Golang 要注意的陷阱和常见错误

原文: 50 Shades of Go: Traps, Gotchas, and Common Mistakes for New Golang Devs
翻译: Go的50度灰：新Golang开发者要注意的陷阱、技巧和常见错误 , 译者: 影风LEY

GoGo

Go

一. 初级篇

1. 开大括号不能放在单独的一行

GoGo

package main
import "fmt"
func main()  
{ //error, can't have the opening brace on a separate line
    fmt.Println("hello there!")
}

编译错误：

/tmp/sandbox826898458/main.go:6: syntax error: unexpected semicolon or newline before {

有效的例子：

package main
import "fmt"
func main() {  
    fmt.Println("works!")
}

2. 未使用的变量

如果你有未使用的变量，代码将编译失败。当然也有例外。在函数内一定要使用声明的变量，但未使用的全局变量是没问题的。
如果你给未使用的变量分配了一个新的值，代码还是会编译失败。你需要在某个地方使用这个变量，才能让编译器愉快的编译。
Fails:

package main
var gvar int //not an error
func main() {  
    var one int   //error, unused variable
    two := 2      //error, unused variable
    var three int //error, even though it's assigned 3 on the next line
    three = 3     
}

Compile Errors:

/tmp/sandbox473116179/main.go:6: one declared and not used 
/tmp/sandbox473116179/main.go:7: two declared and not used 
/tmp/sandbox473116179/main.go:8: three declared and not used

Works:

package main
import "fmt"
func main() {  
    var one int
    _ = one
    two := 2 
    fmt.Println(two)
    var three int 
    three = 3
    one = three
    var four int
    four = four
}

另一个选择是注释掉或者移除未使用的变量 ：-）

3. 未使用的 import

goimports

$ go get golang.org/x/tools/cmd/goimports

_

package main
import (  
    "fmt"
    "log"
    "time"
)
func main() {  
}

Compile Errors:

/tmp/sandbox627475386/main.go:4: imported and not used: "fmt" 
/tmp/sandbox627475386/main.go:5: imported and not used: "log" 
/tmp/sandbox627475386/main.go:6: imported and not used: "time"

Works:

package main
import (  
    _ "fmt"
    "log"
    "time"
)
var _ = log.Println
func main() {  
    _ = time.Now
}

import

4. 简式的变量声明仅可以在函数内部使用

Fails:

package main
myvar := 1 //error
func main() {  
}

Compile Error:

/tmp/sandbox265716165/main.go:3: non-declaration statement outside function body

Works:

package main
var myvar = 1
func main() {  
}

5. 使用简式声明重复声明变量

你不能在一个单独的声明中重复声明一个变量，但在多变量声明中这是允许的，其中至少要有一个新的声明变量。
重复变量需要在相同的代码块内，否则你将得到一个隐藏变量。
Fails:

package main
func main() {  
    one := 0
    one := 1 //error
}

Compile Error:

/tmp/sandbox706333626/main.go:5: no new variables on left side of :=

Works:

package main
func main() {  
    one := 0
    one, two := 1,2
    one,two = two,one
}

6. 偶然的变量隐藏 Accidental Variable Shadowing

短式变量声明的语法如此的方便（尤其对于那些使用过动态语言的开发者而言），很容易让人把它当成一个正常的分配操作。如果你在一个新的代码块中犯了这个错误，将不会出现编译错误，但你的应用将不会做你所期望的事情。

package main
import "fmt"
func main() {  
    x := 1
    fmt.Println(x)     //prints 1
    {
        fmt.Println(x) //prints 1
        x := 2
        fmt.Println(x) //prints 2
    }
    fmt.Println(x)     //prints 1 (bad if you need 2)
}

vetvet-shadowgo tool vet -shadow your_file.go

7. 不使用显式类型，无法使用“nil”来初始化变量

nilinterfacemapssliceschannels

package main
func main() {  
    var x = nil //error
    _ = x
}

Compile Error:

/tmp/sandbox188239583/main.go:4: use of untyped nil

Works:

package main
func main() {  
    var x interface{} = nil
    _ = x
}

8. 使用“nil” Slices and Maps

nilslicemappanic

package main
func main() {  
    var s []int
    s = append(s,1)
}

Fails:

package main
func main() {  
    var m map[string]int
    m["one"] = 1 //error
}

9. map的容量

mapmapcap()

package main
func main() {  
    m := make(map[string]int,99)
    cap(m) //error
}

Compile Error:

/tmp/sandbox326543983/main.go:5: invalid argument m (type map[string]int) for cap

10. 字符串不会为nil

nil

package main
func main() {  
    var x string = nil //error
    if x == nil { //error
        x = "default"
    }
}

Compile Errors:

/tmp/sandbox630560459/main.go:4: cannot use nil as type string in assignment /tmp/sandbox630560459/main.go:6: invalid operation: x == nil (mismatched types string and nil)

Works:

package main
func main() {  
    var x string //defaults to "" (zero value)
    if x == "" {
        x = "default"
    }
}

11. array 函数的参数

CC++

Go

package main
import "fmt"
func main() {  
    x := [3]int{1,2,3}
    func(arr [3]int) {
        arr[0] = 7
        fmt.Println(arr) //prints [7 2 3]
    }(x)
    fmt.Println(x) //prints [1 2 3] (not ok if you need [7 2 3])
}

如果你需要更新原始数组的数据，你可以使用数组指针类型。

package main
import "fmt"
func main() {  
    x := [3]int{1,2,3}
    func(arr *[3]int) {
        (*arr)[0] = 7
        fmt.Println(arr) //prints &[7 2 3]
    }(&x)
    fmt.Println(x) //prints [7 2 3]
}

sliceslice

package main
import "fmt"
func main() {  
    x := []int{1,2,3}
    func(arr []int) {
        arr[0] = 7
        fmt.Println(arr) //prints [7 2 3]
    }(x)
    fmt.Println(x) //prints [7 2 3]
}

