Collect variables over multiple classes

I would like to collect a variable over different classes in Puppet. Is this possible?

For example, I want to assign two classes to a node:

class { "webapplication1": url => 'webapp1.example.com' }
class { "webapplication2": url => 'webapp2.example.com' }

Then I would like to include a different class, webserver which sets up two virtual hosts for each of these two classes. Therefore it should get the two URL variables from both classes. I do not want to pass the URLs as parameters to the new class. For further fun, maybe sometimes I only want to add one class, or some other time I want to add a completely different class (all only with the similarity that they have a url parameter.

How can I collect these? I thought about adding a tag to each class and then collecting them via a resource collector (ok, does not work with classes). But how do I access their parameters?

So then I rather create a defined resource for webapplications, e.g.

webapplication { "webapplication1": url => 'webapp1.example.com' }
webapplication { "webapplication2": url => 'webapp2.example.com' }

But how would I then collect all urls from all webapplications?

Webapplication <| |>

This does not give me any functionality, it seems. Should I put a class around the webapplications which will then create resources?

class webapps (
  Hash $webapps,
) {
  $webapps.each | String $appname, String $parameters | {
    webapp { $appname: 
      * => $parameters,
    }

    apache::vhost { $parameters["url"]:
      #...
    }
  }
}

class { webapps: 
    webapps => {
      'webapplication1' = {
        'url' => 'webapp1.example.com'
      },
      'webapplication2' = {
        'url' => 'webapp2.example.com'
      },
    }
}

(not tested and might look simpler in YAML...). And tips how to start would be greatly appreciated. Just don't know how to nicely do this.

Thanks!