for-range循环中指针和值切片之间的差异

Question

请检查以下代码段:

package main

import (  
    "fmt"
    "time"
)

type field struct {  
    name string
}

func (p *field) print() {  
    fmt.Println(p.name)
}

func main() {
    fmt.Println("use values:")

    // use values in range loop and go rountines
    values := []field{{"one"},{"two"},{"three"}}
    for _, v := range values {
        go v.print()
    }

    time.Sleep(time.Second)

    fmt.Println()
    fmt.Println("use pointers:")

    // use pointers in range loop and go rountines
    poniters := []*field{{"one"},{"two"},{"three"}}
    for _, v := range poniters {
        go v.print()
    }

    time.Sleep(time.Second)
}

链接到这里:https://play.golang.org/p/cdryPmyWt5

上面的代码将检查for循环中指针和值之间的差异,而go语句也同时使用.代码:

values := []field{{"one"},{"two"},{"three"}}
for _, v := range values {
    go v.print()
}

我们知道控制台将打印三个三三作为结果,因为for循环在goroutines开始执行之前运行到它的末尾,将v写为切片的最后一个元素.但指针怎么样？

poniters := []*field{{"one"},{"two"},{"three"}}
for _, v := range poniters {
    go v.print()
}

它似乎打印一两三,为什么？

谢谢.

Answer 1

答:在调用函数之前评估参数.在评估它们之后,调用的参数将通过值传递给函数,并且被调用的函数开始执行,因此:

第一go v.print()是语法糖go (*field).print(&v)和
所述第二go v.print()是语法糖go (*field).print(v).

如果第一个for循环在goroutines启动之前完成,&v则对于调用是相同的,并且这三个调用都是相同的.通过time.Sleep(100)在go v.print()第一个循环之后添加来查看代码2 .或go func(v field) { v.print() }(v)在The Go Playground(代码3 sync.WaitGroup).
此外,您在这里有数据竞争(见B).

而对于第二个,go (*field).print(v)这里v是指针和三个goroutines参数在调用之前被评估print并具有三个不同的地址.

1-在The Go Playground上试试这个:

package main

import (
    "fmt"
    "time"
)

type field struct {
    name string
}

func (p *field) print() {
    fmt.Println(p.name)
}

func main() {
    fmt.Println("use values:")

    // use values in range loop and go rountines
    values := []field{{"one"}, {"two"}, {"three"}}
    for _, v := range values {
        fmt.Println(&v)
        go (*field).print(&v) //go v.print()
    }

    time.Sleep(time.Second)

    fmt.Println()
    fmt.Println("use pointers:")

    // use pointers in range loop and go rountines
    poniters := []*field{{"one"}, {"two"}, {"three"}}
    for _, v := range poniters {
        fmt.Println(v)
        go (*field).print(v) //go v.print()
    }

    time.Sleep(time.Second)
}

输出:

use values:
&{one}
&{two}
&{three}
three
three
three

use pointers:
&{one}
&{two}
&{three}
two
one
three

2-在The Go Playground尝试这个:

package main

import (
    "fmt"
    "time"
)

type field struct {
    name string
}

func (p *field) print() {
    fmt.Println(p.name)
}

func main() {
    fmt.Println("use values:")

    // use values in range loop and go rountines
    values := []field{{"one"}, {"two"}, {"three"}}
    for _, v := range values {
        fmt.Println(&v)
        go v.print() //go (*field).print(&v) //
        time.Sleep(100)
    }

    time.Sleep(time.Second)

    fmt.Println()
    fmt.Println("use pointers:")

    // use pointers in range loop and go rountines
    poniters := []*field{{"one"}, {"two"}, {"three"}}
    for _, v := range poniters {
        fmt.Println(v)
        go v.print() //go (*field).print(v) //
    }

    time.Sleep(time.Second)
}

输出:

use values:
&{one}
one
&{two}
two
&{three}
three

use pointers:
&{one}
&{two}
&{three}
one
two
three

B:你有数据竞争,尝试go build -race 你的代码,然后运行生成的文件,WARNING: DATA RACE:

输出:

use values:
==================
WARNING: DATA RACE
Read at 0x00c042030210 by goroutine 6:
  runtime.convT2E()
      Go/src/runtime/iface.go:155 +0x0
  main.(*field).print()
      .../m.go:14 +0x6c

Previous write at 0x00c042030210 by main goroutine:
  main.main()
      .../m.go:22 +0x1c3

Goroutine 6 (running) created at:
  main.main()
      .../m.go:23 +0x204
==================
two
three
three

use pointers:
one
two
three
Found 1 data race(s)