2022-08-04 06:59:33 +00:00
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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" class = "active" > < 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 = "44-结构体" > < a class = "header" href = "#44-结构体" > 4.4. 结构体< / a > < / h2 >
< p > 结构体是一种聚合的数据类型，是由零个或多个任意类型的值聚合成的实体。每个值称为结构体的成员。用结构体的经典案例是处理公司的员工信息，每个员工信息包含一个唯一的员工编号、员工的名字、家庭住址、出生日期、工作岗位、薪资、上级领导等等。所有的这些信息都需要绑定到一个实体中，可以作为一个整体单元被复制，作为函数的参数或返回值，或者是被存储到数组中，等等。< / p >
< p > 下面两个语句声明了一个叫Employee的命名的结构体类型， ： < / p >
< pre > < code class = "language-Go" > type Employee struct {
ID int
Name string
Address string
DoB time.Time
Position string
Salary int
ManagerID int
}
var dilbert Employee
< / code > < / pre >
< p > dilbert结构体变量的成员可以通过点操作符访问， ， ， ： < / p >
< pre > < code class = "language-Go" > dilbert.Salary -= 5000 // demoted, for writing too few lines of code
< / code > < / pre >
< p > 或者是对成员取地址，然后通过指针访问：< / p >
< pre > < code class = "language-Go" > position := & dilbert.Position
*position = " Senior " + *position // promoted, for outsourcing to Elbonia
< / code > < / pre >
< p > 点操作符也可以和指向结构体的指针一起工作：< / p >
< pre > < code class = "language-Go" > var employeeOfTheMonth *Employee = & dilbert
employeeOfTheMonth.Position += " (proactive team player)"
< / code > < / pre >
< p > 相当于下面语句< / p >
< pre > < code class = "language-Go" > (*employeeOfTheMonth).Position += " (proactive team player)"
< / code > < / pre >
< p > 下面的EmployeeByID函数将根据给定的员工ID返回对应的员工信息结构体的指针。我们可以使用点操作符来访问它里面的成员： < / p >
< pre > < code class = "language-Go" > func EmployeeByID(id int) *Employee { /* ... */ }
fmt.Println(EmployeeByID(dilbert.ManagerID).Position) // " Pointy-haired boss"
id := dilbert.ID
EmployeeByID(id).Salary = 0 // fired for... no real reason
< / code > < / pre >
< p > 后面的语句通过EmployeeByID返回的结构体指针更新了Employee结构体的成员。如果将EmployeeByID函数的返回值从< code > *Employee< / code > 指针类型改为Employee值类型， ， （ ： ， ） < / p >
< p > 通常一行对应一个结构体成员， ， ， ： < / p >
< pre > < code class = "language-Go" > type Employee struct {
ID int
Name, Address string
DoB time.Time
Position string
Salary int
ManagerID int
}
< / code > < / pre >
< p > 结构体成员的输入顺序也有重要的意义。我们也可以将Position成员合并（ ） ， ， ， < / p >
< p > 如果结构体成员名字是以大写字母开头的， ； < / p >
< p > 结构体类型往往是冗长的， ， ， ， < / p >
< p > 一个命名为S的结构体类型将不能再包含S类型的成员： （ ） < code > *S< / code > 指针类型的成员，这可以让我们创建递归的数据结构，比如链表和树结构等。在下面的代码中，我们使用一个二叉树来实现一个插入排序：< / p >
< p > < u > < i > gopl.io/ch4/treesort< / i > < / u > < / p >
< pre > < code class = "language-Go" > type tree struct {
value int
left, right *tree
}
// Sort sorts values in place.
func Sort(values []int) {
var root *tree
for _, v := range values {
root = add(root, v)
}
appendValues(values[:0], root)
}
// appendValues appends the elements of t to values in order
// and returns the resulting slice.
func appendValues(values []int, t *tree) []int {
if t != nil {
values = appendValues(values, t.left)
values = append(values, t.value)
values = appendValues(values, t.right)
}
return values
}
func add(t *tree, value int) *tree {
if t == nil {
// Equivalent to return & tree{value: value}.
t = new(tree)
t.value = value
return t
}
if value < t.value {
t.left = add(t.left, value)
} else {
t.right = add(t.right, value)
}
return t
}
< / code > < / pre >
< p > 结构体类型的零值是每个成员都是零值。通常会将零值作为最合理的默认值。例如， ， ， ， < / p >
< p > 如果结构体没有任何成员的话就是空结构体， ， ， ， ， ， ， ， < / p >
< pre > < code class = "language-Go" > seen := make(map[string]struct{}) // set of strings
// ...
if _, ok := seen[s]; !ok {
seen[s] = struct{}{}
// ...first time seeing s...
