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<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta charset="UTF-8">
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<title itemprop="name">Let's Go 1 — 变量与常量 - 真白的年轮面包</title>
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<h1 class="center-text glitch" data-text="Shiina Mashiro">Shiina Mashiro</h1>
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<h1 class="entry-title">Let's Go 1 — 变量与常量</h1>
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<h1 id="变量">变量</h1>
<h2 id="命名规范">命名规范</h2>
<p>通常采用驼峰式命名，当遇到特有名词（缩写或简称，如DNS）的时候，特有名词根据是否私有全部大写或小写。</p>
<p>形如：<code>apiClient</code> 、<code>URLString</code> 注意这里的 <code>api</code> (全小写) 和 <code>URL</code>（全大写）</p>
<h2 id="全局变量的声明与赋值">全局变量的声明与赋值</h2>
<pre><code class="language-go">package main
import "fmt"
// 指定类型的变量声明：如果没有为变量赋值，就需要指定变量的类型
var a,b,c int
// 变量赋值会自动匹配类型，无需指定
var d = 1 // 等同于 var d int = 1
// 一次赋值多个变量
var e,f,g = 1,2,3
// 变量代码块
var (
h = 4
i = 5
j,k = 6,7
)
func main(){
...
}
</code></pre>
<h2 id="局部变量的声明与赋值">局部变量的声明与赋值</h2>
<pre><code class="language-go">package main
import "fmt"
func main(){
// 除了上面讲到的声明方式外，函数体内可以通过更简单的方式定义临时变量
a := 6
// 也可以一次多定义几个
b,c := 8,10
// 声明的局部变量如果没被使用就会报错
fmt.Println(a,b,c)// 输出结果 6,8,10
}
</code></pre>
<p>**注意：**如果变量类型不足以存储变量值会导致溢出错误</p>
<pre><code class="language-go">var num byte = 1000000 // overflows
</code></pre>
<h3 id="多变量赋值">多变量赋值</h3>
<p><strong>多变量赋值时，先计算所有相关值，然后再从左到右依次赋值</strong></p>
<p>看下面代码</p>
<pre><code class="language-go">package main
import "fmt"
func main() {
data, i := [3]int{1,2,3},0
i,data[i] = 2,5
fmt.Println(data,i,data[i])// 输出结果:[5 2 3] 2 3
}
</code></pre>
<blockquote>
<p>问：为什么不是输出 [1,2,5] 2 3 ？</p>
<p>答：先计算，后赋值，data[i]中的i被当作0看待<br>
<strong>即 <code>i,data[i] = 2, 5</code> 当作 <code>i,data[0] = 2,5</code></strong></p>
</blockquote>
<p>因此，如果你想交换两个元素的值就变得非常简单了</p>
<pre><code class="language-go">//交换i和j的值
i,j := j,i
</code></pre>
<h3 id="多变量赋值的占位符">多变量赋值的占位符</h3>
<p>如果函数返回多个返回值，而你只想要把其中的一个或几个赋值给相应的变量，可以使用占位符 <code>_</code></p>
<pre><code class="language-go">package main
import "fmt"
// 指定方法的返回类型
func test() (int, string) {
return 1, "abc"
}
func main() {
_, s := test()
fmt.Println(s)// s获取到test()的第二个返回值，输出abc
}
</code></pre>
<h2 id="同名变量的优先级">同名变量的优先级</h2>
<p><strong>同java相同，作用域越小的变量优先级就越高</strong></p>
<pre><code class="language-go">package main
import "fmt"
var num = 1
func main() {
fmt.Println(num)// 使用全局变量，输出1
num := 2
fmt.Println(num)// 使用main中的局部变量，输出2
{
num := 3
fmt.Println(num)// 使用代码块中的局部变量，输出3
}
}
</code></pre>
<h1 id="常量">常量</h1>
