如何使用欄位的 String() 方法輕鬆列印 Go 結構，而不需要為每個結構實作 String() ？

使用 String() 列印結構體欄位

此程式碼：

package main

import (
    "fmt"
    "time"
)

type A struct {
    t time.Time
}

func main() {
    a := A{time.Now()}
    fmt.Println(a)
    fmt.Println(a.t)
}
````
generates this output:

{{633930800 0x206da0}}
2009-11-10 23:00:00 0000 UTC

`A` doesn't implement `String()`, so it doesn't conform to the `fmt.Stringer` interface and presents its internal representation. Implementing `String()` for every struct can be tedious, especially upon modifications to those structs. Is there a simpler method to print structs using their fields' `String()` methods?

**The Custom Print Function**

The fmt package's behavior in this case is intrinsic and cannot be changed. However, a helper function can be crafted using reflection (provided by the `reflect` package). The function iterates through a struct's fields and invokes their `String()` methods if they exist:

func PrintStruct(s 介面{}, 名稱 bool) string {

v := reflect.ValueOf(s)
t := v.Type()
if t.Kind() != reflect.Struct {
    return fmt.Sprint(s)
}

var b bytes.Buffer
b.WriteString("{")
for i := 0; i < v.NumField(); i++ {
    if i > 0 {
        b.WriteString(" ")
    }
    v2 := v.Field(i)
    if names {
        b.WriteString(t.Field(i).Name)
        b.WriteString(":")
    }
    if v2.CanInterface() {
        if st, ok := v2.Interface().(fmt.Stringer); ok {
            b.WriteString(st.String())
            continue
        }
    }
    fmt.Fprint(&b, v2)
}
b.WriteString("}")
return b.String()

如果需要，可以將 String() 方法加入結構體中，呼叫 PrintStruct()函數：

**Integrating the Helper Function**

With the custom function in place, structs can be printed by:

```go
fmt.Println(PrintStruct(a, true))

由於我們使用反射，導出結構體欄位以便自訂函數可存取至關重要（為自訂函數添加了附加欄位）測試目的）：

func (a A) String() string {
    return PrintStruct(a, true)
}

實用範例

type A struct {
    T          time.Time
    I          int
    unexported string
}

完整程式碼：

測試此程式碼會產生：

package main

import (
    "fmt"
    "reflect"
    "strings"
    "time"
)

type A struct {
    T          time.Time
    I          int
    unexported string
}

func PrintStruct(s interface{}, names bool) string {
    v := reflect.ValueOf(s)
    t := v.Type()
    if t.Kind() != reflect.Struct {
        return fmt.Sprint(s)
    }

    var b bytes.Buffer
    b.WriteString("{")
    for i := 0; i < v.NumField(); i++ {
        if i > 0 {
            b.WriteString(" ")
        }
        v2 := v.Field(i)
        if names {
            b.WriteString(t.Field(i).Name)
            b.WriteString(":")
        }
        if v2.CanInterface() {
            if st, ok := v2.Interface().(fmt.Stringer); ok {
                b.WriteString(st.String())
                continue
            }
        }
        fmt.Fprint(&b, v2)
    }
    b.WriteString("}")
    return b.String()
}

func (a A) String() string {
    return PrintStruct(a, true)
}

func main() {
    a := A{time.Now(), 2, "hi!"}
    fmt.Println(a)
    fmt.Println(PrintStruct(a, true))
    fmt.Println(PrintStruct(a, false))
    fmt.Println(PrintStruct("I'm not a struct", true))
}

測試此程式碼會產生：

{{63393490800 0 0x206da0}}
2009-11-10 23:00:00 +0000 UTC
{T:2009-11-10 23:00:00 +0000 UTC I:2 unexported:hi!}
{T:2009-11-10 23:00:00 +0000 UTC I:2 unexported:hi!}
{2009-11-10 23:00:00 +0000 UTC 2 hi!}
I'm not a struct

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