"this" as default parameter for a constructor.

[Mathias LANG]

On Sunday, 11 April 2021 at 20:38:10 UTC, Pierre wrote:
> Hi,
>
> I have a class with a reference to the parent object and a 
> constructor that has the parent as parameter
>
> class foo {
> 	this ( foo p /* , other params */ ) {
> 		parent = p;
> 	}
>
> 	foo parent;
> }
>
> Of cause, the parent is almost always the object that creates 
> the new intance. So
>
> auto f = new foo(this);
>
> I'd like to set "this" ( the creator ) as default argument if 
> the constructor :
>
> this ( foo p = this ) {...}
>
> I can't. But however, the function, the argument and the 
> current object in body of the constructor are 3 different 
> things and the compiler can distinguish each one.
>
> Is there a way to pass the reference of the caller to the 
> creator as default argument ?

Depending on what you are trying to do, I would recommend to 
instead go with nested classes if you can. E.g.
```D
class MyClass {
     class MyChild {
         this (int value) { this.value = value; }
         private int value;
     }
}

void main ()
{
     auto mc = new MyClass;
     auto child = mc.new MyChild(42);
}
```

It'll give you an automatic reference to the parent. Of course if 
you are trying to do something like linked list, where all 
elements have the same type, it won't work.
In this case, the `create` approach might be better. You should 
be able to cook something with a template `this` parameter to 
reduce boilerplate.

And regarding allowing `this` as default argument: Definitely no. 
While it could be possible with some stretch (e.g. we'll have to 
delay default parameter semantic to the call site, unlike what is 
currently done, and that would mess with things like overload 
resolutions and template type inference), it wouldn't be sound / 
it'd be very surprising. I for one would expect `this` to be the 
object referencing itself, not the `this` of my caller.