通过反射引用嵌套结构 [英] Pass by reference nested structures through reflection
问题描述
类型客户端结构{
Id int
年龄int
PrimaryContact联系人
名称字符串
}
类型联系struct {
Id int
ClientId int
IsPrimary bool
电子邮件字符串
}
以上是一个示例代码;我想要实现的是以下内容:
- 使用反射循环所有客户端结构字段
- 对于每个primitive字段,使用反射设置默认值
- 对于每个结构字段使用递归应用上述步骤
问题是,当PrimaryContact字段被内省时,并且当我试图为它的任何字段设置一个值时,我最终得到以下
$ b
reflect.Value.Set using unaddressable value
如果我没有弄错的话,原因是PrimaryContact是通过值传递的,而不是通过引用传递的,所以当我在任何字段上调用Set方法时,它会改变副本上的字段值而不是实际参数。我怎样才能克服这个问题?我如何通过引用使用反射将PrimaryContact字段传递给我的方法?
两个指针:
- 为了设置一个结构体的字段值,你必须将它作为一个指针传递给它
- 要获得struct字段的指针值,请使用
Value.Addr()
工作解决方案:
包主
导入(
fmt
反映
错误
)
类型客户端结构{
Id int
年龄int
PrimaryContact联系
名称字符串
}
类型联系结构{
Id int
ClientId int
IsPrimary bool
电子邮件字符串
}
func SetDefault(s interface {})错误{
返回setDefaultValue(reflect.ValueOf(s))
}
func setDefaultValue(v reflect.Value)错误{
如果v.Kind()!= reflect.Ptr {
返回errors.New(不是指针值)
}
v = reflect.Indirect(v)
switch v.Kind(){
case reflect.Int:
v.SetInt(42)
case reflect.String:
v.SetString(Foo)
案例reflect.Bool:
v.SetBool(true)
案例reflect.Struct:
//遍历结构字段
for i:= 0;我< v.NumField(); i ++ {
err:= setDefaultValue(v.Field(i).Addr())
if err!= nil {
return err
}
}
default:
return errors.New(Unsupported kind:+ v.Kind()。String())
}
返回nil
}
func main(){
a:=客户端{}
err:= SetDefault(& a)
if err!= nil {
fmt.Println(Error:,err)
} else {
fmt.Printf(%+ v \ n,a)
}
}
输出:
{Id:42 Age:42 PrimaryContact:{Id:42 ClientId:42 IsPrimary:true Email:Foo} Name:Foo}
Playground: http://play.golang.org/p/-Mpnb7o4vl
type Client struct {
Id int
Age int
PrimaryContact Contact
Name string
}
type Contact struct {
Id int
ClientId int
IsPrimary bool
Email string
}
The above is a sample code; what I am trying to achieve is the following: - loop through all Client struct fields using reflection - for each "primitive" field set a default value using reflection - for each struct field use recursion to apply the above steps
The issue is that when PrimaryContact field is introspected and when I am trying to set a value for any of its fields I end up with the following panic:
reflect.Value.Set using unaddressable value
The reason if I am not mistaken is that PrimaryContact is passed by value and not by reference so when I would call Set method on any of its fields it would change fields values on the copy and not on the actual argument. How can I overcome this issue? How I could pass PrimaryContact field to my method by reference using reflection?
I saw it as a fun exercise to practice reflection.
Two pointers though:
- In order to set field values of a struct, you must pass it as a pointer
- To get the pointer value of a struct field, use
Value.Addr()
Working solution:
package main
import (
"fmt"
"reflect"
"errors"
)
type Client struct {
Id int
Age int
PrimaryContact Contact
Name string
}
type Contact struct {
Id int
ClientId int
IsPrimary bool
Email string
}
func SetDefault(s interface{}) error {
return setDefaultValue(reflect.ValueOf(s))
}
func setDefaultValue(v reflect.Value) error {
if v.Kind() != reflect.Ptr {
return errors.New("Not a pointer value")
}
v = reflect.Indirect(v)
switch v.Kind() {
case reflect.Int:
v.SetInt(42)
case reflect.String:
v.SetString("Foo")
case reflect.Bool:
v.SetBool(true)
case reflect.Struct:
// Iterate over the struct fields
for i := 0; i < v.NumField(); i++ {
err := setDefaultValue(v.Field(i).Addr())
if err != nil {
return err
}
}
default:
return errors.New("Unsupported kind: " + v.Kind().String())
}
return nil
}
func main() {
a := Client{}
err := SetDefault(&a)
if err != nil {
fmt.Println("Error: ", err)
} else {
fmt.Printf("%+v\n", a)
}
}
Output:
{Id:42 Age:42 PrimaryContact:{Id:42 ClientId:42 IsPrimary:true Email:Foo} Name:Foo}
Playground: http://play.golang.org/p/-Mpnb7o4vl
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