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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
<h2 id="25-类型"><a class="header" href="#25-类型">2.5. 类型</a></h2>
<p>变量或表达式的类型定义了对应存储值的属性特征例如数值在内存的存储大小或者是元素的bit个数它们在内部是如何表达的是否支持一些操作符以及它们自己关联的方法集等。</p>
<p>在任何程序中都会存在一些变量有着相同的内部结构但是却表示完全不同的概念。例如一个int类型的变量可以用来表示一个循环的迭代索引、或者一个时间戳、或者一个文件描述符、或者一个月份一个float64类型的变量可以用来表示每秒移动几米的速度、或者是不同温度单位下的温度一个字符串可以用来表示一个密码或者一个颜色的名称。</p>
<p>一个类型声明语句创建了一个新的类型名称,和现有类型具有相同的底层结构。新命名的类型提供了一个方法,用来分隔不同概念的类型,这样即使它们底层类型相同也是不兼容的。</p>
<pre><code class="language-Go">type 类型名字 底层类型
</code></pre>
<p>类型声明语句一般出现在包一级,因此如果新创建的类型名字的首字符大写,则在包外部也可以使用。</p>
<p>译注对于中文汉字Unicode标志都作为小写字母处理因此中文的命名默认不能导出不过国内的用户针对该问题提出了不同的看法根据RobPike的回复在Go2中有可能会将中日韩等字符当作大写字母处理。下面是RobPik在 <a href="https://github.com/golang/go/issues/5763">Issue763</a> 的回复:</p>
<blockquote>
<p>A solution that's been kicking around for a while:</p>
<p>For Go 2 (can't do it before then): Change the definition to “lower case letters and _ are package-local; all else is exported”. Then with non-cased languages, such as Japanese, we can write 日本语 for an exported name and _日本语 for a local name. This rule has no effect, relative to the Go 1 rule, with cased languages. They behave exactly the same.</p>
</blockquote>
<p>为了说明类型声明,我们将不同温度单位分别定义为不同的类型:</p>
<p><u><i>gopl.io/ch2/tempconv0</i></u></p>
<pre><code class="language-Go">// Package tempconv performs Celsius and Fahrenheit temperature computations.
package tempconv
import &quot;fmt&quot;
type Celsius float64 // 摄氏温度
type Fahrenheit float64 // 华氏温度
const (
AbsoluteZeroC Celsius = -273.15 // 绝对零度
FreezingC Celsius = 0 // 结冰点温度
BoilingC Celsius = 100 // 沸水温度
)
func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }
</code></pre>
<p>我们在这个包声明了两种类型Celsius和Fahrenheit分别对应不同的温度单位。它们虽然有着相同的底层类型float64但是它们是不同的数据类型因此它们不可以被相互比较或混在一个表达式运算。刻意区分类型可以避免一些像无意中使用不同单位的温度混合计算导致的错误因此需要一个类似Celsius(t)或Fahrenheit(t)形式的显式转型操作才能将float64转为对应的类型。Celsius(t)和Fahrenheit(t)是类型转换操作它们并不是函数调用。类型转换不会改变值本身但是会使它们的语义发生变化。另一方面CToF和FToC两个函数则是对不同温度单位下的温度进行换算它们会返回不同的值。</p>
<p>对于每一个类型T都有一个对应的类型转换操作T(x)用于将x转为T类型译注如果T是指针类型可能会需要用小括弧包装T比如<code>(*int)(0)</code>。只有当两个类型的底层基础类型相同时才允许这种转型操作或者是两者都是指向相同底层结构的指针类型这些转换只改变类型而不会影响值本身。如果x是可以赋值给T类型的值那么x必然也可以被转为T类型但是一般没有这个必要。</p>
<p>数值类型之间的转型也是允许的并且在字符串和一些特定类型的slice之间也是可以转换的在下一章我们会看到这样的例子。这类转换可能改变值的表现。例如将一个浮点数转为整数将丢弃小数部分将一个字符串转为<code>[]byte</code>类型的slice将拷贝一个字符串数据的副本。在任何情况下运行时不会发生转换失败的错误译注: 错误只会发生在编译阶段)。</p>
<p>底层数据类型决定了内部结构和表达方式也决定是否可以像底层类型一样对内置运算符的支持。这意味着Celsius和Fahrenheit类型的算术运算行为和底层的float64类型是一样的正如我们所期望的那样。</p>
<pre><code class="language-Go">fmt.Printf(&quot;%g\n&quot;, BoilingC-FreezingC) // &quot;100&quot; °C
boilingF := CToF(BoilingC)
fmt.Printf(&quot;%g\n&quot;, boilingF-CToF(FreezingC)) // &quot;180&quot; °F
fmt.Printf(&quot;%g\n&quot;, boilingF-FreezingC) // compile error: type mismatch
</code></pre>
<p>比较运算符<code>==</code><code>&lt;</code>也可以用来比较一个命名类型的变量和另一个有相同类型的变量,或有着相同底层类型的未命名类型的值之间做比较。但是如果两个值有着不同的类型,则不能直接进行比较:</p>
<pre><code class="language-Go">var c Celsius
var f Fahrenheit
fmt.Println(c == 0) // &quot;true&quot;
fmt.Println(f &gt;= 0) // &quot;true&quot;
fmt.Println(c == f) // compile error: type mismatch
fmt.Println(c == Celsius(f)) // &quot;true&quot;!
</code></pre>
<p>注意最后那个语句。尽管看起来像函数调用但是Celsius(f)是类型转换操作它并不会改变值仅仅是改变值的类型而已。测试为真的原因是因为c和f都是零值。</p>
<p>一个命名的类型可以提供书写方便特别是可以避免一遍又一遍地书写复杂类型译注例如用匿名的结构体定义变量。虽然对于像float64这种简单的底层类型没有简洁很多但是如果是复杂的类型将会简洁很多特别是我们即将讨论的结构体类型。</p>
<p>命名类型还可以为该类型的值定义新的行为。这些行为表示为一组关联到该类型的函数集合,我们称为类型的方法集。我们将在第六章中讨论方法的细节,这里只说些简单用法。</p>
<p>下面的声明语句Celsius类型的参数c出现在了函数名的前面表示声明的是Celsius类型的一个名叫String的方法该方法返回该类型对象c带着°C温度单位的字符串</p>
<pre><code class="language-Go">func (c Celsius) String() string { return fmt.Sprintf(&quot;%g°C&quot;, c) }
</code></pre>
<p>许多类型都会定义一个String方法因为当使用fmt包的打印方法时将会优先使用该类型对应的String方法返回的结果打印我们将在7.1节讲述。</p>
<pre><code class="language-Go">c := FToC(212.0)
fmt.Println(c.String()) // &quot;100°C&quot;
fmt.Printf(&quot;%v\n&quot;, c) // &quot;100°C&quot;; no need to call String explicitly
fmt.Printf(&quot;%s\n&quot;, c) // &quot;100°C&quot;
fmt.Println(c) // &quot;100°C&quot;
fmt.Printf(&quot;%g\n&quot;, c) // &quot;100&quot;; does not call String
fmt.Println(float64(c)) // &quot;100&quot;; does not call String
</code></pre>
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