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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" class = "active" > < 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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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 = "23-变量" > < a class = "header" href = "#23-变量" > 2.3. 变量< / a > < / h2 >
< p > var声明语句可以创建一个特定类型的变量， ， ： < / p >
< pre > < code class = "language-Go" > var 变量名字 类型 = 表达式
< / code > < / pre >
< p > 其中“< em > 类型< / em > ”或“< em > = 表达式< / em > ”两个部分可以省略其中的一个。如果省略的是类型信息，那么将根据初始化表达式来推导变量的类型信息。如果初始化表达式被省略，那么将用零值初始化该变量。 数值类型变量对应的零值是0， ， ， （ ） < / p >
< p > 零值初始化机制可以确保每个声明的变量总是有一个良好定义的值， ， ： < / p >
< pre > < code class = "language-Go" > var s string
fmt.Println(s) // " "
< / code > < / pre >
< p > 这段代码将打印一个空字符串， ， < / p >
< p > 也可以在一个声明语句中同时声明一组变量，或用一组初始化表达式声明并初始化一组变量。如果省略每个变量的类型，将可以声明多个类型不同的变量（类型由初始化表达式推导）：< / p >
< pre > < code class = "language-Go" > var i, j, k int // int, int, int
var b, f, s = true, 2.3, " four" // bool, float64, string
< / code > < / pre >
< p > 初始化表达式可以是字面量或任意的表达式。在包级别声明的变量会在main入口函数执行前完成初始化（ < / p >
< p > 一组变量也可以通过调用一个函数，由函数返回的多个返回值初始化：< / p >
< pre > < code class = "language-Go" > var f, err = os.Open(name) // os.Open returns a file and an error
< / code > < / pre >
< h3 id = "231-简短变量声明" > < a class = "header" href = "#231-简短变量声明" > 2.3.1. 简短变量声明< / a > < / h3 >
< p > 在函数内部，有一种称为简短变量声明语句的形式可用于声明和初始化局部变量。它以“名字 := 表达式”形式声明变量， （ ） ： < / p >
< pre > < code class = "language-Go" > anim := gif.GIF{LoopCount: nframes}
freq := rand.Float64() * 3.0
t := 0.0
< / code > < / pre >
< p > 因为简洁和灵活的特点， ， < / p >
< pre > < code class = "language-Go" > i := 100 // an int
var boiling float64 = 100 // a float64
var names []string
var err error
var p Point
< / code > < / pre >
< p > 和var形式声明语句一样， ： < / p >
< pre > < code class = "language-Go" > i, j := 0, 1
< / code > < / pre >
< p > 但是这种同时声明多个变量的方式应该限制只在可以提高代码可读性的地方使用， < / p >
< p > 请记住“:=”是一个变量声明语句，而“=”是一个变量赋值操作。也不要混淆多个变量的声明和元组的多重赋值（ < / p >
< pre > < code class = "language-Go" > i, j = j, i // 交换 i 和 j 的值
< / code > < / pre >
< p > 和普通var形式的变量声明语句一样， ， ： < / p >
< pre > < code class = "language-Go" > f, err := os.Open(name)
if err != nil {
return err
}
// ...use f...
f.Close()
< / code > < / pre >
< p > 这里有一个比较微妙的地方： （ < / p >
< p > 在下面的代码中， ， < / p >
< pre > < code class = "language-Go" > in, err := os.Open(infile)
// ...
out, err := os.Create(outfile)
< / code > < / pre >
< p > 简短变量声明语句中必须至少要声明一个新的变量，下面的代码将不能编译通过：< / p >
< pre > < code class = "language-Go" > f, err := os.Open(infile)
// ...
