ehlxr
/
gopl-zh.github.com
mirror of https://github.com/gopl-zh/gopl-zh.github.com
1
0
Fork 0
Code Issues Projects Releases Wiki Activity
gopl-zh.github.com/ch5/ch5-03.html

325 lines
34 KiB
HTML
Raw Normal View History

deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<!DOCTYPE HTML>
<html lang="zh" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>多返回值 - Go语言圣经</title>
<!-- Custom HTML head -->
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="&lt;The Go Programming Language&gt;中文版">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="icon" href="../favicon.svg">
<link rel="shortcut icon" href="../favicon.png">
<link rel="stylesheet" href="../css/variables.css">
<link rel="stylesheet" href="../css/general.css">
<link rel="stylesheet" href="../css/chrome.css">
<link rel="stylesheet" href="../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="../highlight.css">
<link rel="stylesheet" href="../tomorrow-night.css">
<link rel="stylesheet" href="../ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="../style.css">
</head>
<body>
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
deploy: 92199cf1ffef9cbdbf3767bae53e549b8186e85d
2023-03-31 10:23:29 +00:00
<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
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky bordered">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Go语言圣经</h1>
<div class="right-buttons">
<a href="../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/gopl-zh/gopl-zh.github.com" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
<a href="https://github.com/gopl-zh/gopl-zh.github.com/edit/master/./ch5/ch5-03.md" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<!-- Page table of contents -->
<div class="sidetoc"><nav class="pagetoc"></nav></div>
<main>
<!-- 头部 -->
<ul dir="auto">
deploy: de40458a40ab5babf96252dae1f10d84b068ba16
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>
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
</ul>
<hr>
<h2 id="53-多返回值"><a class="header" href="#53-多返回值">5.3. 多返回值</a></h2>
<p>在Go中，一个函数可以返回多个值。我们已经在之前例子中看到，许多标准库中的函数返回2个值，一个是期望得到的返回值，另一个是函数出错时的错误信息。下面的例子会展示如何编写多返回值的函数。</p>
<p>下面的程序是findlinks的改进版本。修改后的findlinks可以自己发起HTTP请求，这样我们就不必再运行fetch。因为HTTP请求和解析操作可能会失败，因此findlinks声明了2个返回值：链接列表和错误信息。一般而言，HTML的解析器可以处理HTML页面的错误结点，构造出HTML页面结构，所以解析HTML很少失败。这意味着如果findlinks函数失败了，很可能是由于I/O的错误导致的。</p>
<p><u><i>gopl.io/ch5/findlinks2</i></u></p>
<pre><code class="language-Go">func main() {
for _, url := range os.Args[1:] {
links, err := findLinks(url)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;findlinks2: %v\n&quot;, err)
continue
}
for _, link := range links {
fmt.Println(link)
}
}
}
// findLinks performs an HTTP GET request for url, parses the
// response as HTML, and extracts and returns the links.
