2015-12-09 07:57:17 +00:00
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2015-12-14 04:08:47 +00:00
< div class = "book" data-level = "6.3" data-chapter-title = "通過嵌入結構體來擴展類型" data-filepath = "ch6/ch6-03.md" data-basepath = ".." data-revision = "Mon Dec 14 2015 11:30:54 GMT+0800 (中国标准时间)" >
2015-12-09 07:57:17 +00:00
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前言
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< b > 0.1.< / b >
Go語言起源
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< b > 0.2.< / b >
Go語言項目
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< b > 0.3.< / b >
本書的組織
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< b > 0.4.< / b >
更多的信息
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< b > 0.5.< / b >
緻謝
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< b > 1.< / b >
入門
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< b > 1.1.< / b >
Hello, World
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< b > 1.2.< / b >
命令行參數
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< b > 1.3.< / b >
査找重復的行
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< b > 1.4.< / b >
GIF動畫
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< b > 1.5.< / b >
穫取URL
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< b > 1.6.< / b >
併髮穫取多個URL
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< b > 1.7.< / b >
Web服務
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< b > 1.8.< / b >
本章要點
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< b > 2.< / b >
程序結構
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< b > 2.1.< / b >
命名
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< li class = "chapter " data-level = "2.2" data-path = "ch2/ch2-02.html" >
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< b > 2.2.< / b >
聲明
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< b > 2.3.< / b >
變量
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< b > 2.4.< / b >
賦值
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< b > 2.5.< / b >
類型
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< b > 2.6.< / b >
包和文件
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< b > 2.7.< / b >
作用域
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< b > 3.< / b >
基礎數據類型
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< b > 3.1.< / b >
整型
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< b > 3.2.< / b >
浮點數
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< b > 3.3.< / b >
復數
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< b > 3.4.< / b >
佈爾型
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< b > 3.5.< / b >
字符串
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< b > 3.6.< / b >
常量
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< b > 4.< / b >
復閤數據類型
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< b > 4.1.< / b >
數組
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< b > 4.2.< / b >
切片
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< b > 4.3.< / b >
字典
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< b > 4.4.< / b >
結構體
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< b > 4.5.< / b >
JSON
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< b > 4.6.< / b >
文本和HTML模闆
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< b > 5.< / b >
函數
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< b > 5.1.< / b >
函數聲明
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< b > 5.2.< / b >
遞歸