12. 在 slice 和 array 使用“range”语句时的出现的不希望得到的值

for-inforeachGorange

package main
import "fmt"
func main() {  
    x := []string{"a","b","c"}
    for v := range x {
        fmt.Println(v) //prints 0, 1, 2
    }
}

Good:

package main
import "fmt"
func main() {  
    x := []string{"a","b","c"}
    for _, v := range x {
        fmt.Println(v) //prints a, b, c
    }
}

13. slices 和 arrays 是一维的

GoArraySliceGo

你可以使用纯一维数组、“独立”切片的切片，“共享数据”切片的切片来构建动态的多维数组。
如果你使用纯一维的数组，你需要处理索引、边界检查、当数组需要变大时的内存重新分配。

slicesliceslicesliceslice

package main
func main() {  
    x := 2
    y := 4
    table := make([][]int,x)
    for i:= range table {
        table[i] = make([]int,y)
    }
}

slicesliceslicesliceslice

package main
import "fmt"
func main() {  
    h, w := 2, 4
    raw := make([]int,h*w)
    for i := range raw {
        raw[i] = i
    }
    fmt.Println(raw,&raw[4])
    //prints: [0 1 2 3 4 5 6 7] <ptr_addr_x>
    table := make([][]int,h)
    for i:= range table {
        table[i] = raw[i*w:i*w + w]
    }
    fmt.Println(table,&table[1][0])
    //prints: [[0 1 2 3] [4 5 6 7]] <ptr_addr_x>
}

arrayslice

14. 访问不存在的 map keys

nilnilnilmapmapfalsemapmap

package main
import "fmt"
func main() {  
    x := map[string]string{"one":"a","two":"","three":"c"}
    if v := x["two"]; v == "" { //incorrect
        fmt.Println("no entry")
    }
}

Good:

package main
import "fmt"
func main() {  
    x := map[string]string{"one":"a","two":"","three":"c"}
    if _,ok := x["two"]; !ok {
        fmt.Println("no entry")
    }
}

15. Strings 无法修改

stringbyte slicebyte slicestring

package main
import "fmt"
func main() {  
    x := "text"
    x[0] = 'T'
    fmt.Println(x)
}

Compile Error:

/tmp/sandbox305565531/main.go:7: cannot assign to x[0]

Works:

package main
import "fmt"
func main() {  
    x := "text"
    xbytes := []byte(x)
    xbytes[0] = 'T'
    fmt.Println(string(xbytes)) //prints Text
}

stringbytestringrune slicerune sliceruneGobyte

16. String 和 Byte Slice之间的转换

byte sliceslicebyte

Go[]bytestringstring[]byte

[]bytemap[string]m[string(key)][]bytefor rangefor i,v := range []byte(str) {...}

17. String 和索引操作

byte

package main
import "fmt"
func main() {  
    x := "text"
    fmt.Println(x[0]) //print 116
    fmt.Printf("%T",x[0]) //prints uint8
}

如果你需要访问特定的字符串“字符”（unicode编码的points/runes），使用for range。官方的“unicode/utf8”包和实验中的utf8string包（golang.org/x/exp/utf8string）也可以用。utf8string包中包含了一个很方便的At()方法。把字符串转换为rune的切片也是一个选项。

18. 字符串不总是 UTF8 文本

UTF8string literalUTF8UTF8unicode/utf8ValidString()

package main
import (  
    "fmt"
    "unicode/utf8"
)
func main() {  
    data1 := "ABC"
    fmt.Println(utf8.ValidString(data1)) //prints: true
    data2 := "A\xfeC"
    fmt.Println(utf8.ValidString(data2)) //prints: false
}

19. 字符串的长度

Python

data = u'♥'  
print(len(data)) #prints: 1

Go

package main
import "fmt"
func main() {  
    data := "♥"
    fmt.Println(len(data)) //prints: 3
}

len()bytePythonunicodeGounicode/utf8RuneCountInString()

package main
import (  
    "fmt"
    "unicode/utf8"
)
func main() {  
    data := "♥"
    fmt.Println(utf8.RuneCountInString(data)) //prints: 1
}

RuneCountInString()rune

package main
import (  
    "fmt"
    "unicode/utf8"
)
func main() {  
    data := "é"
    fmt.Println(len(data))                    //prints: 3
    fmt.Println(utf8.RuneCountInString(data)) //prints: 2
}

20. 在多行的 Slice、Array 和 Map 语句中遗漏逗号

Fails:

package main
func main() {  
    x := []int{
,
 //error
    }
    _ = x
}

Compile Errors:

/tmp/sandbox367520156/main.go:6: syntax error: need trailing comma before newline in composite literal /tmp/sandbox367520156/main.go:8: non-declaration statement outside function body /tmp/sandbox367520156/main.go:9: syntax error: unexpected }

Works:

package main
func main() {  
    x := []int{
,
,
    }
    x = x
    y := []int{3,4,} //no error
    y = y
}

当你把声明折叠到单行时，如果你没加末尾的逗号，你将不会得到编译错误。

21. log.Fatal 和 log.Panic 不仅仅是 Log

logginglogloggingGologFatal*()Panic*()logGo

package main
import "log"
func main() {  
    log.Fatalln("Fatal Level: log entry") //app exits here
    log.Println("Normal Level: log entry")
}

