在Go中什麼時候使用指針？

1 在方法中使用指針

什麼是receiver？

func (t T) method_name(t T){}      

這裡面的T就是receiver

• 使用receiver作為方法參數

func main() {
   r := receiver{Name: "zs"}
   fmt.Println(r)
   r.methodA()
   fmt.Println(r)
}

type receiver struct {
   Id   int
   Name string
   Age  int
}

func (receiver receiver) methodA() {
   receiver.Name = "ls"
}

func (receiver *receiver) methodB() {
   receiver.Name = "ls"
}      

結果：

{0 zs 0}

{0 zs 0}

• 使用*receiver作為方法參數

func main() {
   r := receiver{Name: "zs"}
   fmt.Println(r)
   r.methodB()
   fmt.Println(r)
}

type receiver struct {
   Id   int
   Name string
   Age  int
}

func (receiver receiver) methodA() {
   receiver.Name = "ls"
}

func (receiver *receiver) methodB() {
   receiver.Name = "ls"
}      

結果：

{0 zs 0}

{0 ls 0}

2 在結構體中使用指針

• 方式一

func main() {
   student := Student{Map: map[string]int{"S": 0}, ReceiverA: receiver{Name: "A"}, ReceiverB: &receiver{Name: "B"}}
   fmt.Println(student, *student.ReceiverB)
   student.updateA()
   fmt.Println(student, *student.ReceiverB)
}

type receiver struct {
   Id   int
   Name string
   Age  int
}

type Student struct {
   No        int
   Map       map[string]int
   ReceiverA receiver
   ReceiverB *receiver
}

func (stu Student) updateA() {
   stu.Map["a"] = 1
   stu.ReceiverA = receiver{Name: "ww"}
   stu.ReceiverB = &receiver{Name: "ww"}
}

func (stu *Student) updateB() {
   stu.Map["b"] = 2
   stu.ReceiverA = receiver{Name: "ww"}
   stu.ReceiverB = &receiver{Name: "ww"}
}      

結果：

{0 map[S:0] {0 A 0} 0xc0000b4000} {0 B 0}

{0 map[S:0 a:1] {0 A 0} 0xc0000b4000} {0 B 0}

• 方式二

func main() {
   student := Student{Map: map[string]int{"S": 0}, ReceiverA: receiver{Name: "A"}, ReceiverB: &receiver{Name: "B"}}
   fmt.Println(student, *student.ReceiverB)
   student.updateA()
   fmt.Println(student, *student.ReceiverB)
}

type receiver struct {
   Id   int
   Name string
   Age  int
}

type Student struct {
   No        int
   Map       map[string]int
   ReceiverA receiver
   ReceiverB *receiver
}

func (stu Student) updateA() {
   stu.Map["a"] = 1
   stu.ReceiverA = receiver{Name: "ww"}
   stu.ReceiverB = &receiver{Name: "ww"}
}

func (stu *Student) updateB() {
   stu.Map["b"] = 2
   stu.ReceiverA = receiver{Name: "ww"}
   stu.ReceiverB = &receiver{Name: "ww"}
}      

結果：

{0 map[S:0] {0 A 0} 0xc0000b4000} {0 B 0}

{0 map[S:0 b:2] {0 ww 0} 0xc0000b4060} {0 ww 0}

3 什麼時候使用指針

一個函數何時該用指針類型做receiver對初學者而言一直是個頭疼的問題。下面是列舉了一些常用的判斷指導。

• 如果receiver是​

  ​map​

  ​​、​

  ​func​

  ​​或者​

  ​chan​

  ​，不要使用指針
• 如果receiver是​

  ​slice​

  ​并且該函數并不會修改此slice，不要使用指針
• 如果該函數會修改receiver，此時一定要用指針
• 如果receiver是​

  ​struct​

  ​​并且包含互斥類型​

  ​sync.Mutex​

  ​，或者是類似的同步變量，receiver必須是指針，這樣可以避免對象拷貝
• 如果receiver是較大的​

  ​struct​

  ​​或者​

  ​array​

  ​，使用指針則更加高效。多大才算大？假設struct内所有成員都要作為函數變量傳進去，如果覺得這時資料太多，就是struct太大
• 如果receiver是​

  ​struct​

  ​​，​

  ​array​

  ​​或者​

  ​slice​

  ​，并且其中某個element指向了某個可變量，則這個時候receiver選指針會使代碼的意圖更加明顯
• 如果receiver使較小的​

  ​struct​

  ​​或者​

  ​array​

  ​​，并且其變量都是些不變量、常量，例如​

  ​time.Time​

  ​，value receiver更加适合，因為value receiver可以減少需要回收的垃圾量。
• 最後，如果不确定用哪個，使用指針類的receiver

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