Re: How do you return a subclass instance from a base class method?

[Steven Schveighoffer via Digitalmars-d-learn]

On 11/16/22 11:25 PM, Daniel Donnelly wrote:

I have SubclassOf derived from PosetRelation.  For any poset relation, the transitivity law applies, however, I'd like to return the correct type:

```
    PosetRelation transitivity(PosetRelation R, PosetRelation S)
    {
       if (R.op == S.op)
       {
          if (R.right is S.left)
             return new SubclassOf(R.left, S.right);
       }

       return null;
    }

```

How does one accomplish this in D?  Because PosetRelation doesn't know about SubclassOf, in general.

I'd use a template:

```d
T transitivity(T : PosetRelation)(T R, T S)
{
      if (R.op == S.op)
      {
         if (R.right is S.left)
            return new T(R.left, S.right);
      }

      return null;
}
```

-Steve