使用 bitly 的 go-simplejson 时，在 simplejson.Json 文字中显示未导出字段“数据”的隐式分配

Question

When I use like &simplejson.Json{v} (v is a interface read from file and it's actual data structure is map[string]interface{}), then show this error. Details:

A json file named abcd

{
    "pids": [
        { 
            "pid": 168043, 
            "target_regions": [
                40, 
                25, 
                43, 
                299, 
                240
            ]
         },
        { 
            "pid": 168044, 
            "target_regions": [
                63, 
                65,
                68
            ]
        }
    ]

}

And the go file is

package main    

import (
    "fmt"
    "io/ioutil"    

    sjson "github.com/bitly/go-simplejson"
)    

type pidInfo struct {
    Pid           uint64   `json:"pid"`
    TargetRegions []uint32 `json:"target_regions"`
}    

type pidUnitInfo struct {
    Pid2Info map[uint64]*pidInfo
}    

func build() error {
    content, _ := ioutil.ReadFile("./abcd")
    json, err := sjson.NewJson(content)
    if err != nil {
        return err
    }
    newPidUnitInfo(json)
    return nil
}    

func newPidUnitInfo(json *sjson.Json) (*pidUnitInfo, error) {
    newInfo := new(pidUnitInfo)
    newInfo.buildPid2Info(json)
    return nil, nil
}    

func (pui *pidUnitInfo) buildPid2Info(json *sjson.Json) error {
    raw, ok := json.CheckGet("pids")
    if !ok {
        return fmt.Errorf("not found json key %v", "pids")
    }
    pids, err := raw.Array()
    if err != nil {
        return err
    }
    pui.Pid2Info = make(map[uint64]*pidInfo, len(pids))
    for _, v := range pids {
        fmt.Println(v)
        m := &sjson.Json{v}
        fmt.Println(m)
    }
    return nil
}    

func main() {
    build()
}

When I execute it, show implicit assignment of unexported field 'data' in simplejson.Json literal at this line m := &sjson.Json{v} .

Answer 1

This line:

m := &sjson.Json{v}

Tries to create a value (and take the address) of the struct type Json from package go-simplejson . The type declaration is:

type Json struct {
    data interface{}
}

It has one field: data which is unexported. That means packages other than go-simplejson cannot refer to this field. When you use a struct literal &sjson.Json{v} , it would try to initialize the Json.data field with value v which is a violation of this. You cannot do this.

The Json type is not designed for you to specify the internal data , it is designed so that the data will be the placeholder of some decoded JSON data (see the NewFromReader() and NewJson() constructor-like functions).

This data field is handled internally by the go-simplejson package, you cannot set it yourself. You may use sjson.New() to obtain a new *Json value which will initialize this data field with an empty map ( map[string]interface{} ). You may also useJson.Map() method which asserts that data is a map and returns it like that:

js := sjson.New()
m, err := js.Map()
if err != nil {
    // Handle error
}

// Now you have a map of type map[string]interface{}

Or to populate the data inside a Json , you can use itsJson.Set() method.