (w/o any semi-colons) ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 9:45pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/8 "2006-04-09T21:45:49Z")
Robert D. wrote: > On 4/9/06, Brian P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > parki… > > I am afraid that the only one-liner is eval ☹ > > suppose s contains the name of your class > Module.const\_get s does not work unless your class is defined on top > level. > In oder to have the _same_ semantics as > > eval(s).new > > we have to write > > m = Module > s.split(%r{::|.}).each do > |name| > m = m.const\_get(name) > end > m.new s.split(‘::’).inject(Object) { |s,k| s.const\_get(k) }.new – Jim W. ---
**Author:** ![parki](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@parki](https://www.ruby-forum.com/u/parki)\ **Post date:** [April 9, 2006, 10:08pm UTC](https://www.ruby-forum.com/t/dumb-reflection-problem-create-object-from-string/57325/9 "2006-04-09T22:08:17Z")
On 4/9/06, Logan C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > > Module.const\_get(s).new > > > > suppose s contains the name of your class > > > > |name| > > > > – > > > > Yup, very nice > > > > What I meant though is that eval seems much simpler and cheaper > > > > Let us see > > > > yet your’s is fine, just look at the mess when searching through > > > > ObjectSpace cat perf.rb; ruby perf.rb require ‘benchmark’ module One class Two class Three end end end s=“One:2️⃣:Three” n = 50000 Benchmark.bm do |x| x.report("eval "){ n.times{ eval(s).new } } x.report("split ") { n.times{ s.split(/::/).inject(Object) { |p, c| p.const\_get(c) }.new } } x.report(“ObjectSpace”){n.times{x=nil;ObjectSpace.each\_object(Class){|cls| x=cls if cls.to\_s == s};x.new } } end user system total real eval 0.280000 0.000000 0.280000 ( 0.372346) split 0.540000 0.000000 0.540000 ( 0.621870) ObjectSpace 21.920000 0.070000 21.990000 ( 24.452318) – Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui concerne l’univers, je n’en ai pas acquis la certitude absolue. - Albert Einstein