22. 内建的数据结构操作不是同步的

Gogoroutineschannelssync

23. String在“range”语句中的迭代值

rangeunicodepoint/runebyterune

stringfor rangeUTF8byteunicodeUTF8stringbyte slice

package main
import "fmt"
func main() {  
    data := "A\xfe\x02\xff\x04"
    for _,v := range data {
        fmt.Printf("%#x ",v)
    }
    //prints: 0x41 0xfffd 0x2 0xfffd 0x4 (not ok)
    fmt.Println()
    for _,v := range []byte(data) {
        fmt.Printf("%#x ",v)
    }
    //prints: 0x41 0xfe 0x2 0xff 0x4 (good)
}

24. 对Map使用“for range”语句迭代

mapGoruntimemap

package main
import "fmt"
func main() {  
    m := map[string]int{"one":1,"two":2,"three":3,"four":4}
    for k,v := range m {
        fmt.Println(k,v)
    }
}

25. "switch"声明中的失效行为

switchcasebreak

package main
import "fmt"
func main() {  
    isSpace := func(ch byte) bool {
        switch(ch) {
        case ' ': //error
        case '\t':
            return true
        }
        return false
    }
    fmt.Println(isSpace('\t')) //prints true (ok)
    fmt.Println(isSpace(' '))  //prints false (not ok)
}

casefallthroughcaseswitchcase

package main
import "fmt"
func main() {  
    isSpace := func(ch byte) bool {
        switch(ch) {
        case ' ', '\t':
            return true
        }
        return false
    }
    fmt.Println(isSpace('\t')) //prints true (ok)
    fmt.Println(isSpace(' '))  //prints true (ok)
}

26. 自增和自减

Go

package main
import "fmt"
func main() {  
    data := []int{1,2,3}
    i := 0
    ++i //error
    fmt.Println(data[i++]) //error
}

Compile Errors:

/tmp/sandbox101231828/main.go:8: syntax error: unexpected ++ /tmp/sandbox101231828/main.go:9: syntax error: unexpected ++, expecting :

Works:

package main
import "fmt"
func main() {  
    data := []int{1,2,3}
    i := 0
    i++
    fmt.Println(data[i])
}

27. 按位NOT操作

~NOTGo^

package main
import "fmt"
func main() {  
    fmt.Println(~2) //error
}

Compile Error:

/tmp/sandbox965529189/main.go:6: the bitwise complement operator is ^

Works:

package main
import "fmt"
func main() {  
    var d uint8 = 2
    fmt.Printf("%08b\n",^d)
}

Go^XOR

XOR^

GoAND NOT&^NOTA AND (NOT B)

package main
import "fmt"
func main() {  
    var a uint8 = 0x82
    var b uint8 = 0x02
    fmt.Printf("%08b [A]\n",a)
    fmt.Printf("%08b [B]\n",b)
    fmt.Printf("%08b (NOT B)\n",^b)
    fmt.Printf("%08b ^ %08b = %08b [B XOR 0xff]\n",b,0xff,b ^ 0xff)
    fmt.Printf("%08b ^ %08b = %08b [A XOR B]\n",a,b,a ^ b)
    fmt.Printf("%08b & %08b = %08b [A AND B]\n",a,b,a & b)
    fmt.Printf("%08b &^%08b = %08b [A 'AND NOT' B]\n",a,b,a &^ b)
    fmt.Printf("%08b&(^%08b)= %08b [A AND (NOT B)]\n",a,b,a & (^b))
}

28. 操作优先级的差异

&^Go

package main
import "fmt"
func main() {  
    fmt.Printf("0x2 & 0x2 + 0x4 -> %#x\n",0x2 & 0x2 + 0x4)
    //prints: 0x2 & 0x2 + 0x4 -> 0x6
    //Go:    (0x2 & 0x2) + 0x4
    //C++:    0x2 & (0x2 + 0x4) -> 0x2
    fmt.Printf("0x2 + 0x2 << 0x1 -> %#x\n",0x2 + 0x2 << 0x1)
    //prints: 0x2 + 0x2 << 0x1 -> 0x6
    //Go:     0x2 + (0x2 << 0x1)
    //C++:   (0x2 + 0x2) << 0x1 -> 0x8
    fmt.Printf("0xf | 0x2 ^ 0x2 -> %#x\n",0xf | 0x2 ^ 0x2)
    //prints: 0xf | 0x2 ^ 0x2 -> 0xd
    //Go:    (0xf | 0x2) ^ 0x2
    //C++:    0xf | (0x2 ^ 0x2) -> 0xf
}

29. 未导出的结构体不会被编码

jsonxmlgob

package main
import (  
    "fmt"
    "encoding/json"
)
type MyData struct {  
    One int
    two string
}
func main() {  
    in := MyData{1,"two"}
    fmt.Printf("%#v\n",in) //prints main.MyData{One:1, two:"two"}
    encoded,_ := json.Marshal(in)
    fmt.Println(string(encoded)) //prints {"One":1}
    var out MyData
    json.Unmarshal(encoded,&out)
    fmt.Printf("%#v\n",out) //prints main.MyData{One:1, two:""}
}

30. 有活动的Goroutines下的应用退出

goroutines

package main
import (  
    "fmt"
    "time"
)
func main() {  
    workerCount := 2
    for i := 0; i < workerCount; i++ {
        go doit(i)
    }
    time.Sleep(1 * time.Second)
    fmt.Println("all done!")
}
func doit(workerId int) {  
    fmt.Printf("[%v] is running\n",workerId)
    time.Sleep(3 * time.Second)
    fmt.Printf("[%v] is done\n",workerId)
}

你将会看到：

[0] is running 
[1] is running 
all done!

