Why does this opCmp function not work

[Simen Kjærås]

On Wednesday, 23 September 2020 at 11:19:15 UTC, Ruby The 
Roobster wrote:
> bool opEquals(shape rhs)
> {
> return(rhs.volume == this.volume);
> }

This is a wrong implementation of opEquals for a class. The 
documentation mentions 
(https://dlang.org/spec/operatoroverloading.html#equals):

> 4. If overridding Object.opEquals() for classes, the class 
> member function signature should look like:
> 
> class C
> {
>     override bool opEquals(Object o) { ... }
> }

A correct implementation would be:

override bool opEquals(Object o) {
     if (!cast(Shape)o) return false;
     return (cast(Shape)o).volume == this.volume;
}

--
   Simen