从JSON中的JSON数组中提取值（以及为什么不反映和编译错误报告同一类型）

Question

I am having trouble unmarshalling some JSON in Go. In the below code, I want to be able to extract the value of "c" ("Apple", "Berry") from the JSON (<- main goal/question). What has me particularly confused is when I tried to index into the v array(?), I get the error: invalid operation: v[0] (type interface {} does not support indexing).

This is what brought me to using reflect and looking at the type of the variable, which as you can see below is "[]interface {}", even if I assert it to be "interface {}". Of note, if I make the line q := interface{}(v) into q := []interface{}(v), I get: cannot convert v (type interface {}) to type []interface {}: need type assertion. What am I missing? Why does reflect say it is "[]interface {}" but errors say it is "interface {}"? (<- secondary question / help me understand)

package main

import (
    "encoding/json"
    "fmt"
    "reflect"
)

func main() {
    data := []byte("{ \"a\" : { \"b\" : [ { \"n\" : 100, \"c\" : \"Apple\", \"m\" : 1, \"bool\" : false }, { \"n\" : 101, \"c\" : \"Berry\", \"m\" : 2, \"bool\" : false } ] }, \"foo\" : false, \"bar\" : \"foobar\" }")
    var a map[string]interface{}

    _ = json.Unmarshal(data, &a)

    b := a["a"].(map[string]interface{})
    for i, v := range b {
        fmt.Println(i, reflect.TypeOf(v))
        q := interface{}(v)
        fmt.Println(q, reflect.TypeOf(q))
    }

}

Output:

b []interface {}
[map[m:1 bool:false n:100 c:Apple] map[c:Berry m:2 bool:false n:101]] []interface {}

Answer 1

Instead of attempting to unmarshal the json into an interface, you can create specific types to model what the data will be. See the example code below.

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    data := []byte("{ \"a\" : { \"b\" : [ { \"n\" : 100, \"c\" : \"Apple\", \"m\" : 1, \"bool\" : false }, { \"n\" : 101, \"c\" : \"Berry\", \"m\" : 2, \"bool\" : false } ] }, \"foo\" : false, \"bar\" : \"foobar\" }")

    type c struct {
        N int    `json:"n"`
        C string `json:"c"`
        M int    `json:"m"`
        B bool   `json:"bool"`
    }
    type b struct {
        C   []c    `json:"b"`
        Foo bool   `json:"foo"`
        Bar string `json:"bar"`
    }
    type a struct {
        A b `json:"a"`
    }


    var result a
    if err := json.Unmarshal(data, &result); err != nil {
        panic(err.Error())
    }

    fmt.Printf("%+v", result)
}

Link to playground.

output: {A:{C:[{N:100 C:Apple M:1 B:false} {N:101 C:Berry M:2 B:false}] Foo:false Bar:}}

Note that the type members are capitalised - this is required for the json unmarshaling to work correctly.

Once the json is unmarshaled into your types, you can refer to the members as you'd expect:

    for _, v := range result.A.C {
        fmt.Println(v)
    }

output:

{100 Apple 1 false}
{101 Berry 2 false}