Golang语法系列——interface接口

NOTE

• 主要概念

  1. interface 可被 struct 等继承 （animalI = catT, catT 实现所有animalI的方法）
  2. 了解规则：最好使用new()生成指针类

• 参考链接

type-switch 类型选择

• 参考链接

  1. 介绍interface的用处
  https://github.com/astaxie/build-web-application-with-golang/blob/master/zh/02.6.md 

• Comma-ok断言 判断类型 element.(T)

  package main
  
  import (
  	"fmt"
  	"strconv"
  )
  
  type Element interface{}
  type List []Element
  
  type Person struct {
  	name string
  	age int
  }
  
  //打印
  func (p Person) String() string {
  	return "(name: " + p.name + " - age: "+strconv.Itoa(p.age)+ " years)"
  }
  
  func main() {
  	list := make(List, 3)
  	list[0] = 1 //an int
  	list[1] = "Hello" //a string
  	list[2] = Person{"Dennis", 70}
  
  	for index, element := range list{
  		switch value := element.(type) {
  			case int:
  				fmt.Printf("list[%d] is an int and its value is %d\n", index, value)
  			case string:
  				fmt.Printf("list[%d] is a string and its value is %s\n", index, value)
  			case Person:
  				fmt.Printf("list[%d] is a Person and its value is %s\n", index, value)
  			default:
  				fmt.Println("list[%d] is of a different type", index)
  		}
  	}
  }

• interface 可以嵌套interface

  例如io标准库
  // io.ReadWriter
  // Reader和Writer两个 interface
  type ReadWriter interface {
  	Reader
  	Writer
  }

类型断言

• 代码

  #NOTE：如果忽略 areaIntf.(*Square) 中的 * 号，会导致编译错误：impossible type assertion: Square does not implement Shaper (Area method has pointer receiver)。
  
  package main
  
  import (
  	"fmt"
  	"math"
  )
  
  type Square struct {
  	side float32
  }
  
  type Circle struct {
  	radius float32
  }
  
  type Shaper interface {
  	Area() float32
  }
  
  func main() {
  	var areaIntf Shaper
  	sq1 := new(Square)
  	sq1.side = 5
  
  	areaIntf = sq1
  	// Is Square the type of areaIntf?
  	if t, ok := areaIntf.(*Square); ok {
  		fmt.Printf("The type of areaIntf is: %T\n", t)
  	}
  	if u, ok := areaIntf.(*Circle); ok {
  		fmt.Printf("The type of areaIntf is: %T\n", u)
  	} else {
  		fmt.Println("areaIntf does not contain a variable of type Circle")
  	}
  }
  
  func (sq *Square) Area() float32 {
  	return sq.side * sq.side
  }
  
  func (ci *Circle) Area() float32 {
  	return ci.radius * ci.radius * math.Pi
  }
  
  输出
  The type of areaIntf is: *main.Square
  areaIntf does not contain a variable of type Circle

调用规则

• 规则

  Go 语言规范定义了接口方法集的调用规则：
  
      类型 *T 的可调用方法集包含接受者为 *T 或 T 的所有方法集
      类型 T 的可调用方法集包含接受者为 T 的所有方法
      类型 T 的可调用方法集不包含接受者为 *T 的方法

• 代码

  #注意new()的用法
  
  package main
  
  import (
  	"fmt"
  )
  
  type List []int
  
  func (l List) Len() int {
  	return len(l)
  }
  
  func (l *List) Append(val int) {
  	*l = append(*l, val)
  }
  
  type Appender interface {
  	Append(int)
  }
  
  func CountInto(a Appender, start, end int) {
  	for i := start; i <= end; i++ {
  		a.Append(i)
  	}
  }
  
  type Lener interface {
  	Len() int
  }
  
  func LongEnough(l Lener) bool {
  	return l.Len()*10 > 42
  }
  
  func main() {
  	// A bare value
  	var lst List
  	// compiler error:
  	// cannot use lst (type List) as type Appender in argument to CountInto:
  	//       List does not implement Appender (Append method has pointer receiver)
  	// CountInto(lst, 1, 10)
  	if LongEnough(lst) { // VALID:Identical receiver type
  		fmt.Printf("- lst is long enough\n")
  	}
  
  	// A pointer value
  	plst := new(List)
  	CountInto(plst, 1, 10) //VALID:Identical receiver type
  	if LongEnough(plst) {
  		// VALID: a *List can be dereferenced for the receiver
  		fmt.Printf("- plst is long enough\n")
  	}
  }
  
  输出
  - plst is long enough

空接口复制

• 代码

  #需逐个显式复制
  var dataSlice []myType = FuncReturnSlice()
  var interfaceSlice []interface{} = make([]interface{}, len(dataSlice))
  for i, d := range dataSlice {
      interfaceSlice[i] = d
  }
  
  #错误语法
  var dataSlice []myType = FuncReturnSlice()
  var interfaceSlice []interface{} = dataSlice