package main

import (
    "fmt"
)

type Person struct {
    FirstName string
    LastName  string
}

func main() {
    // 准备 =========================
    people := make(map[int]Person)
    person := Person{
        FirstName: "John",
        LastName:  "Doe",
    }
    people[1] = person

    // 报错：cannot assign to struct field people[1].FirstName in map
    // people[1].FirstName = "Jim"

    // 方式 1
    p := people[1]
    p.FirstName = "Alice"
    people[1] = p
    fmt.Println(people)
    // map[1:{Alice Doe}]
    fmt.Println(people[1])
    // {Alice Doe}

    // if p, ok := people[1]; ok {
    //  p.Field = 5
    //  people[1] = p
    // }

    // 方式 2
    people2 := make(map[int]*Person)
    people2[1] = &person
    people2[1].FirstName = "Adam"
    fmt.Println(people2)
    // map[1:0xc000060020]
    fmt.Println(people2[1])
    // &{Adam Doe}
}

总之，不能直接通过 key 找到 value（struct），然后修改其中的一个字段。