}
< / code > < / pre >
< h3 id = "441-结构体字面值" > < a class = "header" href = "#441-结构体字面值" > 4.4.1. 结构体字面值< / a > < / h3 >
< p > 结构体值也可以用结构体字面值表示，结构体字面值可以指定每个成员的值。< / p >
< pre > < code class = "language-Go" > type Point struct{ X, Y int }
p := Point{1, 2}
< / code > < / pre >
< p > 这里有两种形式的结构体字面值语法， ， ， ， ， ， ， < / p >
< p > 其实更常用的是第二种写法， ， ， ： < / p >
< pre > < code class = "language-Go" > anim := gif.GIF{LoopCount: nframes}
< / code > < / pre >
< p > 在这种形式的结构体字面值写法中，如果成员被忽略的话将默认用零值。因为提供了成员的名字，所以成员出现的顺序并不重要。< / p >
< p > 两种不同形式的写法不能混合使用。而且，你不能企图在外部包中用第一种顺序赋值的技巧来偷偷地初始化结构体中未导出的成员。< / p >
< pre > < code class = "language-Go" > package p
type T struct{ a, b int } // a and b are not exported
package q
import " p"
var _ = p.T{a: 1, b: 2} // compile error: can't reference a, b
var _ = p.T{1, 2} // compile error: can't reference a, b
< / code > < / pre >
< p > 虽然上面最后一行代码的编译错误信息中并没有显式提到未导出的成员，但是这样企图隐式使用未导出成员的行为也是不允许的。< / p >
< p > 结构体可以作为函数的参数和返回值。例如， ： < / p >
< pre > < code class = "language-Go" > func Scale(p Point, factor int) Point {
return Point{p.X * factor, p.Y * factor}
}
fmt.Println(Scale(Point{1, 2}, 5)) // " {5 10}"
< / code > < / pre >
< p > 如果考虑效率的话，较大的结构体通常会用指针的方式传入和返回，< / p >
< pre > < code class = "language-Go" > func Bonus(e *Employee, percent int) int {
return e.Salary * percent / 100
}
< / code > < / pre >
< p > 如果要在函数内部修改结构体成员的话， ； ， ， < / p >
< pre > < code class = "language-Go" > func AwardAnnualRaise(e *Employee) {
e.Salary = e.Salary * 105 / 100
}
< / code > < / pre >
< p > 因为结构体通常通过指针处理，可以用下面的写法来创建并初始化一个结构体变量，并返回结构体的地址：< / p >
< pre > < code class = "language-Go" > pp := & Point{1, 2}
< / code > < / pre >
< p > 它和下面的语句是等价的< / p >
< pre > < code class = "language-Go" > pp := new(Point)
*pp = Point{1, 2}
< / code > < / pre >
< p > 不过& Point{1, 2}写法可以直接在表达式中使用，比如一个函数调用。< / p >
< h3 id = "442-结构体比较" > < a class = "header" href = "#442-结构体比较" > 4.4.2. 结构体比较< / a > < / h3 >
< p > 如果结构体的全部成员都是可以比较的，那么结构体也是可以比较的，那样的话两个结构体将可以使用==或!=运算符进行比较。相等比较运算符==将比较两个结构体的每个成员，因此下面两个比较的表达式是等价的：< / p >
< pre > < code class = "language-Go" > type Point struct{ X, Y int }
p := Point{1, 2}
q := Point{2, 1}
fmt.Println(p.X == q.X & & p.Y == q.Y) // " false"
fmt.Println(p == q) // " false"
< / code > < / pre >
< p > 可比较的结构体类型和其他可比较的类型一样， < / p >
< pre > < code class = "language-Go" > type address struct {
hostname string
port int
}
hits := make(map[address]int)
hits[address{" golang.org" , 443}]++
< / code > < / pre >
< h3 id = "443-结构体嵌入和匿名成员" > < a class = "header" href = "#443-结构体嵌入和匿名成员" > 4.4.3. 结构体嵌入和匿名成员< / a > < / h3 >
< p > 在本节中， ， < / p >
