标准库fmt包中定义了

// Stringer is implemented by any value that has a String method,
// which defines the ``native'' format for that value.
// The String method is used to print values passed as an operand
// to any format that accepts a string or to an unformatted printer
// such as Print.
type Stringer interface {
    String() string
}

任何对象定义了String方法，即可以Stringer 类型，fmt、log 进行print输出时会识别为Stringer类型，调用String 方法进行打印输出，即我们可以利用String来进行定制化输出。

示例一 结构体Marshal打印

对于结构体打印，直接调用Println,只能打印其值，如下。

示例代码：

package main

import (
    "log"
)

type test struct {
    X string
    Y int
}

func main() {
    a := test{X: "test", Y: 11}
    log.Println(a)
}

输出：

2019/03/20 13:04:06 {test 11}

现定义String方法，实现Marshal打印

示例代码：

package main

import (
    "encoding/json"
    "log"
)

type test struct {
    X string
    Y int
}

func (test test) String() string {
    out, err := json.Marshal(test)
    if err != nil {
        return err.Error()
    }
    return string(out)
}

func main() {
    a := test{X: "test", Y: 11}
    log.Println(a)
}

输出结果

2019/03/20 13:08:02 {"X":"test","Y":11}

示例二 指针打印其指向内容

示例代码：

package main

import (
    "log"
)

type test struct {
    X string
    Y int
}

func main() {
    a := &test{X: "a", Y: 1}
    b := &test{X: "b", Y: 12}
    var testarr []*test
    testarr = append(testarr, a)
    testarr = append(testarr, b)
    log.Println(testarr)
}

输出：

2019/03/20 13:14:06 [0xc042002440 0xc042002460]

现定义String方法，实现打印指针内容

示例代码：

package main

import (
    "encoding/json"
    "log"
)

type test struct {
    X string
    Y int
}

func (test test) String() string {
    out, err := json.Marshal(test)
    if err != nil {
        return err.Error()
    }
    return string(out)
}

func main() {
    a := &test{X: "a", Y: 1}
    b := &test{X: "b", Y: 12}
    var testarr []*test
    testarr = append(testarr, a)
    testarr = append(testarr, b)
    log.Println(testarr)
}

输出结果：

2019/03/20 13:15:03 [{"X":"a","Y":1} {"X":"b","Y":12}]