本文为译文 Interfaces - II

使用值接收器 或 指针接收器，实现一个接口

我们在接口的第一讲当中所有的接口实现都使用的是值类型。我们也可以实现一个接口使用指针类型。

package main

import "fmt"

type Describer interface {  
    Describe()
}
type Person struct {  
    name string
    age  int
}

func (p Person) Describe() { //implemented using value receiver  
    fmt.Printf("%s is %d years old\n", p.name, p.age)
}

type Address struct {  
    state   string
    country string
}

func (a *Address) Describe() { //implemented using pointer receiver  
    fmt.Printf("State %s Country %s", a.state, a.country)
}

func main() {  
    var d1 Describer
    p1 := Person{"Sam", 25}
    d1 = p1
    d1.Describe()
    p2 := Person{"James", 32}
    d1 = &p2
    d1.Describe()

    var d2 Describer
    a := Address{"Washington", "USA"}

    /* compilation error if the following line is
       uncommented
       cannot use a (type Address) as type Describer
       in assignment: Address does not implement
       Describer (Describe method has pointer
       receiver)
    */
    //d2 = a

    d2 = &a //This works since Describer interface
    //is implemented by Address pointer in line 22
    d2.Describe()

}

我们之前在method中学过。值接受器的方法，接受值类型&指针类型。同样的指针接收器，可以接受值类型&指针类型。

但是在上面的程序中，如果我们将一个值类型赋值给指针类型的接口，就报错了。

The reason is that it is legal to call a pointer-valued method on anything that is already a pointer or whose address can be taken. The concrete value stored in an interface is not addressable and hence it is not possible for the compiler to automatically take the address of a in line no. 45 and hence this code fails.

实现多个接口

一个类型可以实心多个接口

package main

import (  
    "fmt"
)

type SalaryCalculator interface {  
    DisplaySalary()
}

type LeaveCalculator interface {  
    CalculateLeavesLeft() int
}

type Employee struct {  
    firstName string
    lastName string
    basicPay int
    pf int
    totalLeaves int
    leavesTaken int
}

func (e Employee) DisplaySalary() {  
    fmt.Printf("%s %s has salary $%d", e.firstName, e.lastName, (e.basicPay + e.pf))
}

func (e Employee) CalculateLeavesLeft() int {  
    return e.totalLeaves - e.leavesTaken
}

func main() {  
    e := Employee {
        firstName: "Naveen",
        lastName: "Ramanathan",
        basicPay: 5000,
        pf: 200,
        totalLeaves: 30,
        leavesTaken: 5,
    }
    var s SalaryCalculator = e
    s.DisplaySalary()
    var l LeaveCalculator = e
    fmt.Println("\nLeaves left =", l.CalculateLeavesLeft())
}

接口嵌套

package main

import (  
    "fmt"
)

type SalaryCalculator interface {  
    DisplaySalary()
}

type LeaveCalculator interface {  
    CalculateLeavesLeft() int
}

type EmployeeOperations interface {  
    SalaryCalculator
    LeaveCalculator
}

type Employee struct {  
    firstName string
    lastName string
    basicPay int
    pf int
    totalLeaves int
    leavesTaken int
}

func (e Employee) DisplaySalary() {  
    fmt.Printf("%s %s has salary $%d", e.firstName, e.lastName, (e.basicPay + e.pf))
}

func (e Employee) CalculateLeavesLeft() int {  
    return e.totalLeaves - e.leavesTaken
}

func main() {  
    e := Employee {
        firstName: "Naveen",
        lastName: "Ramanathan",
        basicPay: 5000,
        pf: 200,
        totalLeaves: 30,
        leavesTaken: 5,
    }
    var empOp EmployeeOperations = e
    empOp.DisplaySalary()
    fmt.Println("\nLeaves left =", empOp.CalculateLeavesLeft())
}

虽然go没有提供继承，但是还是可以通过嵌套的形式实现类似的能力。

上面的程序中，如果哪个类型实现了CalculateLeavesLeft方法和DisplaySalary方法。则我们说他实现了EmployeeOperations接口。

接口的零值

The zero value of a interface is nil

package main

import "fmt"

type Describer interface {  
    Describe()
}

func main() {  
    var d1 Describer
    if d1 == nil {
        fmt.Printf("d1 is nil and has type %T value %v\n", d1, d1)
    }
}

d1 is nil and has type <nil> value <nil>  

If we try to call a method on the nil interface, the program will panic since the nil interface neither has a underlying value nor a concrete type.

package main

type Describer interface {  
    Describe()
}

func main() {  
    var d1 Describer
    d1.Describe()
}

panic: runtime error: invalid memory address or nil pointer dereference