func findLinks(url string) ([]string, error) {
resp, err := http.Get(url)
if err != nil {
return nil, err
}
if resp.StatusCode != http.StatusOK {
resp.Body.Close()
return nil, fmt.Errorf(&quot;getting %s: %s&quot;, url, resp.Status)
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
return nil, fmt.Errorf(&quot;parsing %s as HTML: %v&quot;, url, err)
}
return visit(nil, doc), nil
}
</code></pre>
<p>在findlinks中，有4处return语句，每一处return都返回了一组值。前三处return，将http和html包中的错误信息传递给findlinks的调用者。第一处return直接返回错误信息，其他两处通过fmt.Errorf（§7.8）输出详细的错误信息。如果findlinks成功结束，最后的return语句将一组解析获得的连接返回给用户。</p>
<p>在findlinks中，我们必须确保resp.Body被关闭，释放网络资源。虽然Go的垃圾回收机制会回收不被使用的内存，但是这不包括操作系统层面的资源，比如打开的文件、网络连接。因此我们必须显式的释放这些资源。</p>
<p>调用多返回值函数时，返回给调用者的是一组值，调用者必须显式的将这些值分配给变量:</p>
<pre><code class="language-Go">links, err := findLinks(url)
</code></pre>
<p>如果某个值不被使用，可以将其分配给blank identifier:</p>
<pre><code class="language-Go">links, _ := findLinks(url) // errors ignored
</code></pre>
<p>一个函数内部可以将另一个有多返回值的函数调用作为返回值，下面的例子展示了与findLinks有相同功能的函数，两者的区别在于下面的例子先输出参数：</p>
<pre><code class="language-Go">func findLinksLog(url string) ([]string, error) {
log.Printf(&quot;findLinks %s&quot;, url)
return findLinks(url)
}
</code></pre>
<p>当你调用接受多参数的函数时，可以将一个返回多参数的函数调用作为该函数的参数。虽然这很少出现在实际生产代码中，但这个特性在debug时很方便，我们只需要一条语句就可以输出所有的返回值。下面的代码是等价的：</p>
<pre><code class="language-Go">log.Println(findLinks(url))
links, err := findLinks(url)
log.Println(links, err)
</code></pre>
<p>准确的变量名可以传达函数返回值的含义。尤其在返回值的类型都相同时，就像下面这样：</p>
<pre><code class="language-Go">func Size(rect image.Rectangle) (width, height int)
func Split(path string) (dir, file string)
func HourMinSec(t time.Time) (hour, minute, second int)
</code></pre>
<p>虽然良好的命名很重要，但你也不必为每一个返回值都取一个适当的名字。比如，按照惯例，函数的最后一个bool类型的返回值表示函数是否运行成功，error类型的返回值代表函数的错误信息，对于这些类似的惯例，我们不必思考合适的命名，它们都无需解释。</p>
<p>如果一个函数所有的返回值都有显式的变量名，那么该函数的return语句可以省略操作数。这称之为bare return。</p>
<pre><code class="language-Go">// CountWordsAndImages does an HTTP GET request for the HTML
// document url and returns the number of words and images in it.
func CountWordsAndImages(url string) (words, images int, err error) {
resp, err := http.Get(url)
if err != nil {
return
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
err = fmt.Errorf(&quot;parsing HTML: %s&quot;, err)
return
}
words, images = countWordsAndImages(doc)
return
}
func countWordsAndImages(n *html.Node) (words, images int) { /* ... */ }
</code></pre>
<p>按照返回值列表的次序，返回所有的返回值，在上面的例子中，每一个return语句等价于：</p>
<pre><code class="language-Go">return words, images, err
</code></pre>
<p>当一个函数有多处return语句以及许多返回值时，bare return 可以减少代码的重复，但是使得代码难以被理解。举个例子，如果你没有仔细的审查代码，很难发现前2处return等价于 return 0,0,err（Go会将返回值 words和images在函数体的开始处，根据它们的类型，将其初始化为0），最后一处return等价于 return words, image, nil。基于以上原因，不宜过度使用bare return。</p>
<p><strong>练习 5.5：</strong> 实现countWordsAndImages。（参考练习4.9如何分词）</p>
<p><strong>练习 5.6：</strong> 修改gopl.io/ch3/surface（§3.2）中的corner函数，将返回值命名，并使用bare return。</p>
<!-- 公众号 -->
<hr>
<table>
<tr>
<td>
<img width="222px" src="https://chai2010.cn/advanced-go-programming-book/css.png">
</td>
<td>
<img width="222px" src="https://chai2010.cn/advanced-go-programming-book/cch.png">
</td>
</tr>
</table>
<div id="giscus-container"></div>
<footer class="page-footer">
<span>© 2015-2016 | <a href="https://github.com/gopl-zh"> Go语言圣经中文版</a>, 仅学习交流使用</span>
</footer>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../ch5/ch5-02.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../ch5/ch5-04.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../ch5/ch5-02.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../ch5/ch5-04.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script type="text/javascript">
window.playground_copyable = true;
</script>
<script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="../book.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript" charset="utf-8">
var pagePath = "ch5/ch5-03.md"
</script>
<!-- Custom JS scripts -->
<script type="text/javascript" src="../js/custom.js"></script>
<script type="text/javascript" src="../js/bigPicture.js"></script>
</body>
</html>