Overriding opEquals in classes, for comparison with things that aren't Objects

[Mike Parker via Digitalmars-d-learn]

On Friday, 29 January 2016 at 15:00:59 UTC, pineapple wrote:
> With this bit of code, the first method seems to work fine - 
> things go as expected. But I get a compile error with the 
> second method, and I'm not sure how else to write this.
>
>     override bool opEquals(Object value) const{
>         return this.equals(cast(typeof(this)) value);
>     }
>     override bool opEquals(T)(T value) const if(isNumeric!(T)){
>         return this.equals(value);
>     }
>
> The compile errors I get look like this:
>
> C:\path\to\file.d(477): Error: function 
> units.quantity.opEquals!int.opEquals cannot override a 
> non-virtual function
> C:\path\to\file.d(519): Error: template instance 
> units.quantity.opEquals!int error instantiating
>
> How can I revise this so that I can override comparison 
> operators for things other than class instances?

The first implementation is fine because you're overriding the 
implementation in the base class (Object). However, the second 
one fails because it's a template. Templates are non-virtual and 
cannot override anything. Even if you could, there is no such 
implementation in Object and, therefore, nothing to override.

Templated functions can be used as overloads, though, but I'm not 
sure off the top of my head if the compiler accepts templated 
opEquals on classes at all. My guess is no, but you can find out 
by removing the override from the template declaration. If not, 
you should be able to implement non-templated overloads for the 
types you're interested in (without the override keyword, mind 
you, since they won't be overriding anything).