Get which derived class an object is if it's stored in an array of its base class

[Morimur55 via Digitalmars-d-learn]

On Saturday, 15 July 2017 at 14:04:17 UTC, Adam D. Ruppe wrote:
> On Saturday, 15 July 2017 at 13:45:40 UTC, Morimur55 wrote:
>> Well I want to cast to the derived type so I can use a method 
>> that's defined in the base class, but is overridden in several 
>> of the derived types... and calling it without a cast seems to 
>> give me the base type functionality, but I'd like the derived 
>> type functionality when it's defined.
>
> That means you're doing something wrong. What does your base 
> class method look like? Is it a template?

class Base {
     private static int idcounter = 0;
     int nextid(){ return ++idcounter }
}

class Derrived {