f, err := os.Create(outfile) // compile error: no new variables
< / code > < / pre >
< p > 解决的方法是第二个简短变量声明语句改用普通的多重赋值语句。< / p >
< p > 简短变量声明语句只有对已经在同级词法域声明过的变量才和赋值操作语句等价，如果变量是在外部词法域声明的，那么简短变量声明语句将会在当前词法域重新声明一个新的变量。我们在本章后面将会看到类似的例子。< / p >
< h3 id = "232-指针" > < a class = "header" href = "#232-指针" > 2.3.2. 指针< / a > < / h3 >
< p > 一个变量对应一个保存了变量对应类型值的内存空间。普通变量在声明语句创建时被绑定到一个变量名， ， ， ， ， < / p >
< p > 一个指针的值是另一个变量的地址。一个指针对应变量在内存中的存储位置。并不是每一个值都会有一个内存地址，但是对于每一个变量必然有对应的内存地址。通过指针，我们可以直接读或更新对应变量的值，而不需要知道该变量的名字（如果变量有名字的话）。< / p >
< p > 如果用“var x int”声明语句声明一个x变量， & x表达式（ ） ， < code > *int< / code > ， ， ， < code > *p< / code > 表达式对应p指针指向的变量的值。一般< code > *p< / code > 表达式读取指针指向的变量的值， ， < code > *p< / code > 对应一个变量，所以该表达式也可以出现在赋值语句的左边，表示更新指针所指向的变量的值。< / p >
< pre > < code class = "language-Go" > x := 1
p := & x // p, of type *int, points to x
fmt.Println(*p) // " 1"
*p = 2 // equivalent to x = 2
fmt.Println(x) // " 2"
< / code > < / pre >
< p > 对于聚合类型每个成员——比如结构体的每个字段、或者是数组的每个元素——也都是对应一个变量，因此可以被取地址。< / p >
< p > 变量有时候被称为可寻址的值。即使变量由表达式临时生成，那么表达式也必须能接受< code > & < / code > 取地址操作。< / p >
< p > 任何类型的指针的零值都是nil。如果p指向某个有效变量， < code > p != nil< / code > 测试为真。指针之间也是可以进行相等测试的， < / p >
< pre > < code class = "language-Go" > var x, y int
fmt.Println(& x == & x, & x == & y, & x == nil) // " true false false"
< / code > < / pre >
< p > 在Go语言中， ， ， ， < / p >
< pre > < code class = "language-Go" > var p = f()
func f() *int {
v := 1
return & v
}
< / code > < / pre >
< p > 每次调用f函数都将返回不同的结果： < / p >
< pre > < code class = "language-Go" > fmt.Println(f() == f()) // " false"
< / code > < / pre >
< p > 因为指针包含了一个变量的地址， ， ， ， （ ： < code > ++v< / code > 操作的模拟， ， ） ： < / p >
< pre > < code class = "language-Go" > func incr(p *int) int {
*p++ // 非常重要： ， ！ ！ ！
return *p
}
v := 1
incr(& v) // side effect: v is now 2
fmt.Println(incr(& v)) // " 3" (and v is 3)
< / code > < / pre >
< p > 每次我们对一个变量取地址，或者复制指针，我们都是为原变量创建了新的别名。例如，< code > *p< / code > 就是变量v的别名。指针特别有价值的地方在于我们可以不用名字而访问一个变量， ： ， （ ： ） ， ， ， < / p >
< p > 指针是实现标准库中flag包的关键技术， ， ， ， ： < code > -n< / code > 用于忽略行尾的换行符，< code > -s sep< / code > 用于指定分隔字符（ ） ， < / p >
< p > < u > < i > gopl.io/ch2/echo4< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Echo4 prints its command-line arguments.
