go语言方法Value Receiver, Pointer Receiver各种不同情况的实验笔记

注意： 此文章只是我的个人笔记，如有谬误，错误， 请一定指出！
for range 问题

package main

import (  

    "fmt"

    "time"

)

type field struct {  

    name string

}

func (p *field) print() {  

    fmt.Printf("print: p: %p, v: %s\n", p, p.name)

}

func main() {  

    data := []field{ {"one"},{"two"},{"three"} }

    for _,v := range data { // 注意:for语句中的迭代变量（如: v）在每次迭代时被重新使用, 一直复用

        go v.print() // 注意： 此处可理解为: go (&v).print(), 也就是用v的指针去调用， 而且v会在每次迭代时复用，所以每一个调用的receiver都是共同指向v的指针， 而且v在最后一次迭代后， 被 赋值为:"three", 所以 才有了打印出3个"three"的结果.

    }

    time.Sleep(3 * time.Second)

    //goroutines print: three, three, three

}

package main

import (  

    "fmt"

    "time"

)

type field struct {  

    name string

}

func (p *field) print() {  

    fmt.Println(p.name)

}

func main() {  

    data := []*field{ {"one"},{"two"},{"three"} }

    for _,v := range data {

        go v.print() //v本身就是指针， 指向one, two, three; 迭代时会改变指向， 直接调用没有复制，直接调用，故每次调用时v都是分别指向one, two , three的地址值 ， 当然会打印出正确的结果 ; 与上例不同， 上例中，因为v的复用，每一次调用的pointer receiver都共同指向v, 但v的值在循环后最终赋值为:three, 所以出现错误结果。

    }

    time.Sleep(3 * time.Second)

}

//goroutines print: one, two, three

前提条件：方法定义时为Pointer receiver.

(2)满足前提条件下，若for _,v := range data中的v本身就是指针， 则直接调用.

//stackoverflow中的解释：

这在Go中是个很常见的技巧。for语句中的迭代变量在每次迭代时被重新使用。这就意味着你在for循环中创建的闭包（即函数字面量）将会引用同一个变量（而在那些goroutine开始执行时就会得到那个变量的值）。

In the first loop,v is thevalue of afield item. Becausev is addressable, it is automatically referenced as the pointer receiver for theprint() method. Sov.print() is using the address ofv itself, and the contents of that address is overwritten each iteration of the loop.

When you change the declaration to use a*field,v is now a pointer to afield value. When you callv.print() in this case, you are operating on the value thatv points to, which is stored indata, and the overwriting of v has no effect.

----------------------------------------------------我的试验代码---------------------------------

package main

import "fmt"

import "time"

type A int

func (a A) ValueReceiver(){

fmt.Printf("ValueReceiver, p: %p, v: %d\n", &a, a)

}

func (a A) ValueReceiverIngo(){

fmt.Printf("ValueReceiverIngo, p: %p, v:%d\n", &a, a)

}

func (a A) ValueReceiverInDefer(){

fmt.Printf("ValueReceiverInDefer, p: %p, v:%d\n", &a, a)

}

func (a A) ValueRececiverInforRange(){

fmt.Printf("ValueRececiverInforRange, p: %p, v: %d\n", &a, a)

}

func main() {

var a A = 1

fmt.Printf("main, p: %p, v: %d\n", &a, a)

a.ValueReceiver()

p := &a

p.ValueReceiver()

//------------call in goroutine--------

go a.ValueReceiverIngo()

go p.ValueReceiverIngo()

time.Sleep(3* time.Second)

//---------call in defer----------

defer a.ValueReceiverInDefer()

defer p.ValueReceiverInDefer()

//---call in for range array, value receiver---

as := [5]A{1,2,3,4,5}

fmt.Printf("as[0]: %p, %d\n", &as[0], as[0])

for _, a := range as {

fmt.Printf("as in for: %p, %d\n", &as[0], as[0])

fmt.Printf("a in for: %p, v: %d\n", &a, a)

a.ValueRececiverInforRange()

pf := &a

pf.ValueRececiverInforRange()

