为什么fmt.Println结构体不使用其成员的String（）方法

Question

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.

Answer 1

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.