为什么fmt.Println结构体不使用其成员的String（）方法

package main

import (
    "fmt"
)

type bar struct {
}

func (b bar) String() string {
    return "bar"
}

type foo struct {
    b []*bar
    bb *bar
}

func main() {
    f := foo{b: []*bar{&bar{}}, bb:&bar{}}
    fmt.Println(f, f.b, f.bb)
}

Why the result is

{[0x176f44] 0x176f44} [bar] bar

Not

{[bar] bar} [bar] bar

Are there any reasons behind it? It seems easy to implement and good for readability.

You have several problems in your code. You define Stirng on bar which is unexported, your fields are unexported as well. This works:

type Bar struct {
}

func (b Bar) String() string {
    return "bar"
}

type foo struct {
    B  []Bar
    BB Bar
}

func main() {
    f := foo{B: []Bar{Bar{}}, BB: Bar{}}
    fmt.Println(f)
}

Playground: https://play.golang.org/p/OhoIcB7cA3.

This would also work with *Bar.