public class newIP_Extension{
    
     private final IP__c ip;

     public newIP_Extension(ApexPages.StandardController stdController){
          this.ip = (IP__c)stdController.getRecord();
     }

     <SNIP - many methods>

     public static testMethod void test_newIP_Extension(){
          Test.setCurrentPageReference(new PageReference('Page.newIP'));

          newIP_Extension controller = new newIP_Extension('IP__c'); 
          String nextPage = controller.continueToNext().getUrl();

          <SNIP various tests>
     }

}

This is the error I get "Error: Compile Error: Constructor not defined: [newIP_Extension].<Constructor>(String) at line 148 column 38 ."

If I don't pass 'IP__c' (the name of the standard controller) into the constructor I get "Error: Compile Error: Constructor not defined: [newIP_Extension].<Constructor>() at line 148 column 38 ."

If I don't quote IP__c I get "Error: Compile Error: Variable does not exist: IP__c line 148 column 58. "

The only difference between my code and the sample code is that the sample code is for a custom controller (rather than extension) and thus the controller doesn't have any parameters. What am I missing here?

Thanks for any insights!