WaitGroupgoroutineworker goroutinegoroutinekillworkerchannelgoroutine

package main
import (  
    "fmt"
    "sync"
)
func main() {  
    var wg sync.WaitGroup
    done := make(chan struct{})
    workerCount := 2
    for i := 0; i < workerCount; i++ {
        wg.Add(1)
        go doit(i,done,wg)
    }
    close(done)
    wg.Wait()
    fmt.Println("all done!")
}
func doit(workerId int,done <-chan struct{},wg sync.WaitGroup) {  
    fmt.Printf("[%v] is running\n",workerId)
    defer wg.Done()
    <- done
    fmt.Printf("[%v] is done\n",workerId)
}

如果你运行这个应用，你将会看到：

[0] is running 
[0] is done 
[1] is running 
[1] is done

workergoroutine

fatal error: all goroutines are asleep - deadlock!

wg.Done()WaitGroupwg.Done()goroutineWaitGroup

package main
import (  
    "fmt"
    "sync"
)
func main() {  
    var wg sync.WaitGroup
    done := make(chan struct{})
    wq := make(chan interface{})
    workerCount := 2
    for i := 0; i < workerCount; i++ {
        wg.Add(1)
        go doit(i,wq,done,&wg)
    }
    for i := 0; i < workerCount; i++ {
        wq <- i
    }
    close(done)
    wg.Wait()
    fmt.Println("all done!")
}
func doit(workerId int, wq <-chan interface{},done <-chan struct{},wg *sync.WaitGroup) {  
    fmt.Printf("[%v] is running\n",workerId)
    defer wg.Done()
    for {
        select {
        case m := <- wq:
            fmt.Printf("[%v] m => %v\n",workerId,m)
        case <- done:
            fmt.Printf("[%v] is done\n",workerId)
            return
        }
    }
}

现在它会如预期般工作 😃

31. 向无缓存的Channel发送消息，只要目标接收者准备好就会立即返回

goroutine

package main
import "fmt"
func main() {  
    ch := make(chan string)
    go func() {
        for m := range ch {
            fmt.Println("processed:",m)
        }
    }()
    ch <- "cmd.1"
    ch <- "cmd.2" //won't be processed
}

32. 向已关闭的Channel发送会引起Panic

channelokfalsechannelokfalsechannelpanicGo

package main
import (  
    "fmt"
    "time"
)
func main() {  
    ch := make(chan int)
    for i := 0; i < 3; i++ {
        go func(idx int) {
            ch <- (idx + 1) * 2
        }(i)
    }
    
    //get the first result
    fmt.Println(<-ch)
    close(ch) //not ok (you still have other senders)
    //do other work
    time.Sleep(2 * time.Second)
}

channelchannel

package main
import (  
    "fmt"
    "time"
)
func main() {  
    ch := make(chan int)
    done := make(chan struct{})
    for i := 0; i < 3; i++ {
        go func(idx int) {
            select {
            case ch <- (idx + 1) * 2: fmt.Println(idx,"sent result")
            case <- done: fmt.Println(idx,"exiting")
            }
        }(i)
    }
    //get first result
    fmt.Println("result:",<-ch)
    close(done)
    //do other work
    time.Sleep(3 * time.Second)
}

33. 使用"nil" Channels

nilchannel上Go

package main
import (  
    "fmt"
    "time"
)
func main() {  
    var ch chan int
    for i := 0; i < 3; i++ {
        go func(idx int) {
            ch <- (idx + 1) * 2
        }(i)
    }
    //get first result
    fmt.Println("result:",<-ch)
    //do other work
    time.Sleep(2 * time.Second)
}

如果运行代码你将会看到一个runtime错误：

fatal error: all goroutines are asleep - deadlock!

selectcase

package main
import "fmt"  
import "time"
func main() {  
    inch := make(chan int)
    outch := make(chan int)
    go func() {
        var in <- chan int = inch
        var out chan <- int
        var val int
        for {
           select {
            case out <- val:
                out = nil
                in = inch
            case val = <- in:
                out = outch
                in = nil
            }
        }
    }()
    go func() {
        for r := range outch {
            fmt.Println("result:",r)
        }
    }()
    time.Sleep(0)
    inch <- 1
    inch <- 2
    time.Sleep(3 * time.Second)
}

34. 传值方法的接收者无法修改原有的值

mapslice

package main
import "fmt"
type data struct {  
    num int
    key *string
    items map[string]bool
}
func (this *data) pmethod() {  
    this.num = 7
}
func (this data) vmethod() {  
    this.num = 8
    *this.key = "v.key"
    this.items["vmethod"] = true
}
func main() {  
    key := "key.1"
    d := data{1,&key,make(map[string]bool)}
    fmt.Printf("num=%v key=%v items=%v\n",d.num,*d.key,d.items)
    //prints num=1 key=key.1 items=map[]
    d.pmethod()
    fmt.Printf("num=%v key=%v items=%v\n",d.num,*d.key,d.items) 
    //prints num=7 key=key.1 items=map[]
    d.vmethod()
    fmt.Printf("num=%v key=%v items=%v\n",d.num,*d.key,d.items)
    //prints num=7 key=v.key items=map[vmethod:true]
}