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< b > 5.3.< / b >
多返迴值
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< li class = "chapter " data-level = "5.4" data-path = "ch5/ch5-04.html" >
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< b > 5.4.< / b >
錯誤
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< b > 5.5.< / b >
函數值
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< li class = "chapter " data-level = "5.6" data-path = "ch5/ch5-06.html" >
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< b > 5.6.< / b >
匿名函數
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< li class = "chapter " data-level = "5.7" data-path = "ch5/ch5-07.html" >
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< b > 5.7.< / b >
可變參數
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< b > 5.8.< / b >
Deferred函數
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< b > 5.9.< / b >
Panic異常
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< b > 5.10.< / b >
Recover捕穫異常
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< b > 6.< / b >
方法
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< b > 6.1.< / b >
方法聲明
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< li class = "chapter " data-level = "6.2" data-path = "ch6/ch6-02.html" >
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< b > 6.2.< / b >
基於指鍼對象的方法
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< li class = "chapter active" data-level = "6.3" data-path = "ch6/ch6-03.html" >
< a href = "../ch6/ch6-03.html" >
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< b > 6.3.< / b >
通過嵌入結構體來擴展類型
< / a >
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< li class = "chapter " data-level = "6.4" data-path = "ch6/ch6-04.html" >
< a href = "../ch6/ch6-04.html" >
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< b > 6.4.< / b >
方法值和方法錶達式
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< li class = "chapter " data-level = "6.5" data-path = "ch6/ch6-05.html" >
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< b > 6.5.< / b >
示例: Bit數組
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< li class = "chapter " data-level = "6.6" data-path = "ch6/ch6-06.html" >
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< b > 6.6.< / b >
封裝
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< li class = "chapter " data-level = "7" data-path = "ch7/ch7.html" >
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< b > 7.< / b >
接口
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< b > 7.1.< / b >
接口是閤約
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< li class = "chapter " data-level = "7.2" data-path = "ch7/ch7-02.html" >
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< b > 7.2.< / b >
接口類型
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< li class = "chapter " data-level = "7.3" data-path = "ch7/ch7-03.html" >
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< b > 7.3.< / b >
實現接口的條件
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< li class = "chapter " data-level = "7.4" data-path = "ch7/ch7-04.html" >
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< b > 7.4.< / b >
flag.Value接口
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< b > 7.5.< / b >
接口值
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< b > 7.6.< / b >
sort.Interface接口
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< li class = "chapter " data-level = "7.7" data-path = "ch7/ch7-07.html" >
