public class MyClass
{
	private int value;

	this(int val)
	{
		value = val;
	}

	MyClass opBinary(string op: "+")(MyClass rhs)
	{
		if (rhs is null)
		{
			return null;
		}

		MyClass result = new MyClass(value + rhs.value);
		return result;
	}
}

void main()
{
	MyClass v1 = new MyClass(4);
	MyClass v2 = new MyClass(5);

	auto v3 = v1 + null; // v3 is null
	auto v4 = null + v2; // v4 equals v2
}

in this example v3 is null. i thought v4 would be null too, but i was wrong.
what should i do to make my binary operation commutative?

On Sunday, 21 October 2012 at 20:34:51 UTC, ref2401 wrote:
> what should i do to make my binary operation commutative?

You can overload opBinaryRight:

MyClass opBinaryRight(string op: "+")(MyClass rhs)
{
   if (rhs is null)
   {
     return null;
   }
   MyClass result = new MyClass(value + rhs.value);
   return result;
}

http://dlang.org/operatoroverloading.html

On Sunday, 21 October 2012 at 20:42:08 UTC, cal wrote:
> On Sunday, 21 October 2012 at 20:34:51 UTC, ref2401 wrote:
>> what should i do to make my binary operation commutative?
>
> You can overload opBinaryRight:
>
> MyClass opBinaryRight(string op: "+")(MyClass rhs)
> {
>    if (rhs is null)
>    {
>      return null;
>    }
>    MyClass result = new MyClass(value + rhs.value);
>    return result;
> }
>
> http://dlang.org/operatoroverloading.html

in the following case

auto v5 = v1 + v2;

there will be compile time error:
overloads pure @safe MyClass(MyClass rhs) and pure @safe MyClass(MyClass lhs) both match argument list for opBinary		

On Sunday, 21 October 2012 at 20:54:15 UTC, ref2401 wrote:
> On Sunday, 21 October 2012 at 20:42:08 UTC, cal wrote:
>> On Sunday, 21 October 2012 at 20:34:51 UTC, ref2401 wrote:
>>> what should i do to make my binary operation commutative?

> in the following case
>
> auto v5 = v1 + v2;
>
> there will be compile time error:
> overloads pure @safe MyClass(MyClass rhs) and pure @safe MyClass(MyClass lhs) both match argument list for opBinary		

Oh right, sorry. Well if all you want to do is catch the null on either side of the '+', you could use this overload of opBinaryRight:

MyClass opBinaryRight(string op: "+", T)(T rhs) if (!is(T == MyClass))
{			
  return null;	
}

I'm not sure if this is what you are trying to do though...