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无法设置类型为接口{}的结构的字段

go-reflect go

-1

learningtech · 技术社区 · 6 年前

我一直在为“反思”计划而挣扎。下面的代码符合我的预期:

package main

import (
  "reflect"
  "log"
)
type Car struct {
  Model string
}
type Person struct {
  Name string
  Cars []Car
}

func ModifyIt(parent interface{},fieldName string, val interface{}) {
  slice := reflect.ValueOf(parent).Elem()
  nth := slice.Index(0)
  //row := nth.Interface() // this line causes errors
  row := nth.Interface().(Person)
  elem := reflect.ValueOf(&row).Elem()
  field := elem.FieldByName(fieldName)
  log.Println(field.CanSet())

}

func main() {

  p := []Person{Person{Name:"john"}}
  c := []Car{Car{"corolla"},Car{"jetta"}}

  ModifyIt(&p,"Cars",&c)
}

但是,如果我换掉这条线 row := nth.Interface().(Person) 具有 row := nth.Interface()

在过去的几个小时里,我尝试了很多其他的事情,比如尝试去做 reflect.TypeOf() , reflect.Indirect()

我还读过一些类似的问题:

reflect: call of reflect.Value.FieldByName on ptr Value

Set a struct field with field type of a interface

Golang reflection: Can't set fields of interface wrapping a struct

所以我的问题是,当结构类型化为接口时,如何设置结构的字段?

更新

我发布了一个解决方案作为答案,但我对它是正确的还是安全的做事方式没有信心。我希望有人能解释,或张贴一个更好的解决方案。

2 回复 | 直到 6 年前

1

3

Cerise Limón 6 年前

试试这个:

func ModifyIt(slice interface{}, fieldName string, newVal interface{}) {
    // Create a value for the slice.
    v := reflect.ValueOf(slice)

    // Get the first element of the slice.
    e := v.Index(0)

    // Get the field of the slice element that we want to set.
    f := e.FieldByName(fieldName)

    // Set the value!
    f.Set(reflect.ValueOf(newVal))
}

p := []Person{Person{Name: "john"}}
c := []Car{Car{"corolla"}, Car{"jetta"}}
ModifyIt(p, "Cars", c)

请注意,调用直接传递切片,而不是使用指向切片的指针。不需要指针,增加了额外的复杂性。

Run it on the Playground .

2

0

learningtech 6 年前

纯粹出于运气,我终于找到了工作。

interface{} . 总的来说,我还是不明白我做了什么。

我下面的解决方案充斥着评论,表明我的困惑,以及对我是否正确或安全地做事缺乏信心。

package main

import (
  "reflect"
  "log"
)
type Car struct {
  Model string
}
type Person struct {
  Name string
  Cars []Car
}

func ModifyIt(parent interface{},fieldName string, val interface{}) {
  log.Println(parent)
  slice := reflect.ValueOf(parent).Elem()
  nth := slice.Index(0)
  row := nth.Interface()
  log.Println(nth.CanSet()) // I can set this nth item

  // I think I have a to make a copy, don't fully understand why this is necessary
  newitem := reflect.New(reflect.ValueOf(row).Type())
  newelem := newitem.Elem()
  field := newelem.FieldByName(fieldName)

  // I need to copy the values over from the old nth row to this new item
  for c:=0; c<nth.NumField(); c++ {
    newelem.Field(c).Set(reflect.Indirect(nth.Field(c)))
  }

  // now I can finally set the field for some reason I don't understand
  field.Set(reflect.ValueOf(val).Elem())

  // now that newitem has new contents in the field object, I need to overwrite the nth item with new item
  // I don't know why I'm doing it, but I'll do it
  // I also don't fully understand why I have to use Indirect sometimes, and not other times...it seems interchangeable with ValueOf(something).Elem(), I'm confused....
  nth.Set(reflect.Indirect(newitem))

}

func main() {

  p := []Person{Person{Name:"john"}}
  c := []Car{Car{"corolla"},Car{"jetta"}}

  ModifyIt(&p,"Cars",&c)
  // now parent is up to date, although I have no idea how I got here.
  log.Println(p)
}

如果有人能给出一个更好的答案来澄清我的困惑,那就太好了。我学歌浪一直很辛苦。