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< b > 7.7.< / b >
http.Handler接口
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< b > 7.8.< / b >
error接口
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< a href = "../ch7/ch7-09.html" >
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< b > 7.9.< / b >
示例: 錶達式求值
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< b > 7.10.< / b >
類型斷言
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< b > 7.11.< / b >
基於類型斷言識彆錯誤類型
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< b > 7.12.< / b >
通過類型斷言査詢接口
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< b > 7.13.< / b >
類型分支
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< li class = "chapter " data-level = "7.14" data-path = "ch7/ch7-14.html" >
< a href = "../ch7/ch7-14.html" >
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< b > 7.14.< / b >
示例: 基於標記的XML解碼
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< a href = "../ch7/ch7-15.html" >
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< b > 7.15.< / b >
補充幾點
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< b > 8.< / b >
Goroutines和Channels
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< li class = "chapter " data-level = "8.1" data-path = "ch8/ch8-01.html" >
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< b > 8.1.< / b >
Goroutines
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< a href = "../ch8/ch8-02.html" >
< i class = "fa fa-check" > < / i >
< b > 8.2.< / b >
示例: 併髮的Clock服務
< / a >
< / li >
< li class = "chapter " data-level = "8.3" data-path = "ch8/ch8-03.html" >
< a href = "../ch8/ch8-03.html" >
< i class = "fa fa-check" > < / i >
< b > 8.3.< / b >
示例: 併髮的Echo服務
< / a >
< / li >
< li class = "chapter " data-level = "8.4" data-path = "ch8/ch8-04.html" >
< a href = "../ch8/ch8-04.html" >
< i class = "fa fa-check" > < / i >
< b > 8.4.< / b >
Channels
< / a >
< / li >
< li class = "chapter " data-level = "8.5" data-path = "ch8/ch8-05.html" >
< a href = "../ch8/ch8-05.html" >
< i class = "fa fa-check" > < / i >
< b > 8.5.< / b >
併行的循環
< / a >
< / li >
< li class = "chapter " data-level = "8.6" data-path = "ch8/ch8-06.html" >
< a href = "../ch8/ch8-06.html" >
< i class = "fa fa-check" > < / i >
< b > 8.6.< / b >
示例: 併髮的Web爬蟲
< / a >
< / li >
< li class = "chapter " data-level = "8.7" data-path = "ch8/ch8-07.html" >
< a href = "../ch8/ch8-07.html" >
< i class = "fa fa-check" > < / i >
< b > 8.7.< / b >
基於select的多路復用
< / a >
< / li >
< li class = "chapter " data-level = "8.8" data-path = "ch8/ch8-08.html" >
< a href = "../ch8/ch8-08.html" >
< i class = "fa fa-check" > < / i >
< b > 8.8.< / b >
示例: 併髮的字典遍歷
< / a >
< / li >
< li class = "chapter " data-level = "8.9" data-path = "ch8/ch8-09.html" >
< a href = "../ch8/ch8-09.html" >
< i class = "fa fa-check" > < / i >
< b > 8.9.< / b >
併髮的退齣
< / a >
< / li >
< li class = "chapter " data-level = "8.10" data-path = "ch8/ch8-10.html" >
< a href = "../ch8/ch8-10.html" >
< i class = "fa fa-check" > < / i >
< b > 8.10.< / b >
示例: 聊天服務
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "9" data-path = "ch9/ch9.html" >
< a href = "../ch9/ch9.html" >
< i class = "fa fa-check" > < / i >
< b > 9.< / b >
基於共享變量的併髮
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "9.1" data-path = "ch9/ch9-01.html" >
< a href = "../ch9/ch9-01.html" >
< i class = "fa fa-check" > < / i >
< b > 9.1.< / b >
競爭條件
< / a >
< / li >
< li class = "chapter " data-level = "9.2" data-path = "ch9/ch9-02.html" >