二. 中级篇

35. 关闭HTTP的响应

httphttpGoGo

package main
import (  
    "fmt"
    "net/http"
    "io/ioutil"
)
func main() {  
    resp, err := http.Get("https://api.ipify.org?format=json")
    defer resp.Body.Close()//not ok
    if err != nil {
        fmt.Println(err)
        return
    }
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(body))
}

httprespnilruntime panichttpdefer

package main
import (  
    "fmt"
    "net/http"
    "io/ioutil"
)
func main() {  
    resp, err := http.Get("https://api.ipify.org?format=json")
    if err != nil {
        fmt.Println(err)
        return
    }
    defer resp.Body.Close()//ok, most of the time :-)
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(body))
}

httprespnilerrnon-nilnon-nilhttpnon-nildefer

package main
import (  
    "fmt"
    "net/http"
    "io/ioutil"
)
func main() {  
    resp, err := http.Get("https://api.ipify.org?format=json")
    if resp != nil {
        defer resp.Body.Close()
    }
    if err != nil {
        fmt.Println(err)
        return
    }
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(body))
}

resp.Body.Close()httpkeepalive httphttphttp

_, err = io.Copy(ioutil.Discard, resp.Body)

如果你不立即读取整个响应将是必要的，这可能在你处理 json API响应时会发生：

json.NewDecoder(resp.Body).Decode(&data;)

36. 关闭HTTP的连接

HTTPhttpHTTPClosetruehttpConnectioncloseConnection: closehttp

package main
import (  
    "fmt"
    "net/http"
    "io/ioutil"
)
func main() {  
    req, err := http.NewRequest("GET","http://golang.org",nil)
    if err != nil {
        fmt.Println(err)
        return
    }
    req.Close = true
    //or do this:
    //req.Header.Add("Connection", "close")
    resp, err := http.DefaultClient.Do(req)
    if resp != nil {
        defer resp.Body.Close()
    }
    if err != nil {
        fmt.Println(err)
        return
    }
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(len(string(body)))
}

httphttp

package main
import (  
    "fmt"
    "net/http"
    "io/ioutil"
)
func main() {  
    tr := &http.Transport{DisableKeepAlives: true}
    client := &http.Client{Transport: tr}
    resp, err := client.Get("http://golang.org")
    if resp != nil {
        defer resp.Body.Close()
    }
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(resp.StatusCode)
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(len(string(body)))
}

HTTPHTTP

37. 比较Structs, Arrays, Slices, and Maps

==

package main
import "fmt"
type data struct {  
    num int
    fp float32
    complex complex64
    str string
    char rune
    yes bool
    events <-chan string
    handler interface{}
    ref *byte
    raw [10]byte
}
func main() {  
    v1 := data{}
    v2 := data{}
    fmt.Println("v1 == v2:",v1 == v2) //prints: v1 == v2: true
}

如果结构体中的元素无法比较，那使用等号将导致编译错误。注意数组仅在它们的数据元素可比较的情况下才可以比较。

package main
import "fmt"
type data struct {  
    num int                //ok
    checks [10]func() bool //not comparable
    doit func() bool       //not comparable
    m map[string] string   //not comparable
    bytes []byte           //not comparable
}
func main() {  
    v1 := data{}
    v2 := data{}
    fmt.Println("v1 == v2:",v1 == v2)
}

GoreflectDeepEqual()

package main
import (  
    "fmt"
    "reflect"
)
type data struct {  
    num int                //ok
    checks [10]func() bool //not comparable
    doit func() bool       //not comparable
    m map[string] string   //not comparable
    bytes []byte           //not comparable
}
func main() {  
    v1 := data{}
    v2 := data{}
    fmt.Println("v1 == v2:",reflect.DeepEqual(v1,v2)) //prints: v1 == v2: true
    m1 := map[string]string{"one": "a","two": "b"}
    m2 := map[string]string{"two": "b", "one": "a"}
    fmt.Println("m1 == m2:",reflect.DeepEqual(m1, m2)) //prints: m1 == m2: true
    s1 := []int{1, 2, 3}
    s2 := []int{1, 2, 3}
    fmt.Println("s1 == s2:",reflect.DeepEqual(s1, s2)) //prints: s1 == s2: true
}

DeepEqual()

package main
import (  
    "fmt"
    "reflect"
)
func main() {  
    var b1 []byte = nil
    b2 := []byte{}
    fmt.Println("b1 == b2:",reflect.DeepEqual(b1, b2)) //prints: b1 == b2: false
}

DeepEqual()slicenilslicebytes.Equal()bytes.Equal()nilslice

package main
import (  
    "fmt"
    "bytes"
)
func main() {  
    var b1 []byte = nil
    b2 := []byte{}
    fmt.Println("b1 == b2:",bytes.Equal(b1, b2)) //prints: b1 == b2: true
}

DeepEqual()slice

package main
import (  
    "fmt"
    "reflect"
    "encoding/json"
)
func main() {  
    var str string = "one"
    var in interface{} = "one"
    fmt.Println("str == in:",str == in,reflect.DeepEqual(str, in)) 
    //prints: str == in: true true
    v1 := []string{"one","two"}
    v2 := []interface{}{"one","two"}
    fmt.Println("v1 == v2:",reflect.DeepEqual(v1, v2)) 
    //prints: v1 == v2: false (not ok)
    data := map[string]interface{}{
        "code": 200,
        "value": []string{"one","two"},
    }
    encoded, _ := json.Marshal(data)
    var decoded map[string]interface{}
    json.Unmarshal(encoded, &decoded)
    fmt.Println("data == decoded:",reflect.DeepEqual(data, decoded)) 
    //prints: data == decoded: false (not ok)
}

byte slice==bytes.Equal()bytes.Compare()bytesstringToUpper()ToLower()strings.EqualFold()bytes.EqualFold()reflect.DeepEqual()bytes.Equal()bytes.Compare()subtle.ConstantTimeCompare()

38. 从Panic中恢复

recover()panicdeferrecover()

package main
import "fmt"
func main() {  
    recover() //doesn't do anything
    panic("not good")
    recover() //won't be executed :)
    fmt.Println("ok")
}

