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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" class="active"><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"><s
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="22-声明"><a class="header" href="#22-声明">2.2. 声明</a></h2>
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<p>声明语句定义了程序的各种实体对象以及部分或全部的属性。Go语言主要有四种类型的声明语句：var、const、type和func，分别对应变量、常量、类型和函数实体对象的声明。这一章我们重点讨论变量和类型的声明，第三章将讨论常量的声明，第五章将讨论函数的声明。</p>
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<p>一个Go语言编写的程序对应一个或多个以.go为文件后缀名的源文件。每个源文件中以包的声明语句开始，说明该源文件是属于哪个包。包声明语句之后是import语句导入依赖的其它包，然后是包一级的类型、变量、常量、函数的声明语句，包一级的各种类型的声明语句的顺序无关紧要（译注：函数内部的名字则必须先声明之后才能使用）。例如，下面的例子中声明了一个常量、一个函数和两个变量：</p>
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<p><u><i>gopl.io/ch2/boiling</i></u></p>
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<pre><code class="language-Go">// Boiling prints the boiling point of water.
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package main
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import "fmt"
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const boilingF = 212.0
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func main() {
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var f = boilingF
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var c = (f - 32) * 5 / 9
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fmt.Printf("boiling point = %g°F or %g°C\n", f, c)
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// Output:
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// boiling point = 212°F or 100°C
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}
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</code></pre>
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<p>其中常量boilingF是在包一级范围声明语句声明的，然后f和c两个变量是在main函数内部声明的声明语句声明的。在包一级声明语句声明的名字可在整个包对应的每个源文件中访问，而不是仅仅在其声明语句所在的源文件中访问。相比之下，局部声明的名字就只能在函数内部很小的范围被访问。</p>
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<p>一个函数的声明由一个函数名字、参数列表（由函数的调用者提供参数变量的具体值）、一个可选的返回值列表和包含函数定义的函数体组成。如果函数没有返回值，那么返回值列表是省略的。执行函数从函数的第一个语句开始，依次顺序执行直到遇到return返回语句，如果没有返回语句则是执行到函数末尾，然后返回到函数调用者。</p>
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<p>我们已经看到过很多函数声明和函数调用的例子了，在第五章将深入讨论函数的相关细节，这里只简单解释下。下面的fToC函数封装了温度转换的处理逻辑，这样它只需要被定义一次，就可以在多个地方多次被使用。在这个例子中，main函数就调用了两次fToC函数，分别使用在局部定义的两个常量作为调用函数的参数。</p>
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<p><u><i>gopl.io/ch2/ftoc</i></u></p>
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<pre><code class="language-Go">// Ftoc prints two Fahrenheit-to-Celsius conversions.
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package main
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import "fmt"
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func main() {
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const freezingF, boilingF = 32.0, 212.0
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fmt.Printf("%g°F = %g°C\n", freezingF, fToC(freezingF)) // "32°F = 0°C"
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fmt.Printf("%g°F = %g°C\n", boilingF, fToC(boilingF)) // "212°F = 100°C"
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}
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func fToC(f float64) float64 {
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return (f - 32) * 5 / 9
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}
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</code></pre>
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<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>
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