< a href = "../ch9/ch9-02.html" >
< i class = "fa fa-check" > < / i >
< b > 9.2.< / b >
sync.Mutex互斥鎖
< / a >
< / li >
< li class = "chapter " data-level = "9.3" data-path = "ch9/ch9-03.html" >
< a href = "../ch9/ch9-03.html" >
< i class = "fa fa-check" > < / i >
< b > 9.3.< / b >
sync.RWMutex讀寫鎖
< / a >
< / li >
< li class = "chapter " data-level = "9.4" data-path = "ch9/ch9-04.html" >
< a href = "../ch9/ch9-04.html" >
< i class = "fa fa-check" > < / i >
< b > 9.4.< / b >
內存衕步
< / a >
< / li >
< li class = "chapter " data-level = "9.5" data-path = "ch9/ch9-05.html" >
< a href = "../ch9/ch9-05.html" >
< i class = "fa fa-check" > < / i >
< b > 9.5.< / b >
sync.Once初始化
< / a >
< / li >
< li class = "chapter " data-level = "9.6" data-path = "ch9/ch9-06.html" >
< a href = "../ch9/ch9-06.html" >
< i class = "fa fa-check" > < / i >
< b > 9.6.< / b >
競爭條件檢測
< / a >
< / li >
< li class = "chapter " data-level = "9.7" data-path = "ch9/ch9-07.html" >
< a href = "../ch9/ch9-07.html" >
< i class = "fa fa-check" > < / i >
< b > 9.7.< / b >
示例: 併髮的非阻塞緩存
< / a >
< / li >
< li class = "chapter " data-level = "9.8" data-path = "ch9/ch9-08.html" >
< a href = "../ch9/ch9-08.html" >
< i class = "fa fa-check" > < / i >
< b > 9.8.< / b >
Goroutines和綫程
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "10" data-path = "ch10/ch10.html" >
< a href = "../ch10/ch10.html" >
< i class = "fa fa-check" > < / i >
< b > 10.< / b >
包和工具
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "10.1" data-path = "ch10/ch10-01.html" >
< a href = "../ch10/ch10-01.html" >
< i class = "fa fa-check" > < / i >
< b > 10.1.< / b >
簡介
< / a >
< / li >
< li class = "chapter " data-level = "10.2" data-path = "ch10/ch10-02.html" >
< a href = "../ch10/ch10-02.html" >
< i class = "fa fa-check" > < / i >
< b > 10.2.< / b >
導入路徑
< / a >
< / li >
< li class = "chapter " data-level = "10.3" data-path = "ch10/ch10-03.html" >
< a href = "../ch10/ch10-03.html" >
< i class = "fa fa-check" > < / i >
< b > 10.3.< / b >
包聲明
< / a >
< / li >
< li class = "chapter " data-level = "10.4" data-path = "ch10/ch10-04.html" >
< a href = "../ch10/ch10-04.html" >
< i class = "fa fa-check" > < / i >
< b > 10.4.< / b >
導入聲明
< / a >
< / li >
< li class = "chapter " data-level = "10.5" data-path = "ch10/ch10-05.html" >
< a href = "../ch10/ch10-05.html" >
< i class = "fa fa-check" > < / i >
< b > 10.5.< / b >
匿名導入
< / a >
< / li >
< li class = "chapter " data-level = "10.6" data-path = "ch10/ch10-06.html" >
< a href = "../ch10/ch10-06.html" >
< i class = "fa fa-check" > < / i >
< b > 10.6.< / b >
包和命名
< / a >
< / li >
< li class = "chapter " data-level = "10.7" data-path = "ch10/ch10-07.html" >
< a href = "../ch10/ch10-07.html" >
< i class = "fa fa-check" > < / i >
< b > 10.7.< / b >
工具
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "11" data-path = "ch11/ch11.html" >
< a href = "../ch11/ch11.html" >
< i class = "fa fa-check" > < / i >
< b > 11.< / b >
測試
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "11.1" data-path = "ch11/ch11-01.html" >
< a href = "../ch11/ch11-01.html" >
< i class = "fa fa-check" > < / i >
< b > 11.1.< / b >
go test
< / a >
< / li >
< li class = "chapter " data-level = "11.2" data-path = "ch11/ch11-02.html" >
< a href = "../ch11/ch11-02.html" >
< i class = "fa fa-check" > < / i >
< b > 11.2.< / b >
測試函數
< / a >
< / li >
< li class = "chapter " data-level = "11.3" data-path = "ch11/ch11-03.html" >
< a href = "../ch11/ch11-03.html" >
< i class = "fa fa-check" > < / i >
