英文:
setting a value of a pointer does not work through interface{}
问题
以下是稍作修改的示例，来自于Go语言的反射规则（http://blog.golang.org/laws-of-reflection）。第二个代码段使用了来自map[string]interface{}的指针，但是它没有起作用，我做错了什么？
谢谢
//http://play.golang.org/p/LuMBUWLVT6
package main
import (
"fmt"
"reflect"
)
type T struct {
x float64
}
func (x T) RowMap() map[string]interface{} {
return map[string]interface{}{
"x": &x.x,
}
}
func main() {
// 这一部分按预期工作，当完成时，x.x将为7.1
var x = T{3.4}
p := reflect.ValueOf(&x.x) // 注意：取x的地址。
v := p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 7.1 {7.1}
// 这一部分我不明白为什么x.x没有被设置为7.1
x = T{3.4}
rowmap := x.RowMap()
p = reflect.ValueOf(rowmap["x"]) // rowmap["x"] => &x.x，与上面类似，但包含在interface{}中
v = p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 3.4 {3.4} ?? 嗯，应该是 // 7.1 {7.1}
}
希望对你有帮助！
英文:
Below is slightly modified example from the go laws of reflection http://blog.golang.org/laws-of-reflection. The 2nd code section uses a pointer from a map[string]interface{} and it does not work, what am I doing wrong?
Thanks
//http://play.golang.org/p/LuMBUWLVT6
package main
import (
"fmt"
"reflect"
)
type T struct {
x float64
}
func (x T) RowMap() map[string]interface{} {
return map[string]interface{}{
"x": &x.x,
}
}
func main() {
// this section works as expected, x.x will be 7.1 when done
var x = T{3.4}
p := reflect.ValueOf(&x.x) // Note: take the address of x.
v := p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 7.1 {7.1}
// this section I do not understand why x.x is not being set to 7.1
x = T{3.4}
rowmap := x.RowMap()
p = reflect.ValueOf(rowmap["x"]) // rowmap["x"] => &x.x just like above, but is containted in interface{}
v = p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 3.4 {3.4} ?? huh, should be // 7.1 {7.1}
}
答案1
得分: 2
Elem返回接口v包含的值，或指针v指向的值。
尝试打印以下内容，你会看到你想要看到的内容，但x的含义不会改变，它从未被更新。
fmt.Println(v.Float()) // 7.1
你需要传递一个指针给你的方法。将你的方法签名更改为以下形式：
func (x *T) RowMap() map[string]interface{} {
传递一个指针而不是一个副本。
我添加了一些打印语句，我认为它们会帮助澄清问题 http://play.golang.org/p/xcFMicIPcP
查看你的方法内部和外部的x的地址，看看它们是如何不同的。
英文:
>Elem returns the value that the interface v contains or that the pointer v points to.
Try printing the following and you'll see what you want to see but x does not change meaning it's never being updated.
fmt.Println(v.Float()) // 7.1
You need to pass a pointer to your method. Change your method signature to look like this
func (x *T) RowMap() map[string]interface{} {
Pass a pointer instead of a copy.
I've added some print statements that I think will help clear things up http://play.golang.org/p/xcFMicIPcP
Look at the address of x inside and outside of your method and see how they're different.
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