Go 系列教程 ——第27篇：组合取代继承

Go 不支持继承，但它支持组合（Composition）。组合一般定义为“合并在一起”。汽车就是一个关于组合的例子：一辆汽车由车轮、引擎和其他各种部件组合在一起。

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。每一个博客的帖子都有标题、内容和作者信息。使用组合可以很好地表示它们。通过学习本教程后面的内容，我们会知道如何实现组合。

author

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)
}

authorauthorfirstnamelastnamebiofullName()author

post

type post struct {  
    title     string
    content   string
    author
}

func (p post) details() {  
    fmt.Println("Title: ", p.title)
    fmt.Println("Content: ", p.content)
    fmt.Println("Author: ", p.author.fullName())
    fmt.Println("Bio: ", p.author.bio)
}

posttitlecontentauthorauthorpostpostauthorpostdetails()

p.author.fullName()p.fullName()details()

func (p post) details() {  
    fmt.Println("Title: ", p.title)
    fmt.Println("Content: ", p.content)
    fmt.Println("Author: ", p.fullName())
    fmt.Println("Bio: ", p.bio)
}

authorpost

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)
    fmt.Println("Author: ", p.fullName())
    fmt.Println("Bio: ", p.bio)
}

func main() {  
    author1 := author{        
        "Naveen",        
        "Ramanathan",        
        "Golang Enthusiast",
    }
    post1 := post{        
        "Inheritance in Go",        
        "Go supports composition instead of inheritance",
        author1,
    }
    post1.details()
}

在 playground 上运行

authorauthor1post

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

website

type website struct {  
        []post
}

func (w website) contents() {  
    fmt.Println("Contents of Website\n")    
    for _, v := range w.posts {
        v.details()
        fmt.Println()
    }
}

在你添加上述代码后，当你运行程序时，编译器将会报错，如下所示：

main.go:31:9: syntax error: unexpected [, expecting field name or embedded type

[]post

type website struct {  
        posts []post
}

[]postposts

现在我们来修改主函数，为我们的新网站创建一些帖子吧。

修改后的完整代码如下所示：

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)
    fmt.Println("Author: ", p.fullName())
    fmt.Println("Bio: ", p.bio)
}

type website struct {  
     posts []post
}

func (w website) contents() {  
    fmt.Println("Contents of Website\n")    
    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()
}

在 playground 中运行

author1post1post2post3w

程序会输出：

Contents of Website

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