< b > 11.3.< / b >
測試覆蓋率
< / a >
< / li >
< li class = "chapter " data-level = "11.4" data-path = "ch11/ch11-04.html" >
< a href = "../ch11/ch11-04.html" >
< i class = "fa fa-check" > < / i >
< b > 11.4.< / b >
基準測試
< / a >
< / li >
< li class = "chapter " data-level = "11.5" data-path = "ch11/ch11-05.html" >
< a href = "../ch11/ch11-05.html" >
< i class = "fa fa-check" > < / i >
< b > 11.5.< / b >
剖析
< / a >
< / li >
< li class = "chapter " data-level = "11.6" data-path = "ch11/ch11-06.html" >
< a href = "../ch11/ch11-06.html" >
< i class = "fa fa-check" > < / i >
< b > 11.6.< / b >
示例函數
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "12" data-path = "ch12/ch12.html" >
< a href = "../ch12/ch12.html" >
< i class = "fa fa-check" > < / i >
< b > 12.< / b >
反射
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "12.1" data-path = "ch12/ch12-01.html" >
< a href = "../ch12/ch12-01.html" >
< i class = "fa fa-check" > < / i >
< b > 12.1.< / b >
為何需要反射?
< / a >
< / li >
< li class = "chapter " data-level = "12.2" data-path = "ch12/ch12-02.html" >
< a href = "../ch12/ch12-02.html" >
< i class = "fa fa-check" > < / i >
< b > 12.2.< / b >
reflect.Type和reflect.Value
< / a >
< / li >
< li class = "chapter " data-level = "12.3" data-path = "ch12/ch12-03.html" >
< a href = "../ch12/ch12-03.html" >
< i class = "fa fa-check" > < / i >
< b > 12.3.< / b >
Display遞歸打印
< / a >
< / li >
< li class = "chapter " data-level = "12.4" data-path = "ch12/ch12-04.html" >
< a href = "../ch12/ch12-04.html" >
< i class = "fa fa-check" > < / i >
< b > 12.4.< / b >
示例: 編碼S錶達式
< / a >
< / li >
< li class = "chapter " data-level = "12.5" data-path = "ch12/ch12-05.html" >
< a href = "../ch12/ch12-05.html" >
< i class = "fa fa-check" > < / i >
< b > 12.5.< / b >
通過reflect.Value脩改值
< / a >
< / li >
< li class = "chapter " data-level = "12.6" data-path = "ch12/ch12-06.html" >
< a href = "../ch12/ch12-06.html" >
< i class = "fa fa-check" > < / i >
< b > 12.6.< / b >
示例: 解碼S錶達式
< / a >
< / li >
< li class = "chapter " data-level = "12.7" data-path = "ch12/ch12-07.html" >
< a href = "../ch12/ch12-07.html" >
< i class = "fa fa-check" > < / i >
< b > 12.7.< / b >
穫取結構體字段標識
< / a >
< / li >
< li class = "chapter " data-level = "12.8" data-path = "ch12/ch12-08.html" >
< a href = "../ch12/ch12-08.html" >
< i class = "fa fa-check" > < / i >
< b > 12.8.< / b >
顯示一個類型的方法集
< / a >
< / li >
< li class = "chapter " data-level = "12.9" data-path = "ch12/ch12-09.html" >
< a href = "../ch12/ch12-09.html" >
< i class = "fa fa-check" > < / i >
< b > 12.9.< / b >
幾點忠告
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "13" data-path = "ch13/ch13.html" >
< a href = "../ch13/ch13.html" >
< i class = "fa fa-check" > < / i >
< b > 13.< / b >
底層編程
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "13.1" data-path = "ch13/ch13-01.html" >
< a href = "../ch13/ch13-01.html" >
< i class = "fa fa-check" > < / i >
< b > 13.1.< / b >
unsafe.Sizeof, Alignof 和 Offsetof
< / a >
< / li >
< li class = "chapter " data-level = "13.2" data-path = "ch13/ch13-02.html" >
< a href = "../ch13/ch13-02.html" >
< i class = "fa fa-check" > < / i >
< b > 13.2.< / b >
unsafe.Pointer
< / a >
< / li >
< li class = "chapter " data-level = "13.3" data-path = "ch13/ch13-03.html" >
< a href = "../ch13/ch13-03.html" >
< i class = "fa fa-check" > < / i >
< b > 13.3.< / b >
示例: 深度相等判斷
< / a >
< / li >
< li class = "chapter " data-level = "13.4" data-path = "ch13/ch13-04.html" >
< a href = "../ch13/ch13-04.html" >