Works:

package main
import "fmt"
func main() {  
    defer func() {
        fmt.Println("recovered:",recover())
    }()
    panic("not good")
}

recover()defer

package main
import "fmt"
func doRecover() {  
    fmt.Println("recovered =>",recover()) //prints: recovered => <nil>
}
func main() {  
    defer func() {
        doRecover() //panic is not recovered
    }()
    panic("not good")
}

39. 在Slice, Array, and Map "range"语句中更新引用元素的值

range

package main
import "fmt"
func main() {  
    data := []int{1,2,3}
    for _,v := range data {
        v *= 10 //original item is not changed
    }
    fmt.Println("data:",data) //prints data: [1 2 3]
}

如果你需要更新原有集合中的数据，使用索引操作符来获得数据。

package main
import "fmt"
func main() {  
    data := []int{1,2,3}
    for i,_ := range data {
        data[i] *= 10
    }
    fmt.Println("data:",data) //prints data: [10 20 30]
}

for range

package main
import "fmt"
func main() {  
    data := []*struct{num int} { {1},{2},{3} }
    for _,v := range data {
        v.num *= 10
    }
    fmt.Println(data[0],data[1],data[2]) //prints &{10} &{20} &{30}
}

40. 在Slice中"隐藏"数据

slicesliceslicesliceslice

package main
import "fmt"
func get() []byte {  
    raw := make([]byte,10000)
    fmt.Println(len(raw),cap(raw),&raw[0]) //prints: 10000 10000 <byte_addr_x>
    return raw[:3]
}
func main() {  
    data := get()
    fmt.Println(len(data),cap(data),&data[0]) //prints: 3 10000 <byte_addr_x>
}

sliceslice

package main
import "fmt"
func get() []byte {  
    raw := make([]byte,10000)
    fmt.Println(len(raw),cap(raw),&raw[0]) //prints: 10000 10000 <byte_addr_x>
    res := make([]byte,3)
    copy(res,raw[:3])
    return res
}
func main() {  
    data := get()
    fmt.Println(len(data),cap(data),&data[0]) //prints: 3 3 <byte_addr_y>
}

41. Slice的数据“毁坏”

slice

package main
import (  
    "fmt"
    "bytes"
)
func main() {  
    path := []byte("AAAA/BBBBBBBBB")
    sepIndex := bytes.IndexByte(path,'/')
    dir1 := path[:sepIndex]
    dir2 := path[sepIndex+1:]
    fmt.Println("dir1 =>",string(dir1)) //prints: dir1 => AAAA
    fmt.Println("dir2 =>",string(dir2)) //prints: dir2 => BBBBBBBBB
    dir1 = append(dir1,"suffix"...)
    path = bytes.Join([][]byte{dir1,dir2},[]byte{'/'})
    fmt.Println("dir1 =>",string(dir1)) //prints: dir1 => AAAAsuffix
    fmt.Println("dir2 =>",string(dir2)) //prints: dir2 => uffixBBBB (not ok)
    fmt.Println("new path =>",string(path))
}

sliceslicesliceslice

package main
import (  
    "fmt"
    "bytes"
)
func main() {  
    path := []byte("AAAA/BBBBBBBBB")
    sepIndex := bytes.IndexByte(path,'/')
    dir1 := path[:sepIndex:sepIndex] //full slice expression
    dir2 := path[sepIndex+1:]
    fmt.Println("dir1 =>",string(dir1)) //prints: dir1 => AAAA
    fmt.Println("dir2 =>",string(dir2)) //prints: dir2 => BBBBBBBBB
    dir1 = append(dir1,"suffix"...)
    path = bytes.Join([][]byte{dir1,dir2},[]byte{'/'})
    fmt.Println("dir1 =>",string(dir1)) //prints: dir1 => AAAAsuffix
    fmt.Println("dir2 =>",string(dir2)) //prints: dir2 => BBBBBBBBB (ok now)
    fmt.Println("new path =>",string(path))
}

sliceslicesliceslice

42. 陈旧的(Stale)Slices

sliceslicesliceslicesliceslice

import "fmt"
func main() {  
    s1 := []int{1,2,3}
    fmt.Println(len(s1),cap(s1),s1) //prints 3 3 [1 2 3]
    s2 := s1[1:]
    fmt.Println(len(s2),cap(s2),s2) //prints 2 2 [2 3]
    for i := range s2 { s2[i] += 20 }
    //still referencing the same array
    fmt.Println(s1) //prints [1 22 23]
    fmt.Println(s2) //prints [22 23]
    s2 = append(s2,4)
    for i := range s2 { s2[i] += 10 }
    //s1 is now "stale"
    fmt.Println(s1) //prints [1 22 23]
    fmt.Println(s2) //prints [32 33 14]
}

43. 类型声明和方法

interface

package main
import "sync"
type myMutex sync.Mutex
func main() {  
    var mtx myMutex
    mtx.Lock() //error
    mtx.Unlock() //error  
}

Compile Errors:

/tmp/sandbox106401185/main.go:9: mtx.Lock undefined (type myMutex has no field or method Lock) /tmp/sandbox106401185/main.go:10: mtx.Unlock undefined (type myMutex has no field or method Unlock)

struct

package main
import "sync"
type myLocker struct {  
    sync.Mutex
}
func main() {  
    var lock myLocker
    lock.Lock() //ok
    lock.Unlock() //ok
}

interface

package main
import "sync"
type myLocker sync.Locker
func main() {  
    var lock myLocker = new(sync.Mutex)
    lock.Lock() //ok
    lock.Unlock() //ok
}

