hi,
On Fri Apr 15, 2022 at 08:00 CET, Zhaoxun Yan wrote:
> Here is my note on json & struct, run it somewhere and it may give you a
> hint.
>
> package main
>
> import "fmt"
> import "encoding/json"
>
> type prices struct{
> Price float64 `json:"price"`
> floor float64
> Ceiling float64
> settle float64
> time int64
> }
>
>
> func main() {
> content := prices{
> Price: 6.4 ,
> floor: 3.5,
> Ceiling: 7,
> settle: 8.5,
> time: 1,
> }
>
> js,_ := json.Marshal( content)
> fmt.Println(string(js))
>
> var back prices
> json.Unmarshal(js, &back)
> fmt.Println(back)
> fmt.Println(back.Price)
> }
thanks for the offer.
but that's not really what I am after.
my issue is two-fold:
- how to convey the constraint of the Unmarshaler interface (which is
logically implemented on a pointer receiver, so *T, not T)
- how to create values of T (where *T implements Unmarshaler)
I have managed to something along these lines, with some reflect help:
https://go.dev/play/p/-CK2Bk-LU8y
```
type T1 struct {
v string
}
func (t *T1) Unmarshal(p []byte) error {
t.v = string(p)
return nil
}
type Unmarshaler interface {
Unmarshal(p []byte) error
}
func f[T Unmarshaler]() T {
t := alloc[T]()
unmarshal(t)
return t
}
func alloc[T any]() T {
var t T
rv := reflect.ValueOf(&t).Elem()
rv.Set(reflect.New(rv.Type().Elem()))
return t
}
func unmarshal[T Unmarshaler](t T) {
err := t.Unmarshal([]byte("hello"))
if err != nil {
panic(err)
}
}
func main() {
t := *f[*T1]()
log.Printf("t1: %+v", t)
}
```
so: passing *T to the "factory function", and using reflect to reach
into the guts of underlying type, allocate and return the newly
populated value.
it's not super robust.
but I can do.
perhaps I am not holding it right?
I am probably missing the type manipulation vocabulary (akin to
reflect's PointerTo and Elem/Indirect facilities).
-s
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