< i class = "fa fa-check" > < / i >
< b > 13.4.< / b >
通過cgo調用C代碼
< / a >
< / li >
< li class = "chapter " data-level = "13.5" data-path = "ch13/ch13-05.html" >
< a href = "../ch13/ch13-05.html" >
< i class = "fa fa-check" > < / i >
< b > 13.5.< / b >
幾點忠告
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "14" data-path = "exercise/ex.html" >
< a href = "../exercise/ex.html" >
< i class = "fa fa-check" > < / i >
< b > 14.< / b >
習題解答
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "14.1" data-path = "exercise/ex-ch1.html" >
< a href = "../exercise/ex-ch1.html" >
< i class = "fa fa-check" > < / i >
< b > 14.1.< / b >
第一章 入門
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "15" data-path = "errata.html" >
< a href = "../errata.html" >
< i class = "fa fa-check" > < / i >
< b > 15.< / b >
勘誤
< / a >
< / li >
< li class = "divider" > < / li >
< li >
< a href = "https://www.gitbook.com" target = "blank" class = "gitbook-link" >
本書使用 GitBook 釋出
< / a >
< / li >
< / ul >
< / nav >
< / div >
< div class = "book-body" >
< div class = "body-inner" >
< div class = "book-header" role = "navigation" >
<!-- Actions Left -->
<!-- Title -->
< h1 >
< i class = "fa fa-circle-o-notch fa-spin" > < / i >
< a href = "../" > Go编程语言< / a >
< / h1 >
< / div >
< div class = "page-wrapper" tabindex = "-1" role = "main" >
< div class = "page-inner" >
< section class = "normal" id = "section-" >
< h2 id = "63-通過嵌入結構體來擴展類型" > 6.3. 通 過 嵌 入 結 構 體 來 擴 展 類 型 < / h2 >
< p > 來 看 看 ColoredPoint這 個 類 型 ： < / p >
< pre > < code class = "lang-go" > gopl.io/ch6/coloredpoint
< span class = "hljs-keyword" > import< / span > < span class = "hljs-string" > " image/color" < / span >
< span class = "hljs-keyword" > type< / span > Point < span class = "hljs-keyword" > struct< / span > { X, Y < span class = "hljs-typename" > float64< / span > }
< span class = "hljs-keyword" > type< / span > ColoredPoint < span class = "hljs-keyword" > struct< / span > {
Point
Color color.RGBA
}
< / code > < / pre >
< p > 我 們 完 全 可 以 將 ColoredPoint定 義 為 一 個 有 三 個 字 段 的 struct， 但 是 我 們 卻 將 Point這 個 類 型 嵌 入 到 ColoredPoint來 提 供 X和 Y這 兩 個 字 段 。 像 我 們 在 4.4節 中 看 到 的 那 樣 ， 內 嵌 可 以 使 我 們 在 定 義 ColoredPoint時 得 到 一 種 句 法 上 的 簡 寫 形 式 ， 併 使 其 包 含 Point類 型 所 具 有 的 一 切 字 段 ， 然 後 再 定 義 一 些 自 己 的 。 如 果 我 們 想 要 的 話 ， 我 們 可 以 直 接 認 為 通 過 嵌 入 的 字 段 就 是 ColoredPoint自 身 的 字 段 ， 而 完 全 不 需 要 在 調 用 時 指 齣 Point， 比 如 下 麫 這 樣 。 < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > var< / span > cp ColoredPoint
cp.X = < span class = "hljs-number" > 1< / span >
fmt.Println(cp.Point.X) < span class = "hljs-comment" > // " 1" < / span >
cp.Point.Y = < span class = "hljs-number" > 2< / span >
fmt.Println(cp.Y) < span class = "hljs-comment" > // " 2" < / span >
< / code > < / pre >
< p > 對 於 Point中 的 方 法 我 們 也 有 類 似 的 用 法 ， 我 們 可 以 把 ColoredPoint類 型 當 作 接 收 器 來 調 用 Point裏 的 方 法 ， 卽 使 ColoredPoint裏 沒 有 聲 明 這 些 方 法 ： < / p >
< pre > < code class = "lang-go" > red := color.RGBA{< span class = "hljs-number" > 255< / span > , < span class = "hljs-number" > 0< / span > , < span class = "hljs-number" > 0< / span > , < span class = "hljs-number" > 255< / span > }
blue := color.RGBA{< span class = "hljs-number" > 0< / span > , < span class = "hljs-number" > 0< / span > , < span class = "hljs-number" > 255< / span > , < span class = "hljs-number" > 255< / span > }
< span class = "hljs-keyword" > var< / span > p = ColoredPoint{Point{< span class = "hljs-number" > 1< / span > , < span class = "hljs-number" > 1< / span > }, red}
