如何在golang中初始化嵌套结构数组的值

Question

My struct

type Result struct {
   name   string
   Objects []struct {
       id int
   }
}

Initialize values for this

func main() {
   var r Result;
   r.name  = "Vanaraj";
   r.Objects[0].id = 10;
   fmt.Println(r)
}

I got this error. "panic: runtime error: index out of range"

How to fix this?

Answer 1

Firstly, I'd say it's more idiomatic to define a type for your struct, regardless of how simple the struct is. For example:

type MyStruct struct {
    MyField int
}

This would mean changing your Result struct to be as follows:

type Result struct {
    name   string
    Objects []MyStruct
}

The reason your program panics is because you're trying to access an area in memory (an item in your Object s array) that hasn't been allocated yet.

For arrays of structs, this needs to be done with make .

r.Objects = make([]MyStruct, 0)

Then, in order to add to your array safely, you're better off instantiating an individual MyStruct , ie

ms := MyStruct{
    MyField: 10,
}

And then append ing this to your r.Objects array

r.Objects = append(r.Objects, ms)

For more information about make , see the docs

Answer 2

After defining the struct as suggested in another answer :

type MyStruct struct {
    MyField int
}


type Result struct {
    Name   string
    Objects []MyStruct
}

Then you can initialize a Result object like this:

result := Result{
    Name: "I am Groot",
    Objects: []MyStruct{
        {
            MyField: 1,
        },
        {
            MyField: 2,
        },
        {
            MyField: 3,
        },
    },
}

Full code:

package main

import "fmt"

func main() {
    result := Result{
        Name: "I am Groot",
        Objects: []MyStruct{
            {
                MyField: 1,
            },
            {
                MyField: 2,
            },
            {
                MyField: 3,
            },
        },
    }

    fmt.Println(result)

}

type MyStruct struct {
    MyField int
}

type Result struct {
    Name    string
    Objects []MyStruct
}

You can verify this in this Go playground .

Answer 3

Objects contains no elements. You need to append element first. Like this:

r.Objects = append(r.Objects, struct{ id int }{}) 

Also you can omit r.Objects[0].id = 10; using initialization of your struct like this:

r.Objects = append(r.Objects, struct{ id int }{ 10 })

Answer 4

This is probably a dumb idea, but you can roundtrip it through JSON:

package main

import (
   "bytes"
   "encoding/json"
)

type (
   a []interface{}
   m map[string]interface{}
)

func decode(in, out interface{}) {
   var b bytes.Buffer
   json.NewEncoder(&b).Encode(in)
   json.NewDecoder(&b).Decode(out)
}

Example:

package main
import "fmt"

type result struct {
   Name string
   Objects []struct {
      Id int
   }
}

func main() {
   r := m{
      "Name": "Vanaraj",
      "Objects": a{
         m{"Id": 10},
      },
   }
   var s result
   decode(r, &s)
   fmt.Printf("%+v\n", s) // {Name:Vanaraj Objects:[{Id:10}]}
}