Thread overview
overload binary + operator to work with different types
Mar 13
Marc
Mar 13
Marc
March 13
I want to basically make this work:

>auto l = new List();
>l += 5;

I managed to do this:

>class List
>{
>	int[] items;
>	ref List opBinary(string op)(int rhs) if(op == "+")
>	{
>		items ~= rhs;
>		return *this;
>	}
}

Note the ref in the fucntion return, I also want to return a reference to the class so that this Works:

> l += 5 + 8 + 9 ... ;

Could someone point out how do that/what's wrong with my attempy?
March 13
On Tuesday, March 13, 2018 18:55:35 Marc via Digitalmars-d-learn wrote:
> I want to basically make this work:
> >auto l = new List();
> >l += 5;
>
> I managed to do this:
> >class List
> >{
> >
> > int[] items;
> > ref List opBinary(string op)(int rhs) if(op == "+")
> > {
> >
> >     items ~= rhs;
> >     return *this;
> >
> > }
>
> }
>
> Note the ref in the fucntion return, I also want to return a
>
> reference to the class so that this Works:
> > l += 5 + 8 + 9 ... ;
>
> Could someone point out how do that/what's wrong with my attempy?

+ is overloaded with opBinary, and += is overloaded using opOpAssign:

https://dlang.org/spec/operatoroverloading.html#op-assign http://ddili.org/ders/d.en/operator_overloading.html

- Jonathan M Davis

March 13
On Tuesday, 13 March 2018 at 19:05:00 UTC, Jonathan M Davis wrote:
> On Tuesday, March 13, 2018 18:55:35 Marc via Digitalmars-d-learn wrote:
>> I want to basically make this work:
>> >[...]
>>
>> I managed to do this:
>> > [...]
>>
>> }
>>
>> Note the ref in the fucntion return, I also want to return a
>>
>> reference to the class so that this Works:
>> > [...]
>>
>> Could someone point out how do that/what's wrong with my attempy?
>
> + is overloaded with opBinary, and += is overloaded using opOpAssign:
>
> https://dlang.org/spec/operatoroverloading.html#op-assign http://ddili.org/ders/d.en/operator_overloading.html
>
> - Jonathan M Davis

Soon as I posted I found out opOpAssign.
I've had got close than I thought:

>	List opOpAssign(string op)(int rhs) if(op == "+")
>	{
>		items ~= rhs;
>		return this;
>	}

Thanks!
March 13
On Tuesday, 13 March 2018 at 18:55:35 UTC, Marc wrote:
> I want to basically make this work:
>
>>auto l = new List();
>>l += 5;
>
> I managed to do this:
>
>>class List
>>{
>>	int[] items;
>>	ref List opBinary(string op)(int rhs) if(op == "+")
>>	{
>>		items ~= rhs;
>>		return *this;
>>	}
> }
>
> Note the ref in the fucntion return, I also want to return a reference to the class so that this Works:
>
>> l += 5 + 8 + 9 ... ;
>
> Could someone point out how do that/what's wrong with my attempy?

First, D has a concatenation operator (~). Use it instead of + if concatenation is what you want.

Now, to your question: classes are reference types in D, so you don't need a 'ref' on function:

List opBinary(string op : "~")(int rhs) {
    items ~= rhs;
    return this;
}

This will return the exact same List that you've just appended to. However, that won't do what you're asking, since you specifically asked for += (which I will interpret as ~=, for reasons explained above). When the operation you want to overload is an assignment operator, as in +=, -=, ~=, etc, you'll need to write a function called opOpAssign:

List opOpAssign(string op : "~")(int rhs) {
    items ~= rhs;
    return this;
}

With opOpAssign, this code will work:

unittest {
    List a = new List();
    a ~= 1;
    a ~= 2;
    a ~= 3;
}

However, we still haven't gotten code on the format 'a ~= 1 ~ 2 ~ 3;' to work, and that's because we can't. When you have an assignment expression (something that looks like 'lhs = rhs'), the left-hand side is evaluated separately from the right-hand side, and then the assignment is performed. Since '1 ~ 2 ~ 3' doesn't do what you want it to, neither will 'a ~= 1 ~ 2 ~ 3', since it's essentially '(a) = (1 ~ 2 ~ 3);'.

Overloading just opBinary though (as in my first example), we can make this work: 'a ~ 1 ~ 2 ~ 3;'. The problem, as you probably notice, is that there's no assignment there. It looks as though you're just concatenating a bunch of items, and then discarding the result.

Now, since we've established that there's no way to do exactly what you want, maybe it's time to take a look at what you actually want. :p Why do you want to write that code? Why would a.append(1, 2, 3); not be good enough?

And given that seasoned D veterans know that the lhs and rhs of an assignment are evaluated separately, and that 'a ~ b' generally doesn't have side effects, you should think very carefully through your reasoning for breaking that intuition.

--
  Simen
March 13
On 3/13/18 3:28 PM, Simen Kjærås wrote:

> Now, since we've established that there's no way to do exactly what you want, maybe it's time to take a look at what you actually want. :p Why do you want to write that code? Why would a.append(1, 2, 3); not be good enough?

Typically, we can do this with arrays quite easily, especially strings:

arr ~= [1, 2, 3];
str ~= "123";

It would be nice to have another way to do this. a.append(1, 2, 3) would work , and you can do it in a nice variadic way with:

append(T[] vals...)

And this won't allocate anything on the heap (this is essentially how I did it in dcollections, see https://github.com/schveiguy/dcollections/blob/master/dcollections/model/Addable.d#L55).

But most D containers, you would expect to be able to append a range. This is what I would recommend. It will work for arrays as well.

-Steve