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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< hr >
< h2 id = "58-deferred函数" > < a class = "header" href = "#58-deferred函数" > 5.8. Deferred函数< / a > < / h2 >
< p > 在findLinks的例子中， ， ， ， ， ， ， < / p >
< p > 下面的例子获取HTML页面并输出页面的标题。title函数会检查服务器返回的Content-Type字段， ， ， < / p >
< p > < u > < i > gopl.io/ch5/title1< / i > < / u > < / p >
< pre > < code class = "language-Go" > func title(url string) error {
resp, err := http.Get(url)
if err != nil {
return err
}
// Check Content-Type is HTML (e.g., " text/html;charset=utf-8" ).
ct := resp.Header.Get(" Content-Type" )
if ct != " text/html" & & !strings.HasPrefix(ct," text/html;" ) {
resp.Body.Close()
return fmt.Errorf(" %s has type %s, not text/html" ,url, ct)
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
return fmt.Errorf(" parsing %s as HTML: %v" , url,err)
}
visitNode := func(n *html.Node) {
if n.Type == html.ElementNode & & n.Data == " title" & & n.FirstChild != nil {
fmt.Println(n.FirstChild.Data)
}
}
forEachNode(doc, visitNode, nil)
return nil
}
< / code > < / pre >
< p > 下面展示了运行效果：< / p >
< pre > < code > $ go build gopl.io/ch5/title1
$ ./title1 http://gopl.io
The Go Programming Language
$ ./title1 https://golang.org/doc/effective_go.html
Effective Go - The Go Programming Language
$ ./title1 https://golang.org/doc/gopher/frontpage.png
title1: https://golang.org/doc/gopher/frontpage.png has type image/png, not text/html
< / code > < / pre >
< p > resp.Body.close调用了多次， （ ） ， ， < / p >
< p > 你只需要在调用普通函数或方法前加上关键字defer， ， ， ， ， ， ， < / p >
< p > defer语句经常被用于处理成对的操作， ， ， ， ， < / p >
< p > < u > < i > gopl.io/ch5/title2< / i > < / u > < / p >
< pre > < code class = "language-Go" > func title(url string) error {
resp, err := http.Get(url)
if err != nil {
return err
}
defer resp.Body.Close()
ct := resp.Header.Get(" Content-Type" )
if ct != " text/html" & & !strings.HasPrefix(ct," text/html;" ) {
return fmt.Errorf(" %s has type %s, not text/html" ,url, ct)
}
doc, err := html.Parse(resp.Body)
if err != nil {
return fmt.Errorf(" parsing %s as HTML: %v" , url,err)
}
// ...print doc's title element…
return nil
}
< / code > < / pre >
< p > 在处理其他资源时， ， ： < / p >
< p > < u > < i > io/ioutil< / i > < / u > < / p >
< pre > < code class = "language-Go" > package ioutil
func ReadFile(filename string) ([]byte, error) {
f, err := os.Open(filename)
if err != nil {
return nil, err
}
defer f.Close()
return ReadAll(f)
}
< / code > < / pre >
< p > 或是处理互斥锁（ < / p >
< pre > < code class = "language-Go" > var mu sync.Mutex
var m = make(map[string]int)
func lookup(key string) int {
mu.Lock()
defer mu.Unlock()
return m[key]
}
< / code > < / pre >
< p > 调试复杂程序时， ， ， ， ， ， ， ： ， ， ， < / p >
< p > < u > < i > gopl.io/ch5/trace< / i > < / u > < / p >
< pre > < code class = "language-Go" > func bigSlowOperation() {
defer trace(" bigSlowOperation" )() // don't forget the extra parentheses
// ...lots of work…
time.Sleep(10 * time.Second) // simulate slow operation by sleeping
}
func trace(msg string) func() {
start := time.Now()
log.Printf(" enter %s" , msg)
return func() {
log.Printf(" exit %s (%s)" , msg,time.Since(start))
}
}
< / code > < / pre >
< p > 每一次bigSlowOperation被调用， ， ， （ ） < / p >
< pre > < code > $ go build gopl.io/ch5/trace
$ ./trace
2015/11/18 09:53:26 enter bigSlowOperation
2015/11/18 09:53:36 exit bigSlowOperation (10.000589217s)
< / code > < / pre >
< p > 我们知道， ， ， ， ， < / p >
< p > 以double函数为例： < / p >
< pre > < code class = "language-Go" > func double(x int) int {
return x + x
}
< / code > < / pre >
< p > 我们只需要首先命名double的返回值， ， ， < / p >
< pre > < code class = "language-Go" > func double(x int) (result int) {
defer func() { fmt.Printf(" double(%d) = %d\n" , x,result) }()
return x + x
}
_ = double(4)
// Output:
// " double(4) = 8"
< / code > < / pre >
< p > 可能double函数过于简单， ， ， < / p >
< p > 被延迟执行的匿名函数甚至可以修改函数返回给调用者的返回值：< / p >
< pre > < code class = "language-Go" > func triple(x int) (result int) {
defer func() { result += x }()
return double(x)
}
fmt.Println(triple(4)) // " 12"
< / code > < / pre >
< p > 在循环体中的defer语句需要特别注意， ， ， ， < / p >
< pre > < code class = "language-Go" > for _, filename := range filenames {
f, err := os.Open(filename)
if err != nil {
return err
}
defer f.Close() // NOTE: risky; could run out of file descriptors
// ...process f…
}
< / code > < / pre >
< p > 一种解决方法是将循环体中的defer语句移至另外一个函数。在每次循环时， < / p >
< pre > < code class = "language-Go" > for _, filename := range filenames {
if err := doFile(filename); err != nil {
return err
}
}
func doFile(filename string) error {
f, err := os.Open(filename)
if err != nil {
return err
}
defer f.Close()
// ...process f…
}
< / code > < / pre >
< p > 下面的代码是fetch（ ） ， < / p >
< p > < u > < i > gopl.io/ch5/fetch< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Fetch downloads the URL and returns the
// name and length of the local file.
func fetch(url string) (filename string, n int64, err error) {
resp, err := http.Get(url)
if err != nil {
return " " , 0, err
}
defer resp.Body.Close()
local := path.Base(resp.Request.URL.Path)
if local == " /" {
local = " index.html"
}
f, err := os.Create(local)
if err != nil {
return " " , 0, err
}
n, err = io.Copy(f, resp.Body)
// Close file, but prefer error from Copy, if any.
if closeErr := f.Close(); err == nil {
err = closeErr
}
return local, n, err
}
< / code > < / pre >
< p > 对resp.Body.Close延迟调用我们已经见过了， ， ， ， ， ， ， ， ， ， ， ， < / p >
2022-09-05 04:28:24 +00:00
< p > < strong > 练习5.18： < / strong > 不修改fetch的行为， ， < / p >
2022-08-04 06:59:33 +00:00
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