< p > 考虑一个二维的绘图程序，提供了一个各种图形的库，例如矩形、椭圆形、星形和轮形等几何形状。这里是其中两个的定义：< / p >
< pre > < code class = "language-Go" > type Circle struct {
X, Y, Radius int
}
type Wheel struct {
X, Y, Radius, Spokes int
}
< / code > < / pre >
< p > 一个Circle代表的圆形类型包含了标准圆心的X和Y坐标信息， ， ： < / p >
< pre > < code class = "language-Go" > var w Wheel
w.X = 8
w.Y = 8
w.Radius = 5
w.Spokes = 20
< / code > < / pre >
< p > 随着库中几何形状数量的增多，我们一定会注意到它们之间的相似和重复之处，所以我们可能为了便于维护而将相同的属性独立出来：< / p >
< pre > < code class = "language-Go" > type Point struct {
X, Y int
}
type Circle struct {
Center Point
Radius int
}
type Wheel struct {
Circle Circle
Spokes int
}
< / code > < / pre >
< p > 这样改动之后结构体类型变的清晰了，但是这种修改同时也导致了访问每个成员变得繁琐：< / p >
< pre > < code class = "language-Go" > var w Wheel
w.Circle.Center.X = 8
w.Circle.Center.Y = 8
w.Circle.Radius = 5
w.Spokes = 20
< / code > < / pre >
< p > Go语言有一个特性让我们只声明一个成员对应的数据类型而不指名成员的名字； ， ， < / p >
< pre > < code class = "language-Go" > type Circle struct {
Point
Radius int
}
type Wheel struct {
Circle
Spokes int
}
< / code > < / pre >
< p > 得益于匿名嵌入的特性，我们可以直接访问叶子属性而不需要给出完整的路径：< / p >
< pre > < code class = "language-Go" > var w Wheel
w.X = 8 // equivalent to w.Circle.Point.X = 8
w.Y = 8 // equivalent to w.Circle.Point.Y = 8
w.Radius = 5 // equivalent to w.Circle.Radius = 5
w.Spokes = 20
< / code > < / pre >
< p > 在右边的注释中给出的显式形式访问这些叶子成员的语法依然有效， < / p >
< p > 不幸的是，结构体字面值并没有简短表示匿名成员的语法， 因此下面的语句都不能编译通过：< / p >
< pre > < code class = "language-Go" > w = Wheel{8, 8, 5, 20} // compile error: unknown fields
w = Wheel{X: 8, Y: 8, Radius: 5, Spokes: 20} // compile error: unknown fields
< / code > < / pre >
< p > 结构体字面值必须遵循形状类型声明时的结构，所以我们只能用下面的两种语法，它们彼此是等价的：< / p >
< p > < u > < i > gopl.io/ch4/embed< / i > < / u > < / p >
< pre > < code class = "language-Go" > w = Wheel{Circle{Point{8, 8}, 5}, 20}
w = Wheel{
Circle: Circle{
Point: Point{X: 8, Y: 8},
Radius: 5,
},
Spokes: 20, // NOTE: trailing comma necessary here (and at Radius)
}
fmt.Printf(" %#v\n" , w)
// Output:
// Wheel{Circle:Circle{Point:Point{X:8, Y:8}, Radius:5}, Spokes:20}
w.X = 42
fmt.Printf(" %#v\n" , w)
// Output:
// Wheel{Circle:Circle{Point:Point{X:42, Y:8}, Radius:5}, Spokes:20}
< / code > < / pre >
< p > 需要注意的是Printf函数中%v参数包含的#副词， ， < / p >
< p > 因为匿名成员也有一个隐式的名字， ， ， ， ， （ ） ， < / p >
< pre > < code class = "language-Go" > w.X = 8 // equivalent to w.circle.point.X = 8
< / code > < / pre >
< p > 但是在包外部， ， ， < / p >
< p > 到目前为止，我们看到匿名成员特性只是对访问嵌套成员的点运算符提供了简短的语法糖。稍后，我们将会看到匿名成员并不要求是结构体类型；其实任何命名的类型都可以作为结构体的匿名成员。但是为什么要嵌入一个没有任何子成员类型的匿名成员类型呢？< / p >
< p > 答案是匿名类型的方法集。简短的点运算符语法可以用于选择匿名成员嵌套的成员， ， ， ， < / p >
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