44. 从"for switch"和"for select"代码块中跳出

breakswitch/selectreturn

package main
import "fmt"
func main() {  
    loop:
        for {
            switch {
            case true:
                fmt.Println("breaking out...")
                break loop
            }
        }
    fmt.Println("out!")
}

goto

45. "for"声明中的迭代变量和闭包

Goforforgoroutine

package main
import (  
    "fmt"
    "time"
)
func main() {  
    data := []string{"one","two","three"}
    for _,v := range data {
        go func() {
            fmt.Println(v)
        }()
    }
    time.Sleep(3 * time.Second)
    //goroutines print: three, three, three
}

goroutinefor

package main
import (  
    "fmt"
    "time"
)
func main() {  
    data := []string{"one","two","three"}
    for _,v := range data {
        vcopy := v //
        go func() {
            fmt.Println(vcopy)
        }()
    }
    time.Sleep(3 * time.Second)
    //goroutines print: one, two, three
}

goroutine

package main
import (  
    "fmt"
    "time"
)
func main() {  
    data := []string{"one","two","three"}
    for _,v := range data {
        go func(in string) {
            fmt.Println(in)
        }(v)
    }
    time.Sleep(3 * time.Second)
    //goroutines print: one, two, three
}

下面这个陷阱稍微复杂一些的版本。
Incorrect:

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()
    }
    time.Sleep(3 * time.Second)
    //goroutines print: three, three, three
}

Works:

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 {
        v := v
        go v.print()
    }
    time.Sleep(3 * time.Second)
    //goroutines print: one, two, three
}

在运行这段代码时你认为会看到什么结果？（原因是什么？）

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()
    }
    time.Sleep(3 * time.Second)
}

46. Defer函数调用参数的求值

deferdefer

Arguments for a deferred function call are evaluated when the defer statement is evaluated (not when the function is actually executing).

package main
import "fmt"
func main() {  
    var i int = 1
    defer fmt.Println("result =>",func() int { return i * 2 }())
    i++
    //prints: result => 2 (not ok if you expected 4)
}

47. 被Defer的函数调用执行

deferGodeferfordefer

package main
import (  
    "fmt"
    "os"
    "path/filepath"
)
func main() {  
    if len(os.Args) != 2 {
        os.Exit(-1)
    }
    start, err := os.Stat(os.Args[1])
    if err != nil || !start.IsDir(){
        os.Exit(-1)
    }
    var targets []string
    filepath.Walk(os.Args[1], func(fpath string, fi os.FileInfo, err error) error {
        if err != nil {
            return err
        }
        if !fi.Mode().IsRegular() {
            return nil
        }
        targets = append(targets,fpath)
        return nil
    })
    for _,target := range targets {
        f, err := os.Open(target)
        if err != nil {
            fmt.Println("bad target:",target,"error:",err) //prints error: too many open files
            break
        }
        defer f.Close() //will not be closed at the end of this code block
        //do something with the file...
    }
}

解决这个问题的一个方法是把代码块写成一个函数。

package main
import (  
    "fmt"
    "os"
    "path/filepath"
)
func main() {  
    if len(os.Args) != 2 {
        os.Exit(-1)
    }
    start, err := os.Stat(os.Args[1])
    if err != nil || !start.IsDir(){
        os.Exit(-1)
    }
    var targets []string
    filepath.Walk(os.Args[1], func(fpath string, fi os.FileInfo, err error) error {
        if err != nil {
            return err
        }
        if !fi.Mode().IsRegular() {
            return nil
        }
        targets = append(targets,fpath)
        return nil
    })
    for _,target := range targets {
        func() {
            f, err := os.Open(target)
            if err != nil {
                fmt.Println("bad target:",target,"error:",err)
                return
            }
            defer f.Close() //ok
            //do something with the file...
        }()
    }
}

defer

48. 失败的类型断言

失败的类型断言返回断言声明中使用的目标类型的“零值”。这在与隐藏变量混合时，会发生未知情况。
Incorrect:

package main
import "fmt"
func main() {  
    var data interface{} = "great"
    if data, ok := data.(int); ok {
        fmt.Println("[is an int] value =>",data)
    } else {
        fmt.Println("[not an int] value =>",data) 
        //prints: [not an int] value => 0 (not "great")
    }
}

Works:

package main
import "fmt"
func main() {  
    var data interface{} = "great"
    if res, ok := data.(int); ok {
        fmt.Println("[is an int] value =>",res)
    } else {
        fmt.Println("[not an int] value =>",data) 
        //prints: [not an int] value => great (as expected)
    }
}

49. 阻塞的Goroutine和资源泄露

Rob Pike在2012年的Google I/O大会上所做的“Go Concurrency Patterns”的演讲上，说道过几种基础的并发模式。从一组目标中获取第一个结果就是其中之一。

func First(query string, replicas ...Search) Result {  
    c := make(chan Result)
    searchReplica := func(i int) { c <- replicas[i](query) }
    for i := range replicas {
        go searchReplica(i)
    }
    return <-c
}

goroutinegoroutinechannelchannelgoroutinegoroutineFirst()channelgoroutinegoroutinegoroutinechannel

func First(query string, replicas ...Search) Result {  
    c := make(chan Result,len(replicas))
    searchReplica := func(i int) { c <- replicas[i](query) }
    for i := range replicas {
        go searchReplica(i)
    }
    return <-c
}

defaultselectchanneldefaultchannelgoroutine

func First(query string, replicas ...Search) Result {  
    c := make(chan Result,1)
    searchReplica := func(i int) { 
        select {
        case c <- replicas[i](query):
        default:
        }
    }
    for i := range replicas {
        go searchReplica(i)
    }
    return <-c
}

channelworkers

func First(query string, replicas ...Search) Result {  
    c := make(chan Result)
    done := make(chan struct{})
    defer close(done)
    searchReplica := func(i int) { 
        select {
        case c <- replicas[i](query):
        case <- done:
        }
    }
    for i := range replicas {
        go searchReplica(i)
    }
    return <-c
}