package main
import (
" flag"
" fmt"
" strings"
)
var n = flag.Bool(" n" , false, " omit trailing newline" )
var sep = flag.String(" s" , " " , " separator" )
func main() {
flag.Parse()
fmt.Print(strings.Join(flag.Args(), *sep))
if !*n {
fmt.Println()
}
}
< / code > < / pre >
< p > 调用flag.Bool函数会创建一个新的对应布尔型标志参数的变量。它有三个属性： ， （ ） ， ， < code > -h< / code > 或< code > -help< / code > 参数， ， ， < code > sep< / code > 和< code > n< / code > 变量分别是指向对应命令行标志参数变量的指针，因此必须用< code > *sep< / code > 和< code > *n< / code > 形式的指针语法间接引用它们。< / p >
< p > 当程序运行时， ， （ ） ， ， ， < / p >
< p > 让我们运行一些echo测试用例： < / p >
< pre > < code > $ go build gopl.io/ch2/echo4
$ ./echo4 a bc def
a bc def
$ ./echo4 -s / a bc def
a/bc/def
$ ./echo4 -n a bc def
a bc def$
$ ./echo4 -help
Usage of ./echo4:
-n omit trailing newline
-s string
separator (default " " )
< / code > < / pre >
< h3 id = "233-new函数" > < a class = "header" href = "#233-new函数" > 2.3.3. new函数< / a > < / h3 >
< p > 另一个创建变量的方法是调用内建的new函数。表达式new(T)将创建一个T类型的匿名变量， ， ， < code > *T< / code > 。< / p >
< pre > < code class = "language-Go" > p := new(int) // p, *int 类型, 指向匿名的 int 变量
fmt.Println(*p) // " 0"
*p = 2 // 设置 int 匿名变量的值为 2
fmt.Println(*p) // " 2"
< / code > < / pre >
< p > 用new创建变量和普通变量声明语句方式创建变量没有什么区别， ， ， ， < / p >
< p > 下面的两个newInt函数有着相同的行为： < / p >
< pre > < code class = "language-Go" > func newInt() *int {
return new(int)
}
func newInt() *int {
var dummy int
return & dummy
}
< / code > < / pre >
< p > 每次调用new函数都是返回一个新的变量的地址， ： < / p >
< pre > < code class = "language-Go" > p := new(int)
q := new(int)
fmt.Println(p == q) // " false"
< / code > < / pre >
< p > 当然也可能有特殊情况： ， ， < code > struct{}< / code > 和< code > [0]int< / code > ， （ ） （ ： ， ， ， < code > runtime.SetFinalizer< / code > 函数相关文档）。< / p >
< p > new函数使用通常相对比较少， ， （ < / p >
< p > 由于new只是一个预定义的函数， ， ： < / p >
< pre > < code class = "language-Go" > func delta(old, new int) int { return new - old }
< / code > < / pre >
< p > 由于new被定义为int类型的变量名， < / p >
< h3 id = "234-变量的生命周期" > < a class = "header" href = "#234-变量的生命周期" > 2.3.4. 变量的生命周期< / a > < / h3 >
< p > 变量的生命周期指的是在程序运行期间变量有效存在的时间段。对于在包一级声明的变量来说，它们的生命周期和整个程序的运行周期是一致的。而相比之下，局部变量的生命周期则是动态的：每次从创建一个新变量的声明语句开始，直到该变量不再被引用为止，然后变量的存储空间可能被回收。函数的参数变量和返回值变量都是局部变量。它们在函数每次被调用的时候创建。< / p >
< p > 例如， ： < / p >
< pre > < code class = "language-Go" > for t := 0.0; t < cycles*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
blackIndex)
}
< / code > < / pre >
< p > 译注：函数的右小括弧也可以另起一行缩进，同时为了防止编译器在行尾自动插入分号而导致的编译错误，可以在末尾的参数变量后面显式插入逗号。像下面这样：< / p >
< pre > < code class = "language-Go" > for t := 0.0; t < cycles*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
img.SetColorIndex(
size+int(x*size+0.5), size+int(y*size+0.5),
blackIndex, // 最后插入的逗号不会导致编译错误，
) // 小括弧另起一行缩进，和大括弧的风格保存一致
}
< / code > < / pre >
< p > 在每次循环的开始会创建临时变量t， < / p >
< p > 那么Go语言的自动垃圾收集器是如何知道一个变量是何时可以被回收的呢？ ， ， ， ， ， ， < / p >
< p > 因为一个变量的有效周期只取决于是否可达，因此一个循环迭代内部的局部变量的生命周期可能超出其局部作用域。同时，局部变量可能在函数返回之后依然存在。< / p >
< p > 编译器会自动选择在栈上还是在堆上分配局部变量的存储空间， ， < / p >
< pre > < code class = "language-Go" > var global *int
func f() {
var x int
x = 1
global = & x
}
func g() {
y := new(int)
*y = 1
}
< / code > < / pre >
< p > f函数里的x变量必须在堆上分配， ， ； ， ， ， < code > *y< / code > 将是不可达的，也就是说可以马上被回收的。因此，< code > *y< / code > 并没有从函数g中逃逸， < code > *y< / code > 的存储空间（ ： ， ） ， ， ， ， ， < / p >
< p > Go语言的自动垃圾收集器对编写正确的代码是一个巨大的帮助， ， ， ， ， （ ） < / p >
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