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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" > < 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" > < strong aria-hidd
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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 = "62-基于指针对象的方法" > < a class = "header" href = "#62-基于指针对象的方法" > 6.2. 基于指针对象的方法< / a > < / h2 >
< p > 当调用一个函数时，会对其每一个参数值进行拷贝，如果一个函数需要更新一个变量，或者函数的其中一个参数实在太大我们希望能够避免进行这种默认的拷贝，这种情况下我们就需要用到指针了。对应到我们这里用来更新接收器的对象的方法，当这个接受者变量本身比较大时，我们就可以用其指针而不是对象来声明方法，如下：< / p >
< pre > < code class = "language-go" > func (p *Point) ScaleBy(factor float64) {
p.X *= factor
p.Y *= factor
}
< / code > < / pre >
< p > 这个方法的名字是< code > (*Point).ScaleBy< / code > 。这里的括号是必须的；没有括号的话这个表达式可能会被理解为< code > *(Point.ScaleBy)< / code > 。< / p >
< p > 在现实的程序里， ， ， < / p >
< p > 只有类型（ ） < code > (*Point)< / code > ，才可能是出现在接收器声明里的两种接收器。此外，为了避免歧义，在声明方法时，如果一个类型名本身是一个指针的话，是不允许其出现在接收器中的，比如下面这个例子：< / p >
< pre > < code class = "language-go" > type P *int
func (P) f() { /* ... */ } // compile error: invalid receiver type
< / code > < / pre >
< p > 想要调用指针类型方法< code > (*Point).ScaleBy< / code > ， ， < / p >
< pre > < code class = "language-go" > r := & Point{1, 2}
r.ScaleBy(2)
fmt.Println(*r) // " {2, 4}"
< / code > < / pre >
< p > 或者这样：< / p >
< pre > < code class = "language-go" > p := Point{1, 2}
pptr := & p
pptr.ScaleBy(2)
fmt.Println(p) // " {2, 4}"
< / code > < / pre >
< p > 或者这样:< / p >
< pre > < code class = "language-go" > p := Point{1, 2}
(& p).ScaleBy(2)
fmt.Println(p) // " {2, 4}"
< / code > < / pre >
< p > 不过后面两种方法有些笨拙。幸运的是， ， ， ： < / p >
< pre > < code class = "language-go" > p.ScaleBy(2)
< / code > < / pre >
< p > 编译器会隐式地帮我们用& p去调用ScaleBy这个方法。这种简写方法只适用于“变量”， ， < / p >
< pre > < code class = "language-go" > Point{1, 2}.ScaleBy(2) // compile error: can't take address of Point literal
< / code > < / pre >
< p > 但是我们可以用一个< code > *Point< / code > 这样的接收器来调用Point的方法， ， < code > *< / code > 来取到该变量即可。编译器在这里也会给我们隐式地插入< code > *< / code > 这个操作符，所以下面这两种写法等价的：< / p >
< pre > < code class = "language-Go" > pptr.Distance(q)
(*pptr).Distance(q)
< / code > < / pre >
< p > 这里的几个例子可能让你有些困惑，所以我们总结一下：在每一个合法的方法调用表达式中，也就是下面三种情况里的任意一种情况都是可以的：< / p >
< p > 要么接收器的实际参数和其形式参数是相同的类型， < code > *T< / code > ： < / p >
< pre > < code class = "language-go" > Point{1, 2}.Distance(q) // Point
pptr.ScaleBy(2) // *Point
< / code > < / pre >
< p > 或者接收器实参是类型T， < code > *T< / code > ，这种情况下编译器会隐式地为我们取变量的地址：< / p >
< pre > < code class = "language-go" > p.ScaleBy(2) // implicit (& p)
< / code > < / pre >
< p > 或者接收器实参是类型< code > *T< / code > ， ， ： < / p >
< pre > < code class = "language-go" > pptr.Distance(q) // implicit (*pptr)
< / code > < / pre >
< p > 如果命名类型T（ ： ） （ < code > *T< / code > ） ， ； ， ， ， ， ， ， < / p >
< p > < strong > 译注：< / strong > 作者这里说的比较绕，其实有两点：< / p >
< ol >
< li > 不管你的method的receiver是指针类型还是非指针类型， < / li >
< li > 在声明一个method的receiver该是指针还是非指针类型时， ， ， ， ； ， ， ， < / li >
< / ol >
< h3 id = "621-nil也是一个合法的接收器类型" > < a class = "header" href = "#621-nil也是一个合法的接收器类型" > 6.2.1. Nil也是一个合法的接收器类型< / a > < / h3 >
< p > 就像一些函数允许nil指针作为参数一样， ， ， ， ： < / p >
< pre > < code class = "language-go" > // An IntList is a linked list of integers.
// A nil *IntList represents the empty list.
type IntList struct {
Value int
Tail *IntList
}
// Sum returns the sum of the list elements.
func (list *IntList) Sum() int {
if list == nil {
return 0
}
return list.Value + list.Tail.Sum()
}
< / code > < / pre >
< p > 当你定义一个允许nil作为接收器值的方法的类型时， ， < / p >
< p > 下面是net/url包里Values类型定义的一部分。< / p >
< p > < u > < i > net/url< / i > < / u > < / p >
< pre > < code class = "language-go" > package url
// Values maps a string key to a list of values.
type Values map[string][]string
// Get returns the first value associated with the given key,
// or " " if there are none.
func (v Values) Get(key string) string {
if vs := v[key]; len(vs) > 0 {
return vs[0]
}
return " "
}
// Add adds the value to key.
// It appends to any existing values associated with key.
func (v Values) Add(key, value string) {
v[key] = append(v[key], value)
}
< / code > < / pre >
< p > 这个定义向外部暴露了一个map的命名类型， ， （ ， ， < / p >
< p > < u > < i > gopl.io/ch6/urlvalues< / i > < / u > < / p >
< pre > < code class = "language-go" > m := url.Values{" lang" : {" en" }} // direct construction
m.Add(" item" , " 1" )
m.Add(" item" , " 2" )
fmt.Println(m.Get(" lang" )) // " en"
fmt.Println(m.Get(" q" )) // " "
fmt.Println(m.Get(" item" )) // " 1" (first value)
fmt.Println(m[" item" ]) // " [1 2]" (direct map access)
m = nil
fmt.Println(m.Get(" item" )) // " "
m.Add(" item" , " 3" ) // panic: assignment to entry in nil map
< / code > < / pre >
< p > 对Get的最后一次调用中， " item" )， " item" )的话是无法通过编译的， ， ， < / p >
< p > 由于url.Values是一个map类型， ， ， ， ， ， ， ， （ ： ， ， ， ） < / p >
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