need `this` for `this` of type `ref @safe Test(string reg_arg)

[Paul Backus]

On Sunday, 18 June 2023 at 17:43:01 UTC, rempas wrote:
> Ok, so I'm having a struct that has a constructor that takes a 
> template parameter. I suppose this question could also be named 
> `how to initialize constructors with template parameters` but 
> whatever! The funny thing is, I think that I may have already 
> found how to do it in the past but I forgot... This time, I'm 
> going to add the code in a big module in my project so I'm not 
> going to have it as a reference and I won't forget it.
>
> Here is a small sample code:
>
> ```d
> import std.stdio;
>
> struct Test {
>   this(string type = "def")(string reg_arg) {
>     writeln("reg_arg: ", reg_arg);
>   }
> }
>
> void main() {
>   auto test = Test.__ctor!("non_def")("Hello");
> }
> ```
>
> And the error I get: `test.d(10): Error: need `this` for `this` 
> of type `ref @safe Test(string reg_arg)`

`__ctor` doesn't create a new object, it initializes an existing 
object. You need to create the object first, then call `__ctor` 
on it:

```d
void main() {
   Test test;
   test.__ctor!("non_def")("Hello");
}
```