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digitalmars.D.learn - overload binary + operator to work with different types

reply Marc <jckj33 gmail.com> writes:
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?
Mar 13 2018
next sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
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
Mar 13 2018
parent Marc <jckj33 gmail.com> writes:
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!
Mar 13 2018
prev sibling parent reply Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
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
Mar 13 2018
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
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
Mar 13 2018