为何在演讲中会包含这些bug？Rob Pike仅仅是不想把演示复杂化。这么作是合理的，但对于Go新手而言，可能会直接使用代码，而不去思考它可能有问题。

三. 高级篇

50. 使用指针接收方法的值的实例

mapinterface

package main
import "fmt"
type data struct {  
    name string
}
func (p *data) print() {  
    fmt.Println("name:",p.name)
}
type printer interface {  
    print()
}
func main() {  
    d1 := data{"one"}
    d1.print() //ok
    var in printer = data{"two"} //error
    in.print()
    m := map[string]data {"x":data{"three"}}
    m["x"].print() //error
}

Compile Errors:

/tmp/sandbox017696142/main.go:21: cannot use data literal (type data) as type printer in assignment: data does not implement printer (print method has pointer receiver) 
/tmp/sandbox017696142/main.go:25: cannot call pointer method on m["x"] 
/tmp/sandbox017696142/main.go:25: cannot take the address of m["x"]

51. 更新 map 的值

structmapstruct

package main
type data struct {  
    name string
}
func main() {  
    m := map[string]data {"x":{"one"}}
    m["x"].name = "two" //error
}

Compile Error:

/tmp/sandbox380452744/main.go:9: cannot assign to m["x"].name

mapGoslice

package main
import "fmt"
type data struct {  
    name string
}
func main() {  
    s := []data one
    s[0].name = "two" //ok
    fmt.Println(s)    //prints: [{two}]
}

map

package main
import "fmt"
type data struct {  
    name string
}
func main() {  
    m := map[string]data {"x":{"one"}}
    r := m["x"]
    r.name = "two"
    m["x"] = r
    fmt.Printf("%v",m) //prints: map[x:{two}]
}

另一个有效的方法是使用指针的map。

package main
import "fmt"
type data struct {  
    name string
}
func main() {  
    m := map[string]*data {"x":{"one"}}
    m["x"].name = "two" //ok
    fmt.Println(m["x"]) //prints: &{two}
}

顺便说下，当你运行下面的代码时会发生什么？

package main
type data struct {  
    name string
}
func main() {  
    m := map[string]*data {"x":{"one"}}
    m["z"].name = "what?" //???
}

52. “nil” Interfaces 和 “nil” Interfaces的值

Gointerfaceinterfacenilnilinterfaceinterfaceinterfacenil

package main
import "fmt"
func main() {  
    var data *byte
    var in interface{}
    fmt.Println(data,data == nil) //prints: <nil> true
    fmt.Println(in,in == nil)     //prints: <nil> true
    in = data
    fmt.Println(in,in == nil)     //prints: <nil> false
    //'data' is 'nil', but 'in' is not 'nil'
}

interface

package main
import "fmt"
func main() {  
    doit := func(arg int) interface{} {
        var result *struct{} = nil
        if(arg > 0) {
            result = &struct;{}{}
        }
        return result
    }
    if res := doit(-1); res != nil {
        fmt.Println("good result:",res) //prints: good result: <nil>
        //'res' is not 'nil', but its value is 'nil'
    }
}

Works:

package main
import "fmt"
func main() {  
    doit := func(arg int) interface{} {
        var result *struct{} = nil
        if(arg > 0) {
            result = &struct;{}{}
        } else {
            ret

53. 栈和堆变量

C++newGonew()make()CC++go buildgo run-mgo run -gcflags -m app.go

54. GOMAXPROCS, 并发, 和并行

GoGOMAXPROCSGOMAXPROCSCPUGogoroutineruntime.GOMAXPROCS()GOMAXPROCSGOMAXPROCSCPUGOMAXPROCS

package main
import (  
    "fmt"
    "runtime"
)
func main() {  
    fmt.Println(runtime.GOMAXPROCS(-1)) //prints: 1
    fmt.Println(runtime.NumCPU())       //prints: 1 (on play.golang.org)
    runtime.GOMAXPROCS(20)
    fmt.Println(runtime.GOMAXPROCS(-1)) //prints: 20
    runtime.GOMAXPROCS(300)
    fmt.Println(runtime.GOMAXPROCS(-1)) //prints: 256
}

55. 读写操作的重排顺序

Gogoroutinegoroutine

package main
import (  
    "runtime"
    "time"
)
var _ = runtime.GOMAXPROCS(3)
var a, b int
func u1() {  
    a = 1
    b = 2
}
func u2() {  
    a = 3
    b = 4
}
func p() {  
    println(a)
    println(b)
}
func main() {  
    go u1()
    go u2()
    go p()
    time.Sleep(1 * time.Second)
}

abbagoroutinechannelsync

56. 优先调度

goroutinegoroutinefor

package main
import "fmt"
func main() {  
    done := false
    go func(){
        done = true
    }()
    for !done {
    }
    fmt.Println("done!")
}

forGCgochannellock

package main
import "fmt"
func main() {  
    done := false
    go func(){
        done = true
    }()
    for !done {
        fmt.Println("not done!") //not inlined
    }
    fmt.Println("done!")
}

forgo buildgo run-mgo build -gcflags -mruntimeGoshed()

package main
import (  
    "fmt"
    "runtime"
)
func main() {  
    done := false
    go func(){
        done = true
    }()
    for !done {
        runtime.Gosched()
    }
    fmt.Println("done!")
}