}

}

//结果

main, p: 0x10434114, v: 1
ValueReceiver, p: 0x1043411c, v: 1
ValueReceiver, p: 0x10434134, v: 1
ValueReceiverIngo, p: 0x1043413c, v:1
ValueReceiverIngo, p: 0x10434144, v:1
as[0]: 0x10430240, 1
as in for: 0x10430240, 1
a in for: 0x10434150, v: 1
ValueRececiverInforRange, p: 0x1043415c, v: 1
ValueRececiverInforRange, p: 0x10434164, v: 1
as in for: 0x10430240, 1
a in for: 0x10434150, v: 2
ValueRececiverInforRange, p: 0x10434174, v: 2
ValueRececiverInforRange, p: 0x1043417c, v: 2
as in for: 0x10430240, 1
a in for: 0x10434150, v: 3
ValueRececiverInforRange, p: 0x1043418c, v: 3
ValueRececiverInforRange, p: 0x10434194, v: 3
as in for: 0x10430240, 1
a in for: 0x10434150, v: 4
ValueRececiverInforRange, p: 0x104341a4, v: 4
ValueRececiverInforRange, p: 0x104341ac, v: 4
as in for: 0x10430240, 1
a in for: 0x10434150, v: 5
ValueRececiverInforRange, p: 0x104341bc, v: 5
ValueRececiverInforRange, p: 0x104341c4, v: 5
ValueReceiverInDefer, p: 0x104341cc, v:1
ValueReceiverInDefer, p: 0x104341d4, v:1

结论：通过分析以上地址， 对于value receciver method， 各种调用方式下， 都是对于原值的复制， 也就是说，以副本为receiver调用， 即使是以指针方式调用，也是以*pointer 生成副本后再调用 ；同时也注意到for range中复用了a (for _, a := range as ).--------------------------------------------------------------------------------------

-------------------------------------Pointer Receiver Test--------------------------

package main

import "fmt"

import "time"

type A int

func(p *A)PointerReceiver(){

fmt.Printf("PointerReceiver, p: %p, v: %d\n", p, *p)

}

func(p *A)PointerReceiveringo(){

fmt.Printf("PointerReceiveringo, p: %p, v: %d\n", p, *p)

}

func(p *A)PointerReceiverinDefer(){

fmt.Printf("PointerReceiverinDefer, p: %p, v: %d\n", p, *p)

}

func(p *A)PointerReceiverinforRange(){

fmt.Printf("PointerReceiverinforRange, p: %p, v: %d\n", p, *p)

}

func main() {

var a A = 1

fmt.Printf("main, p: %p, v: %d\n", &a, a)

a.PointerReceiver()

p := &a

p.PointerReceiver()

//------------------

go a.PointerReceiveringo()

go p.PointerReceiveringo()

time.Sleep(3* time.Second)

//------------------

defer a.PointerReceiverinDefer()

defer p.PointerReceiverinDefer()

//------------------

as := [5]A{1,2,3,4,5}

fmt.Printf("as[0], p: %p, v: %d\n", &as[0], as[0])

for _, a := range as {

fmt.Printf("as in for: p: %p, v: %d\n", &as[0], as[0])

fmt.Printf("a in for: p: %p, v: %d\n", &a, a)

a.PointerReceiverinforRange()

p := &a

p.PointerReceiverinforRange()

}

}

结果：

main, p: 0x10434114, v: 1
PointerReceiver, p: 0x10434114, v: 1
PointerReceiver, p: 0x10434114, v: 1
PointerReceiveringo, p: 0x10434114, v: 1
PointerReceiveringo, p: 0x10434114, v: 1
as[0], p: 0x10430240, v: 1
as in for: p: 0x10430240, v: 1
a in for: p: 0x10434140, v: 1
PointerReceiverinforRange, p: 0x10434140, v: 1
PointerReceiverinforRange, p: 0x10434140, v: 1
as in for: p: 0x10430240, v: 1
a in for: p: 0x10434140, v: 2
PointerReceiverinforRange, p: 0x10434140, v: 2
PointerReceiverinforRange, p: 0x10434140, v: 2
as in for: p: 0x10430240, v: 1
a in for: p: 0x10434140, v: 3
PointerReceiverinforRange, p: 0x10434140, v: 3
PointerReceiverinforRange, p: 0x10434140, v: 3
as in for: p: 0x10430240, v: 1
a in for: p: 0x10434140, v: 4
PointerReceiverinforRange, p: 0x10434140, v: 4
PointerReceiverinforRange, p: 0x10434140, v: 4
as in for: p: 0x10430240, v: 1
a in for: p: 0x10434140, v: 5
PointerReceiverinforRange, p: 0x10434140, v: 5
PointerReceiverinforRange, p: 0x10434140, v: 5
PointerReceiverinDefer, p: 0x10434114, v: 1
PointerReceiverinDefer, p: 0x10434114, v: 1

结论： 通过以上地址的分析， 对于pointer receiver method， 在调用时， 不会生成副本，

而是原对象本身的地址；也就是说没有复制; 在（for _, a := range as） 中的a会被复用。

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参考资料：

注意： 此文章只是我个人笔记， 如有错漏，请一定指正， 共同学习， 我的邮箱: htyu_0203_39@sina.com

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