为什么％v为嵌套结构打印出意外的值？

Question

When printing a struct with nested struct which has a String() implemented, the %v format prints an 'unexpected' value per our understanding.

Below is the code snippet.

package main

import (
  "fmt"
)

type Inner struct {
}

type A struct {
    Inner
    FieldA string
}

func (i Inner) String() string {
    return "anything"
}

func main() {
    myA := A{FieldA: "A"}
    fmt.Printf("%v", myA)
}

We expect the output to be

{anything A}

But the actual result is

anything

Why this result? It seems FieldA is ignored? To make it more confusing, if we have two nested structs where both of them have String() implemented, the output is expected.

package main

import (
  "fmt"
)

type Inner struct {
}

type InnerAgain struct {
}

type A struct {
    Inner
    InnerAgain
    FieldA string
}

func (i Inner) String() string {
    return "anything"
}

func (i InnerAgain) String() string {
    return "nothing"
}

func main() {
    myA := A{FieldA: "A"}
    fmt.Printf("%v", myA)
}

The output is

{anything nothing A}

...

Answer 1

Since you are embedding Inner, you inherent all its properties - including the String() - function. So %v is actually calling Inner.String()

From the docs (https://golang.org/pkg/fmt/) :

1. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).