我一直在努力使用反射包。下面的代码符合我的期望:
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,如果我用row := nth.Interface().(Person)
替换行row := nth.Interface()
,那就是我删除类型断言,然后我得到错误:
panic:reflect:调用reflect.Value.FieldByName on interface value on line“field:= elem.FieldByName(fieldName)
我在过去的几个小时里尝试了很多其他的事情,比如尝试做reflect.TypeOf()
,reflect.Indirect()
等......在其他一些变量上却没有成功。
我读过其他一些问题:
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
他们似乎暗示我对指针或接口的工作方式没有很好的理解。
所以我的问题是,当结构类型作为接口输入时,如何设置结构的字段?
UPDATE
我发布了一个解决方案作为答案,但我不相信它是否是正确或安全的做事方式。我希望有人可以解释,或发布更好的解决方案。
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))
}
像这样称呼它:
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。指针不是必需的,增加了额外的复杂性。
纯粹的运气,我终于得到了一些工作。
我用很少的押韵或理由拼凑了一堆随意的东西。我甚至尝试在Golang网站上阅读反思法则,但我认为我没有很好地理解它与我为什么不能设置类型为interface{}
的变量有关。总的来说,我仍然不明白我做了什么。
我的解决方案充满了评论,表明我的困惑,对我是否正确或安全地做事情缺乏信心。
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)
}
如果有人能发布一个更好的答案来解决我的困惑,那将是很棒的。我一直很难学习golang。