Golang基础-Structs与Methods、Interfaces
Structs
- 类型定义(type)相当于一种别名
- 将struct定义为一种类型Car
- NewCar函数return &Car{},返回指针
// car.go
package elon
// Car implements a remote controlled car.
type Car struct {
speed int
batteryDrain int
battery int
distance int
}
// NewCar creates a new car with given specifications.
func NewCar(speed, batteryDrain int) *Car {
return &Car{
speed: speed,
batteryDrain: batteryDrain,
battery: 100,
}
}
struct的零值,When constructing the zero value for a struct type, all of the struct's fields will be set to their zero value:
type Person struct {
Name string
Age int
}
var myPerson Person
fmt.Printf("Zero value Person: %#v", myPerson)
// Output: Zero value Person: main.Person{Name:"", Age:0}
// 给有数值的struct赋零值
myPerson = Person{}
Methods
- 给Car类型绑定方法
- 绑定方法到*Car,说明在指针上,函数内可以改变结构体的值;如果是Car,则是拷贝,不能改变原始变量的值。
package elon
import "fmt"
// TODO: define the 'Drive()' method
func (car *Car) Drive() {
if car.battery < car.batteryDrain {
return
}
car.distance += car.speed
car.battery -= car.batteryDrain
}
// TODO: define the 'DisplayDistance() string' method
func (car *Car) DisplayDistance() string {
return fmt.Sprintf("Driven %d meters", car.distance)
}
// TODO: define the 'DisplayBattery() string' method
func (car *Car) DisplayBattery() string {
return fmt.Sprintf("Battery at %d%%", car.battery)
}
// TODO: define the 'CanFinish(trackDistance int) bool' method
func (car *Car) CanFinish(trackDistance int) bool {
if (trackDistance+car.speed-1)/car.speed*car.batteryDrain > car.battery {
return false
} else {
return true
}
}
- 给新的自定义类型绑定方法不一定要求类型是struct,也可以是int等一般类型。
下面定义的DistanceUnit类型的原型是int。给两个自定义类型实现了Stringer interface,也就是自定义打印时输出什么。Stringer接口只有一个方法,String() string
type DistanceUnit int
const (
Kilometer DistanceUnit = 0
Mile DistanceUnit = 1
)
type Distance struct {
number float64
unit DistanceUnit
}
func (sc DistanceUnit) String() string {
units := []string{"km", "mi"}
return units[sc]
}
func (d Distance) String() string {
return fmt.Sprintf("%v %v", d.number, d.unit)
}
Interfaces
- 定义了Greeter interface
- 定义了Italian和Portuguese两种类型
- 两种类型均实现了Greeter interface
- SayHello参数有一个Greeter,一个接口,但是却可以传入实现了Greeter所有接口的类型的变量。
package airportrobot
import "fmt"
// Write your code here.
// This exercise does not have tests for each individual task.
// Try to solve all the tasks first before running the tests.
type Greeter interface {
LanguageName() string
Greet(username string) string
}
func SayHello(username string, g Greeter) string {
return fmt.Sprintf("I can speak %s: %s", g.LanguageName(), g.Greet(username))
}
type Italian struct {
}
func (I Italian) LanguageName() string {
return "Italian"
}
func (I Italian) Greet(username string) string {
return fmt.Sprintf("Ciao %s!", username)
}
type Portuguese struct {
}
func (P Portuguese) LanguageName() string {
return "Portuguese"
}
func (P Portuguese) Greet(username string) string {
return fmt.Sprintf("Olá %s!", username)
}
这里的Italian{}和Portuguese{}牵涉到golang中空结构体的一个作用,实现方法接收者。Italian和Portuguese都是struct{},一个没有任何字段的结构体,的别名,作为了两种新的类型,虽然本质是一样的,但这两种新类型将作为不同类型的存在。而这两种新类型,则各自实现了Greeter接口的两个函数。
Italian{}和Portuguese{}则都相当于struct{}{},struct{}是类型,后面的{}是初始化,最终出来的是一个空结构体。本质是空结构体,但实际上是Italian和Portuguese两种类型的两个值。
空结构体还有两个作用,将map的value作为空结构体实现set;实现空通道,常常会遇到通知型 channel,其不需要发送任何数据,只是用于协调 Goroutine 的运行,用于流转各类状态或是控制并发情况,由于该 channel 使用的是空结构体,因此也不会带来额外的内存开销。
fmt.Println(SayHello("mlg", Italian{}))
fmt.Println(SayHello("glm", Portuguese{}))
为什么这里给type绑定方法用的不是指针?
For implementing the interface, it does not matter whether the method has a value or pointer receiver.
如果要改成指针形式的,则相应的调用函数也应改变。
type Portuguese struct {
}
func (P *Portuguese) LanguageName() string {
return "Portuguese"
}
func (P *Portuguese) Greet(username string) string {
return fmt.Sprintf("Olá %s!", username)
}
// 这里的调用传参也换成了指针(地址)
fmt.Println(SayHello("glm", &Portuguese{}))
所以interface类型的参数可以传值也可以传指针?
经过实验,即使上面代码中Portuguese绑定的是值而不是指针,在调用SayHello时仍然可以传入&Portuguese{}。但是如果实现的方法绑定的是指针,则在调用SayHello时只能传入指针,不能传值。
Type Assertion
Interfaces in Go can introduce ambiguity about the underlying type. In fact, the empty interface can take any concrete value at all (including primitives). A type assertion allows us to extract the interface value's underlying concrete value using this syntax: interfaceVariable.(concreteType)
.
上面的SayHello函数的第二个参数是Greeter interface,但传入的值是却是其他类型的变量,可以使用Type Assertion来判断或转化为相应的类型。
func seeType(g Greeter) {
switch g.(type) {
case Italian:
fmt.Println("Italian type")
case Portuguese:
fmt.Println("Portuguese type")
}
}
seeType(Italian{}) // => Italian type
seeType(Portuguese{}) // => Portuguese type
var i interface{} = 12 // try: 12.3, true, int64(12), []int{}, map[string]int{}
switch v := i.(type) {
case int:
fmt.Printf("the integer %d\n", v)
case string:
fmt.Printf("the string %s\n", v)
default:
fmt.Printf("type, %T, not handled explicitly: %#v\n", v, v)
}