<p><strong>常量值</strong> <strong>必须</strong>是<strong>编译器可确定</strong>的 <strong>数字</strong>、<strong>字符串</strong>、<strong>布尔值。</strong><br>
这意味着，<strong>常量必须在声明的时候初始化</strong></p>
<h2 id="命名规范-1">命名规范</h2>
<p>同变量规则，力求语义表达完整清楚，不要嫌名字长。<br>
如果模块复杂，为避免混淆，可按功能统一定义在package下的一个文件中。<br>
通常也使用驼峰命名法</p>
<h2 id="常量与常量组的声明">常量与常量组的声明</h2>
<pre><code class="language-go">const x, y int = 1, 2 // 多常量初始化
const s = "Hello, World!" // 类型推断
const ( // 常量组
a, b = 10, 100
c bool = false
d //如果不提供类型和初始化值则视为和上一常量相同，即 d = false
//常量值可以使用len、cap、unsafe.Sizeof等编译器可确定的函数返回值
e = len(a)
f = unsafe.Sizeof(b)
)
func main() {
const x = "xxx" // 未使⽤用局部常量不会引发编译错误。
}
</code></pre>
<p>**注意：**如果常量类型不足以存储变量值会导致溢出错误</p>
<pre><code class="language-go">const num byte = 1000000 // overflows
</code></pre>
<h2 id="枚举">枚举</h2>
<p>枚举其实就是一个带自增特性的常量组，而这个自增特性是通过关键字 <code>iota</code> 实现的。</p>
<blockquote>
<p>iota是golang语言的常量计数器,只能在常量的表达式中使用。<br>
iota 定义常量组中从 0 开始按行计数的自增枚举值。</p>
</blockquote>
<pre><code class="language-go">const (
Sunday = iota // 0
Monday // 1，通常省略后续⾏行表达式。
Tuesday // 2
Wednesday // 3
Thursday // 4
Friday // 5
Saturday // 6
)
</code></pre>
<p>由于常量赋值是可以用编译器能解释的计算式表达的，所以就出现了下面的骚操作</p>
<pre><code class="language-go">const (
_ = iota // iota = 0
KB int64 = 1 << (10 * iota) // iota = 1
MB // 与 KB 表达式相同，但 iota = 2
GB // 同上，但 iota = 3
TB // 同上，但 iota = 4
)
</code></pre>
<p>上述方式在用于表达具有<strong>数量级关系</strong>的枚举时特别有用</p>
<h3 id="多个iota的各自增长">多个iota的各自增长</h3>
<p>在同一常量组中，可以提供多个 iota，它们<strong>各自增长，互不影响</strong>。</p>
<pre><code class="language-go">const (
A, B = iota, iota << 10 // 0, 0 << 10
C, D // 1, 1 << 10
)
</code></pre>
<p>**技巧：**一列一列的看，同一列上的枚举项共用计数器 <code>iota</code></p>
<h3 id="自增中出现显示赋值">自增中出现显示赋值</h3>
<pre><code class="language-go">const (
A = iota // 0
B // 1
C = "c" // c
D // c，与上⼀一⾏行相同。
E = iota // 4，显式恢复。注意计数包含了 C、D 两⾏行。
F // 5
)
</code></pre>
<h3 id="通过自定义类型实现枚举类型限制">通过自定义类型实现枚举类型限制</h3>
<pre><code class="language-go">package main
import "fmt"
// 声明一种叫Color的类型，其本质是一个int
type Color int
const (
Black Color = iota // 定义一个Color类型的枚举项
Red // ...
Blue // ...
)
//必须传入枚举类型Color的函数
func test(c Color) {
fmt.Println(c)
}
func main() {
c := Black
test(c) //输出0
x := 1
test(x) // Error: cannot use x (type int) as type Color in function argument
test(1) // 常量会被编译器自动转换，不会报错
test(10) // 常量只要类型与枚举类型一致即可，即便其值不被枚举所包括也不会报错
const numd = 10
test(numd) // 常量只要类型与枚举类型一致即可，即便其值不被枚举所包括也不会报错
}
</code></pre>
<p>也就是说<strong>类型限制只能限制变量</strong></p>
<h1 id="参考与补充">参考与补充</h1>
<ul>
<li><a href="javascript:;">Go 边看边练</a></li>
<li><a href="javascript:;">Golang命名规范和开发规范</a></li>
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