golang--组合取代继承

Go 不支持继承，但它支持组合（Composition）。

通过嵌套结构体进行组合

在 Go 中，通过在结构体内嵌套结构体，可以实现组合。

组合的典型例子就是博客帖子。每一个博客的帖子(Post)都有标题、内容和作者信息(author)。“作者信息”这个结构体内嵌在“帖子”这个结构体中。

下面写一个这个例子。

package main
import "fmt"
type author struct {
    firstName string
    lastName  string
    bio       string
}
func (a author)fullName() string {
    return fmt.Sprintf("%s %s", a.firstName, a.lastName)
}
type post struct {
    title string
    content string
    author
}
func (p post)details()  {
    fmt.Println("title: ", p.title)
    fmt.Println("content: ", p.content)
    //一旦结构体内嵌套了一个结构体字段，Go 可以使我们访问其嵌套的字段，好像这些字段属于外部结构体一样。
    fmt.Println("author: ", p.author.fullName()) //可替换为p.fullName()
    fmt.Println("bio: ", p.author.bio) //可替换为p.bio
}
func main()  {
    author1 := author{
        "Naveen",
        "Ramanathan",
        "Golang Enthusiast",
    }
    post1 := post{
        "Inheritance in Go",
        "Go supports composition instead of inheritance",
        author1,
    }
    post1.details()
}

改程序的执行结果为：

title:  Inheritance in Go
content:  Go supports composition instead of inheritance
author:  Naveen Ramanathan
bio:  Golang Enthusiast

结构体切片的嵌套

可以进一步处理这个示例，使用博客帖子的切片来创建一个网站。

package main
import "fmt"
type author struct {
    firstName string
    lastName  string
    bio       string
}
func (a author)fullName() string {
    return fmt.Sprintf("%s %s", a.firstName, a.lastName)
}
type post struct {
    title string
    content string
    author
}
func (p post)details()  {
    fmt.Println("title: ", p.title)
    fmt.Println("content: ", p.content)
    //一旦结构体内嵌套了一个结构体字段，Go 可以使我们访问其嵌套的字段，好像这些字段属于外部结构体一样。
    fmt.Println("author: ", p.author.fullName()) //可替换为p.fullName()
    fmt.Println("bio: ", p.author.bio) //可替换为p.bio
}
type website struct {
    posts []post
}
func (w website)contents()  {
    fmt.Println("website contents: ")
    fmt.Println()
    for _, v := range w.posts{
        v.details()
        fmt.Println()
    }
}
func main()  {
    author1 := author{
        "Naveen",
        "Ramanathan",
        "Golang Enthusiast",
    }
    post1 := post{
        "Inheritance in Go",
        "Go supports composition instead of inheritance",
        author1,
    }
    post2 := post{
        "Struct instead of Classes in Go",
        "Go does not support classes but methods can be added to structs",
        author1,
    }
    post3 := post{
        "Concurrency",
        "Go is a concurrent language and not a parallel one",
        author1,
    }
    w := website{
        posts: []post{post1, post2, post3},
    }
    w.contents()
}

上述程序的执行结果为：

website contents: 
title:  Inheritance in Go
content:  Go supports composition instead of inheritance
author:  Naveen Ramanathan
bio:  Golang Enthusiast
title:  Struct instead of Classes in Go
content:  Go does not support classes but methods can be added to structs
author:  Naveen Ramanathan
bio:  Golang Enthusiast
title:  Concurrency
content:  Go is a concurrent language and not a parallel one
author:  Naveen Ramanathan
bio:  Golang Enthusiast

需要注意的是，结构体不能嵌套一个匿名切片。如果上面website结构体像下面的写法：

type website struct {
    []post
}

就会报错，syntax error: unexpected [, expecting field name or embedded type