CODEWITHSUNDEEP
reflectiongo
BinaryLeeward
BinaryLeeward

Reputation: 43

Golang create struct in different way

guys! I am a beginner in Go. I have some doubts When I learning reflect package ,here's the code:

package main

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

func checkError(err error) {
    if err != nil {
        panic(err)
    }
}

type Test struct {
    X int
    Y string
}

func main() {
    fmt.Println("hello world!")
    test1()
    test2()
}

func test1() {
    a := Test{}
    fmt.Printf("a: %v %T \n", a, a)
    fmt.Println(a)
    err := json.Unmarshal([]byte(`{"X":1,"Y":"x"}`), &a)
    checkError(err)
    fmt.Printf("a: %v %T \n", a, a)
}

func test2() {
    fmt.Println("===========================")
    m := make(map[string]reflect.Type)
    m["test"] = reflect.TypeOf(Test{})
    a := reflect.New(m["test"]).Elem().Interface()
    fmt.Printf("a: %v %T \n", a, a)
    fmt.Println(a)
    err := json.Unmarshal([]byte(`{"X":1,"Y":"x"}`), &a)
    checkError(err)
    fmt.Printf("a: %v %T \n", a, a)
}

and the result :

a: {0 } main.Test 
{0 }
a: {1 x} main.Test 
===========================
a: {0 } main.Test 
{0 }
a: map[X:1 Y:x] map[string]interface {}

Why these two way make different result, Could anyone tell me why, many thanks.

Upvotes: 4

Views: 676

Answers (1)

Mr_Pink
Mr_Pink

Reputation: 109440

In test2 you're passing in the address of the interface{} containing a Test value. When the value is dereferenced by the json package it only sees an interface{}, and therefor it unmarshals into the default types.

What you need is an interface{} containing a pointer to a Test value. 

// reflect.New is creating a *Test{} value.
// You don't want to dereference that with Elem()
a := reflect.New(m["test"]).Interface()

// 'a' contains a *Test value. You already have a pointer, and you
// don't want the address of the interface value.
err := json.Unmarshal([]byte(`{"X":1,"Y":"x"}`), a)

Upvotes: 2

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