在golang的struct中嵌入的接口感到困惑

Question

Like code here, when embedding an interface A into struct B, and then set A to aa which is an instance of AA. Both B and AA have element X, when call b.X I just got B.X. How can I get b.AA.X? I know this syntax is wired, but I just want to figure out how var b is stored in the memory, I tried some unsafe syntax, no way to get b.A.X.:

package main

import (
    "fmt"
    "unsafe"
)

type A interface {
    Hello() string
}

type B struct {
    A
    X string
}

type AA struct {
    num int
    X   string
}

func (aa AA) Hello() string {
    return fmt.Sprintf("hello %d from %s", aa.num, aa.X)
}

func main() {
    aa := AA{200, "golang"}
    b := B{A: aa, X: "python"}
    fmt.Println(b.X)                                          // output: python
    fmt.Printf("--->%d
", ((*AA)(unsafe.Pointer(&b.A)).num)) // output:  --->17667104
    fmt.Printf("===>%+v
", b.A)                              // output: ===>{num:200 X:golang}
}

Answer 1

Embedded fields are accessed by their type name, and fields of an interface are only accessible by asserting the underlying type, so b.A.(AA).X will get you field X of the underlying AA that's in b.A. But if you're needing to access fields, you probably shouldn't be embedding the interface type anyway, you should be embedding the concrete type.