go - json.Unmarshal interface pointer with later type assertion

Question

Because I often unmarshal http.Response.Body, I thought I could write a function which handles all the hassle of reading, closing and unmarshaling into various different structs. That's why I introduced a function func unmarhalInterface(closer *io.ReadCloser, v *interface{}) error and can then assert the return value with t:=i.(T).

According to this answer I already wrapped it into a value of type *interface{}, but because the overlying type is interface{}and not myStruct, the json package implementation chooses map[string]interface{}. After that a type assertion fails (of course). Is there anything i am missing or requires this implementation a type assertion "by hand", that means look for all fields in the map and assign those that I want into my struct.

Code below has minimal example with notation in comments. If my explanation is not sufficient, please ask away.

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "io"
    "io/ioutil"
    "log"
)

type myStruct struct {
    A string `json:"a"`
    B string `json:"b"`
}

func main() {
    jsonBlob := []byte(`{"a":"test","b":"test2"}`)

    var foo = interface{}(myStruct{})
    closer := ioutil.NopCloser(bytes.NewReader(jsonBlob))

    err := unmarshalCloser(&closer, &foo)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(fmt.Sprintf("%v", foo))

    // That′s what i want:
    foo2 := foo.(myStruct)
    fmt.Println(foo2.A)
}

func unmarshalCloser(closer *io.ReadCloser, v *interface{}) error {
    defer func() { _ = (*closer).Close() }()

    data, err := ioutil.ReadAll(*closer)
    if err != nil {
        return err
    }

    err = json.Unmarshal(data, v)
    if err != nil {
        return err
    }
    return nil
}

Golang Playground

Answer 1

Waitting for answers