如何知道结构或指向结构的指针是否实现了接口

Question

I need to know if either the struct or the pointer to that struct implements a given interface.

// You can edit this code!
// Click here and start typing.
package main

import "fmt"

func main() {
 var a A = A{
  i: 5,
 }
 Serialize(a)
 Serialize(&a)
}

type Serializable interface {
 //Serialize() string
 //Deserialize(string)

 Serializebyte() []byte
 Deserializebyte(b []byte) (bytesRead int)
}

type A struct {
 i int
}

func (*A) Serializebyte() []byte {
 return []byte{0x00}
}

func (*A) Deserializebyte(b []byte) (bytesRead int) {
 return 0
}

func Serialize(objInt interface{}) []byte {
 // this doesn't work

 switch v := (objInt).(type) {
 case Serializable:
  fmt.Printf("I'm Serializable
")
  return v.Serializebyte()
 }

 fmt.Printf("I'm not Serializable
")
 return []byte{0x00}
}

// this other way also dont work
func Serialize2(objInt interface{}) []byte {
// this doesn't work
    _, isSerializable := objInt.(Serializable)
    if isSerializable{
        fmt.Printf("I'm Serializable
")
        return objInt.(Serializable).Serializebyte()
}

    fmt.Printf("I'm not Serializable
")
    return []byte{0x00}
}



// Stdout:
// I'm not Serializable
// I'm Serializable

Edit: You can run the code above to see what I mean.

Because (*A) implements Serializable not A, the assertion above does not pass but I want to know if either (*A) implements Serializable or A implements it.

Why do I want that? Because if I can do that, the programmers do not need to know how Serializable works. If not the programers should always need to pass a pointer to Serializable and implement Serializable in the struct pointer rather than the struct itself.

Answer 1

It is usually a bad idea to use *T when the user gives you T. All modifies on *T will NOT take effect on user's data.

But if that is what you really want, you can use reflect.

func testFool(a interface{}) bool {
    if _, ok := a.(Fool); ok {
        return true
    }
    t := reflect.PtrTo(reflect.TypeOf(a))
    FoolType := reflect.TypeOf((*Fool)(nil)).Elem()
    return t.Implements(FoolType)
}

Playground: https://play.golang.org/p/rqJe5_KAP6e

EDIT：If you need to make use of that method with pointer reciever, you can use reflect.Value instead of reflect.Type. However, it makes an extra copy of the param.

func testFool(a interface{}) bool {
    if _, ok := a.(Fool); ok {
        return true
    }
    t := reflect.TypeOf(a)
    v := reflect.New(t)
    v.Elem().Set(reflect.ValueOf(a))
    ptrA := v.Interface()
    if foo, ok := ptrA.(Fool); ok {
        foo.Foo()
        return true
    }
    return false
}

It is hackable to write a copy-free version code by using refelct.NewAt and ·reflect.Value.InterfaceData`. But it is highly un-recommended: It will very likely break your future code and is hard to maintain; It uses unsafe package under the hood.