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2023-06-26 17:30:52 +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-zh.html" > 译者序< / 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" class = "active" > < 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.<
2022-08-04 06:59:33 +00:00
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2024-06-12 07:20:07 +00:00
< 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 >
2023-06-26 17:46:27 +00:00
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2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "27-作用域" > < a class = "header" href = "#27-作用域" > 2.7. 作用域< / a > < / h2 >
< p > 一个声明语句将程序中的实体和一个名字关联，比如一个函数或一个变量。声明语句的作用域是指源代码中可以有效使用这个名字的范围。< / p >
< p > 不要将作用域和生命周期混为一谈。声明语句的作用域对应的是一个源代码的文本区域；它是一个编译时的属性。一个变量的生命周期是指程序运行时变量存在的有效时间段，在此时间区域内它可以被程序的其他部分引用；是一个运行时的概念。< / p >
< p > 句法块是由花括弧所包含的一系列语句， （ ） ， ， ， ， ； ； ， ； ； （ ） < / p >
< p > 声明语句对应的词法域决定了作用域范围的大小。对于内置的类型、函数和常量， ， （ ） ， ， ， ， ， ， ， （ ） < / p >
< p > 控制流标号， ， < / p >
< p > 一个程序可能包含多个同名的声明， ， ， ， ， ， ， < / p >
< p > 当编译器遇到一个名字引用时，它会对其定义进行查找，查找过程从最内层的词法域向全局的作用域进行。如果查找失败，则报告“未声明的名字”这样的错误。如果该名字在内部和外部的块分别声明过，则内部块的声明首先被找到。在这种情况下，内部声明屏蔽了外部同名的声明，让外部的声明的名字无法被访问：< / p >
< pre > < code class = "language-Go" > func f() {}
var g = " g"
func main() {
f := " f"
fmt.Println(f) // " f" ; local var f shadows package-level func f
fmt.Println(g) // " g" ; package-level var
fmt.Println(h) // compile error: undefined: h
}
< / code > < / pre >
< p > 在函数中词法域可以深度嵌套， ， （ ， ） < / p >
< pre > < code class = "language-Go" > func main() {
x := " hello!"
for i := 0; i < len(x); i++ {
x := x[i]
if x != '!' {
x := x + 'A' - 'a'
fmt.Printf(" %c" , x) // " HELLO" (one letter per iteration)
}
}
}
< / code > < / pre >
< p > 在< code > x[i]< / code > 和< code > x + 'A' - 'a'< / code > 声明语句的初始化的表达式中都引用了外部作用域声明的x变量， （ ， ） < / p >
< p > 正如上面例子所示， ； ： ， ， ， （ < code > i++< / code > ），当然也包含循环体词法域。< / p >
< p > 下面的例子同样有三个不同的x变量， ， ， ， ； ： < / p >
< pre > < code class = "language-Go" > func main() {
x := " hello"
for _, x := range x {
x := x + 'A' - 'a'
fmt.Printf(" %c" , x) // " HELLO" (one letter per iteration)
}
}
< / code > < / pre >
< p > 和for循环类似， ， ： < / p >
< pre > < code class = "language-Go" > if x := f(); x == 0 {
fmt.Println(x)
} else if y := g(x); x == y {
fmt.Println(x, y)
} else {
fmt.Println(x, y)
}
fmt.Println(x, y) // compile error: x and y are not visible here
< / code > < / pre >
< p > 第二个if语句嵌套在第一个内部， ： ， < / p >
< p > 在包级别，声明的顺序并不会影响作用域范围，因此一个先声明的可以引用它自身或者是引用后面的一个声明，这可以让我们定义一些相互嵌套或递归的类型或函数。但是如果一个变量或常量递归引用了自身，则会产生编译错误。< / p >
< p > 在这个程序中：< / p >
< pre > < code class = "language-Go" > if f, err := os.Open(fname); err != nil { // compile error: unused: f
return err
}
f.ReadByte() // compile error: undefined f
f.Close() // compile error: undefined f
< / code > < / pre >
< p > 变量f的作用域只在if语句内， ， ， < / p >
< p > 通常需要在if之前声明变量， ： < / p >
< pre > < code class = "language-Go" > f, err := os.Open(fname)
if err != nil {
return err
}
f.ReadByte()
f.Close()
< / code > < / pre >
< p > 你可能会考虑通过将ReadByte和Close移动到if的else块来解决这个问题： < / p >
< pre > < code class = "language-Go" > if f, err := os.Open(fname); err != nil {
return err
} else {
// f and err are visible here too
f.ReadByte()
f.Close()
}
< / code > < / pre >
< p > 但这不是Go语言推荐的做法， ， < / p >
< p > 要特别注意短变量声明语句的作用域范围， ， ， ， ， < / p >
< pre > < code class = "language-Go" > var cwd string
func init() {
cwd, err := os.Getwd() // compile error: unused: cwd
if err != nil {
log.Fatalf(" os.Getwd failed: %v" , err)
}
}
< / code > < / pre >
< p > 虽然cwd在外部已经声明过， < code > :=< / code > 语句还是将cwd和err重新声明为新的局部变量。因为内部声明的cwd将屏蔽外部的声明， < / p >
< p > 由于当前的编译器会检测到局部声明的cwd并没有使用， ， ， ， < / p >
< pre > < code class = "language-Go" > var cwd string
func init() {
cwd, err := os.Getwd() // NOTE: wrong!
if err != nil {
log.Fatalf(" os.Getwd failed: %v" , err)
}
log.Printf(" Working directory = %s" , cwd)
}
< / code > < / pre >
< p > 全局的cwd变量依然是没有被正确初始化的， < / p >
< p > 有许多方式可以避免出现类似潜在的问题。最直接的方法是通过单独声明err变量， < code > :=< / code > 的简短声明方式：< / p >
< pre > < code class = "language-Go" > var cwd string
func init() {
var err error
cwd, err = os.Getwd()
if err != nil {
log.Fatalf(" os.Getwd failed: %v" , err)
}
}
< / code > < / pre >
< p > 我们已经看到包、文件、声明和语句如何来表达一个程序结构。在下面的两个章节，我们将探讨数据的结构。< / p >
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