英文:
Having a hard time with reflection in golang
问题
我正在尝试动态设计一个协议测试。
我需要使用的函数是go-ethereum的Decode：
https://github.com/ethereum/go-ethereum/blob/master/p2p/message.go#L54
然后我的一些代码使用它：
msg <- receive() //发送给我一个类型为p2p.Msg的消息var message MyTargetTypemsg.Decode(&message) //这个工作正常，这显然是使用函数的正确方式，使用变量的指针anotherMessage := output.Msg //这是一个interface{}，见下面msg.Decode(&anotherMessage) //这个失败了
我不明白为什么Decode方法处理这两个不同。一个小测试程序：
package mainimport ("fmt""reflect")type mystruct struct {hello string}func main() {var first mystructsecond := mystruct{}fmt.Println(reflect.TypeOf(first))fmt.Println(reflect.TypeOf(second))}
这将打印相同的类型：
main.mystructmain.mystruct
但是不知何故，上面的Decode，它在内部使用反射，却以不同的方式处理它们。
我的问题是什么？对于我的协议测试，我想定义预期的输出类型：
type Output struct {Msg interface{}}
由于消息的类型可能非常不同，我认为唯一的方法是使用interface{}。因此：
output := Output{Msg: MyTargetType{}.}//anotherOutput := Output{Msg: AnotherType{}.}//等等
这样，我稍后可以检查接收到的输出是否符合预期。但是，那个Decode方法让我发疯。
我尝试了几种使用反射的方法，例如：
var decodedMsg = reflect.TypeOf(output.Msg)msg.Decode(&decodedMsg)
甚至是
var decodedMsg = reflect.New(reflect.TypeOf(output.Msg)).Elem().Interface()fmt.Println(reflect.TypeOf(decodedMsg)) //这实际上打印出了正确的类型！！！// 但是Decode仍然失败，显示：// 给Decode的接口必须是指针
英文:
I am trying to dynamically design a protocol test.
The function which I need to use is go-ethereum's Decode:
https://github.com/ethereum/go-ethereum/blob/master/p2p/message.go#L54
Then some of my code uses it:
msg <- receive() //sends me a message of type p2p.Msgvar message MyTargetTypemsg.Decode(&message) // this works correctly and this is apparently the correct way to use the function, with a pointer to the variableanotherMessage := output.Msg // this is an interface{}, see belowmsg.Decode(&anotherMessage) // this fails
I don't understand why the Decode method handles the two differently. A little test program:
package mainimport ("fmt""reflect")type mystruct struct {hello string}func main() {var first mystructsecond := mystruct{}fmt.Println(reflect.TypeOf(first))fmt.Println(reflect.TypeOf(second))}
This prints the types to be the same:
main.mystructmain.mystruct
But somehow, Decode above, which uses reflection internally, handles them differently.
What's my problem? For my protocol test, I want to define the type of the output to be expected:
type Output struct {Msg interface{}}
Since the messages can be of very different type, I thought the only way is to use interface{}. Therefore:
output := Output{Msg: MyTargetType{}.}//anotherOutput := Output{Msg: AnotherType{}.}// and so on
so that I then later can check that the output received is the one expected. But that Decode method is driving me crazy.
I have tried several things with reflection, e.g.
var decodedMsg = reflect.TypeOf(output.Msg)msg.Decode(&decodedMsg)
or even
var decodedMsg = reflect.New(reflect.TypeOf(output.Msg)).Elem().Interface()fmt.Println(reflect.TypeOf(decodedMsg)) //this actually prints the correct type!!!// But then Decode fails nonetheless with:// interface given to Decode must be a pointer
答案1
得分: 1
将Decode的参数改为指向目标类型的指针。使用以下代码：
output := Output{Msg: &MyTargetType{}, // 注意指针}msg.Decode(output.Msg)
使用反射API处理任何类型：
output := Output{Msg: MyTargetType{},}...// 分配新值。v := reflect.New(reflect.TypeOf(output.Msg))// 解码到该值。注意v是指向新值的指针。msg.Decode(v.Interface()))// 将新值赋给字段。reflect.ValueOf(&output.Msg).Elem().Set(v.Elem())
英文:
The argument to Decode must be a pointer to the target type. Use this code:
output := Output{Msg: &MyTargetType{}, // note pointer}msg.Decode(output.Msg)
Use the reflect API to handle any type:
output := Output{Msg: MyTargetType{},}...// Allocate new value.v := reflect.New(reflect.TypeOf(output.Msg))// Decode to that value. Note that v has pointer to new value.msg.Decode(v.Interface()))// Assign new value to field.reflect.ValueOf(&output.Msg).Elem().Set(v.Elem())
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