New type in generics

[Daniel Theophanes]

I have a situation where I would like to create a type, then set a property on it within a container. To set the type, I envisioned using a method "SetName" which would need to take a pointer receiver: (goplay share is down right now so I'll post inline:

type Namer interface {

SetName(name string)

}

I wish to create a new Namer type as well, ideally without using reflect. If I use [*Ob ] as the type parameter, that works, but then `new(T)` returns `**Ob`, which I can deference to get `*Ob`, but then the value is nil (Ob isn't created).

I'm working around this, but it surprised me, the interaction of a pointer receiver interface type constraint and then I can't create the desired type.

```

package main

import "fmt"

func main() {
ar := NewAppResult()
fmt.Printf("AR: %#v\n", *ar)
ar.Observation.Get("X1")
}

type Ob struct {
Gene  string
Value string
}

func (o *Ob) SetName(name string) {
// o is nil and this will panic.
o.Gene = name
}

type Namer interface {
SetName(name string)
}

type OrderedLookup[T Namer] struct {
List   []T
lookup map[string]T
}

func (ol *OrderedLookup[T]) Get(name string) T {
v, ok := ol.lookup[name]
if !ok {
var v T // T is a pointer, new(T) creates **Ob, but I cant use generic type of [Ob] because then Namer
v.SetName(name)
ol.lookup[name] = v
ol.List = append(ol.List, v)
}
return v
}

type AppResult struct {
Observation *OrderedLookup[*Ob]
}

func NewAppResult() *AppResult {
return &AppResult{
Observation: &OrderedLookup[*Ob]{},
}
}

```

[Axel Wagner]

The way to do that is to add another level of indirection (as everything in Software Engineering):

type Namer[T any] interface {

    *T

    SetName(name string)

}

func WithName[T any, PT Namer[T]](name string) T {

    var v T

    PT(&v).SetName(name)

    return v

}

I will say, though, that it's not unlikely that you'll be happier if you don't do this and instead accept a plain interface value and let the caller allocate the value and pass in a pointer. But if you want to do it, this is the way.

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[Daniel Theophanes]

Thank you. Yeah, that seems overly complicated. I can allocate interior to the generic object, but I can't use pointer-receiver methods, which is fine in my case, I'll just adjust the interior manually.

Thank you.