go interface 的坑

一、概述

[root@node175 demo]# tree 
.
├── lib
│   └── world.go
├── README
└── server.go

1 directory, 3 files

#server.go code
package main

import "fmt"
import "demo/lib"

type MyPrint struct {         
    name string               
}

type Interface interface {
    Print(string) error
}

func (this *MyPrint) Print(who string) error {
    fmt.Printf("%s name is %s
", who, this.name)
    return nil
}

func NewMyPrint(name string) Interface {
    return MyPrint{name: name}
}

func main() {
    fmt.Println("Hi, " + lib.World())
    MyInterface := NewMyPrint("bob")
    fmt.Printf("MyInterface type: %T
", MyInterface)
    MyInterface.Print("my")
}

运行：

[root@node175 demo]# go run server.go
# command-line-arguments
./server.go:20: cannot use MyPrint literal (type MyPrint) as type Interface in return argument:
MyPrint does not implement Interface (Print method has pointer receiver)

　　为了解决这个问题，首先得先了解一下Golang 中 方法的集合的概念，一个struct虽然可以通过值类型和引用类型两种方式定义方法，但是不同的对象类型对应了不同的方法集：

Values                    Methods Receivers
-----------------------------------------------
 T                        (t T)
*T                        (t T) and (t *T) 

　　值类型的对象只有（t T) 结构的方法，虽然值类型的对象也可以调用(t *T) 方法，但这实际上是Golang编译器自动转化成了&t的形式来调用方法，并不是表明值类型的对象拥有该方法。

　　换一个维度来看上面的表格可能更加直观：

Methods Receivers         Values
-----------------------------------------------
(t T)                     T and *T

(t *T)                    *T 

　　这就意味着指针类型的receiver 方法实现接口时，只有指针类型的对象实现了该接口。

　　对应上面的例子来说，只有&MyPrint实现了Interface接口，而MyPrint根本没有实现该接口。所以上面代码会报出这样的异常。

MyPrint method has pointer receiver

　　解决这个问题也很容易，直接使用&MyPrint去代替MyPrint调用方法即可：

package main

import "fmt"
import "demo/lib"

type MyPrint struct {         
    name string               
}

type Interface interface {
    Print(string) error
}

func (this *MyPrint) Print(who string) error {
    fmt.Printf("%s name is %s
", who, this.name)
    return nil
}

func MyselfPrint(name string) Interface { 
    return &MyPrint{name: name}
}

func main() {
    fmt.Println("Hi, " + lib.World())
    MyInterface := MyselfPrint("bob")
    fmt.Printf("MyInterface type: %T
", MyInterface)
    MyInterface.Print("my")
}

或者：

package main

import "fmt"
import "demo/lib"

type MyPrint struct {         
    name string               
}

type Interface interface {
    Print(string) error
}

func (this MyPrint) Print(who string) error {
    fmt.Printf("%s name is %s
", who, this.name)
    return nil
}

func MyselfPrint(name string) Interface { 
    return MyPrint{name: name}
}

func main() {
    fmt.Println("Hi, " + lib.World())
    MyInterface := MyselfPrint("bob")
    fmt.Printf("MyInterface type: %T
", MyInterface)
    MyInterface.Print("my")
}

 再或者：

package main

import "fmt"
import "demo/lib"

type MyPrint struct {
    name string
}

type Interface interface {
    Print(string) error
}

func (this MyPrint) Print(who string) error {
    fmt.Printf("%s name is %s
", who, this.name)
    return nil 
}

func MyselfPrint(name string) Interface {
    return &MyPrint{name: name}
}

func main() {
    fmt.Println("Hi, " + lib.World())
    MyInterface := MyselfPrint("bob")
    fmt.Printf("MyInterface type: %T
", MyInterface)
    MyInterface.Print("my")
}