无法修改数组内部的结构？

Question

I'm just starting to learn golang and I have the following code:

https://play.golang.org/p/OBsf9MRLD8

package main

import (
    "encoding/json"
    "os"
)

type ResourceUsage struct {
    Type string
}
type Node struct {
    Resources []ResourceUsage
}

func main(){
    encoder := json.NewEncoder(os.Stdout)
    nodes := make([]Node, 2)
    nodes[0] = Node{}
    nodes[1] = Node{}

    for _,n := range nodes {
        n.Resources = append(n.Resources, ResourceUsage{Type: "test"})
    }
    encoder.Encode(nodes)
}

I was hoping it to print

[{"Resources":[{"Type:"test"}]},{"Resources":[{"Type:"test"}]}]

But instead I get:

[{"Resources":null},{"Resources":null}]

How can I accomplish the expected output?

Answer 1

Structs are copied in range loops. You need to access by index.

for i := range nodes {
    nodes[i].Resources = append(nodes[i].Resources, ResourceUsage{Type: "test"})
}

You could also choose to use pointers, which would not copy data.

nodes := make([]*Node, 2)
nodes[0] = &Node{}
nodes[1] = &Node{}

@Kaedys points out, you can't take the address of the iterated value.

for _, v := range nodes {
    &v // mistake
}