I'm trying to figure out how to do the equivalent of Java instanceof in D. Is anybody familiar with how to do it? It should be possible.

sample:

java :

class Square {
	int x,y;

	public boolean equals (Object object) {
		if (object instanceof Square) {
			Square p = (Square)object;
			p.x = this.x;
			p.y = this.y;
			return true;
		}
		return false;
	}
}

> 
> class Square {
> 	int x,y;
> 
> 	public boolean equals (Object object) {
> 		if (object instanceof Square) {
> 			Square p = (Square)object;
> 			p.x = this.x;
> 			p.y = this.y;
> 			return true;
> 		}
> 		return false;
> 	}
> 	}

A little mistake here, but the question still is valid. Lines with p.x, p.y, and "return true" should be replaced with the following:

return (p.x == this.x) && (p.y == this.y);

John Reimer wrote:
>>class Square {
>>	int x,y;
>>
>>	public boolean equals (Object object) {
>>		if (object instanceof Square) {
>>			Square p = (Square)object;
>>			p.x = this.x;
>>			p.y = this.y;
>>			return true;
>>		}
>>		return false;
>>	}
>>	}
> 
> 
> A little mistake here, but the question still is valid. Lines with
> p.x, p.y, and "return true" should be replaced with the following: 
> 
> return (p.x == this.x) && (p.y == this.y);


Cast to the type and check for null:

Square p = cast(Square)object;
if(p) { is instance } else { is not }

> 
> Cast to the type and check for null:
> 
> Square p = cast(Square)object;
> if(p) { is instance } else { is not }

Hmmm... that's it?

Much appreciated! :)

If cast() fails, it will return null?  Or do you need a try/catch?

Vathix wrote:

> John Reimer wrote:
> 
>>> class Square {
>>>     int x,y;
>>>
>>>     public boolean equals (Object object) {
>>>         if (object instanceof Square) {
>>>             Square p = (Square)object;
>>>             p.x = this.x;
>>>             p.y = this.y;
>>>             return true;
>>>         }
>>>         return false;
>>>     }
>>>     }
>>
>>
>>
>> A little mistake here, but the question still is valid. Lines with
>> p.x, p.y, and "return true" should be replaced with the following:
>> return (p.x == this.x) && (p.y == this.y);
> 
> 
> 
> Cast to the type and check for null:
> 
> Square p = cast(Square)object;
> if(p) { is instance } else { is not }

Brad Anderson wrote:

> If cast() fails, it will return null?  Or do you need a try/catch?
> 


Just returns null.

Vathix wrote:

> Brad Anderson wrote:
>
>> If cast() fails, it will return null?  Or do you need a try/catch?
>>
>
> Just returns null.

BTW: In C++ you'd have to use dynamic_cast<Square>(object)

-- 
-Anderson: http://badmama.com.au/~anderson/

> 
> BTW: In C++ you'd have to use dynamic_cast<Square>(object)
> 

That's an interesting point. D way seems so much simpler. :)

> >
> > BTW: In C++ you'd have to use dynamic_cast<Square>(object)
> >
>
> That's an interesting point. D way seems so much simpler. :)

How do you work that? It's the same number of expressions/line. And the C++ is *far* clearer in highlighting that something nasty is happening by the intentionally ugly dynamic_cast<>

Brad Anderson says

> If cast() fails, it will return null?  Or do you need a try/catch?

Maybe a construct can be made that supports throwing an exception. This would be in the form:

try {
<instance2> = safeCast(<class2>)<instance1>;
} catch (ClassCastException e) {
..
}

Though it could also be done using a template I think.

Regards,
Sjoerd van Leent