在 Golang 中传递结构类型？

Question

Please forgive my question, I'm new to Golang and possibly have the wrong approach.

I'm currently implementing a Terraform provider for an internal service.

As probably expected, that requires unmarshalling JSON data in to pre-defined Struct Types, eg:

type SomeTypeIveDefined struct {
    ID   string `json:"id"`
    Name String `json:"name"`
}

I've got myself in to a situation where I have a lot of duplicate code that looks like this

    res := r.(*http.Response)
    var tempThing SomeTypeIveDefined
    dec := json.NewDecoder(res.Body)
    err := dec.Decode(&tempThing)

In an effort to reduce duplication, I decided what I wanted to do was create a function which does the JSON unmarshalling, but takes in the Struct Type as a parameter.

I've trawled through several StackOverflow articles and Google Groups trying to make sense of some of the answers around using the reflect package , but I've not had much success in using it.

My latest attempt was using reflect.StructOf and passing in a set of StructField s, but that still seems to require using myReflectedStruct.Field(0) rather than myReflectedStruct.ID .

I suspect there may be no way until something like Generics are widely available in Golang.

I considered perhaps an interface for the structs which requires implementing an unmarshal method, then I could pass the interface to the function and call the unmarshal method. But then I'm still implementing unmarshal on all the structs anyway.

I'm just wondering what suggestions there may be for achieving what I'm after, please?

Answer 1

Create a helper function with the repeated code. Pass the destination value as a pointer.

func decode(r *http.Repsonse, v interface{}) error {
     return json.NewDecoder(res.Body).Decode(v)
}

Call the helper function with a pointer to your thing:

var tempThing SomeTypeIveDefined
err := deocde(r, &tempThing)

Answer 2

You can do this with interfaces:

func decodeResponse(r *http.Response, dest interface{}) error {
    dec := json.NewDecoder(r.Body)
    return dec.Decode(dest)
}

func handler(...) {
    res := r.(*http.Response)
    var tempThing SomeTypeIveDefined
    if err:=decodeResponse(res,&tempThing); err!=nil {
      // handle err
    }
   ...
}

You don't need to implement an unmarshal for the structs, because the stdlib decoder will use reflection to set the struct fields.