Starting to play around with golang and was looking at a custom json.Unmarshal. In a blog post the had the following:
type FlexInt int
func (fi *FlexInt) UnmarshalJSON(data []byte) error {
if data[0] != '"' {
return json.Unmarshal(data, (*int)(fi))
}
var s string
if err := json.Unmarshal(data, &s); err != nil {
return err
}
i, err := strconv.Atoi(s)
if err != nil {
return err
}
*fi = FlexInt(i)
return nil
}
And I understand what it is doing - but I dont understand (*int)(fi) part. Looking at the value of the fi pointer it is the same as (*int)(fi) - (*int)(fi) == fi. Yet when I change that line to simply fi it then does an infinite loop
The expression converts fi to an *int. The result contains the same address, but has a different type.
If a *FlexInt is passed to json.Unmarshal, then json.Unmarshal will call the *FlexInt.UnmarshalJSON method which calls json.Unmarshal and so on.
It's the same pointer, but with a different type. When json.Unmarshal is called with an interface{} that contains a FlexInt*, it calls FlexInt's UnmarshalJSON method. When it's called with an interface{} that contains an int*, it uses the builtin behavior. Since FlexInt and int have the same underlying type, it's acceptable to convert a pointer to one into a pointer to the other, but it's the "actual" type that the interface will be marked with.