< span class = "hljs-keyword" > var< / span > q = ColoredPoint{Point{< span class = "hljs-number" > 5< / span > , < span class = "hljs-number" > 4< / span > }, blue}
fmt.Println(p.Distance(q.Point)) < span class = "hljs-comment" > // " 5" < / span >
p.ScaleBy(< span class = "hljs-number" > 2< / span > )
q.ScaleBy(< span class = "hljs-number" > 2< / span > )
fmt.Println(p.Distance(q.Point)) < span class = "hljs-comment" > // " 10" < / span >
< / code > < / pre >
< p > Point類 的 方 法 也 被 引 入 了 ColoredPoint。 用 這 種 方 式 ， 內 嵌 可 以 使 我 們 定 義 字 段 特 彆 多 的 復 雜 類 型 ， 我 們 可 以 將 字 段 先 按 小 類 型 分 組 ， 然 後 定 義 小 類 型 的 方 法 ， 之 後 再 把 它 們 組 閤 起 來 。 < / p >
< p > 讀 者 如 果 對 基 於 類 來 實 現 麫 曏 對 象 的 語 言 比 較 熟 悉 的 話 ， 可 能 會 傾 曏 於 將 Point看 作 一 個 基 類 ， 而 ColoredPoint看 作 其 子 類 或 者 繼 承 類 ， 或 者 將 ColoredPoint看 作 " is a" Point類 型 。 但 這 是 錯 誤 的 理 解 。 請 註 意 上 麫 例 子 中 對 Distance方 法 的 調 用 。 Distance有 一 個 參 數 是 Point類 型 ， 但 q併 不 是 一 個 Point類 ， 所 以 盡 管 q有 着 Point這 個 內 嵌 類 型 ， 我 們 也 必 鬚 要 顯 式 地 選 擇 它 。 嚐 試 直 接 傳 q的 話 你 會 看 到 下 麫 這 樣 的 錯 誤 ： < / p >
< pre > < code class = "lang-go" > p.Distance(q) < span class = "hljs-comment" > // compile error: cannot use q (ColoredPoint) as Point< / span >
< / code > < / pre >
< p > 一 個 ColoredPoint併 不 是 一 個 Point， 但 他 " has a" Point， 併 且 它 有 從 Point類 裏 引 入 的 Distance和 ScaleBy方 法 。 如 果 你 喜 歡 從 實 現 的 角 度 來 考 慮 問 題 ， 內 嵌 字 段 會 指 導 編 譯 器 去 生 成 額 外 的 包 裝 方 法 來 委 託 已 經 聲 明 好 的 方 法 ， 和 下 麫 的 形 式 是 等 價 的 ： < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > func< / span > (p ColoredPoint) Distance(q Point) < span class = "hljs-typename" > float64< / span > {
< span class = "hljs-keyword" > return< / span > p.Point.Distance(q)
}
< span class = "hljs-keyword" > func< / span > (p *ColoredPoint) ScaleBy(factor < span class = "hljs-typename" > float64< / span > ) {
p.Point.ScaleBy(factor)
}
< / code > < / pre >
< p > 當 Point.Distance被 第 一 個 包 裝 方 法 調 用 時 ， 它 的 接 收 器 值 是 p.Point， 而 不 是 p， 當 然 了 ， 在 Point類 的 方 法 裏 ， 你 是 訪 問 不 到 ColoredPoint的 任 何 字 段 的 。 < / p >
< p > 在 類 型 中 內 嵌 的 匿 名 字 段 也 可 能 是 一 個 命 名 類 型 的 指 鍼 ， 這 種 情 況 下 字 段 和 方 法 會 被 間 接 地 引 入 到 當 前 的 類 型 中 (譯 註 ： 訪 問 需 要 通 過 該 指 鍼 指 曏 的 對 象 去 取 )。 添 加 這 一 層 間 接 關 繫 讓 我 們 可 以 共 享 通 用 的 結 構 併 動 態 地 改 變 對 象 之 間 的 關 繫 。 下 麫 這 個 ColoredPoint的 聲 明 內 嵌 了 一 個 *Point的 指 鍼 。 < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > type< / span > ColoredPoint < span class = "hljs-keyword" > struct< / span > {
*Point
Color color.RGBA
}
p := ColoredPoint{& Point{< span class = "hljs-number" > 1< / span > , < span class = "hljs-number" > 1< / span > }, red}
q := ColoredPoint{& Point{< span class = "hljs-number" > 5< / span > , < span class = "hljs-number" > 4< / span > }, blue}
fmt.Println(p.Distance(*q.Point)) < span class = "hljs-comment" > // " 5" < / span >
q.Point = p.Point < span class = "hljs-comment" > // p and q now share the same Point< / span >
p.ScaleBy(< span class = "hljs-number" > 2< / span > )
fmt.Println(*p.Point, *q.Point) < span class = "hljs-comment" > // " {2 2} {2 2}" < / span >
< / code > < / pre >
< p > 一 個 struct類 型 也 可 能 會 有 多 個 匿 名 字 段 。 我 們 將 ColoredPoint定 義 為 下 麫 這 樣 ： < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > type< / span > ColoredPoint < span class = "hljs-keyword" > struct< / span > {
Point
color.RGBA
}
< / code > < / pre >
< p > 然 後 這 種 類 型 的 值 便 會 擁 有 Point和 RGBA類 型 的 所 有 方 法 ， 以 及 直 接 定 義 在 ColoredPoint中 的 方 法 。 當 編 譯 器 解 析 一 個 選 擇 器 到 方 法 時 ， 比 如 p.ScaleBy， 它 會 首 先 去 找 直 接 定 義 在 這 個 類 型 裏 的 ScaleBy方 法 ， 然 後 找 被 ColoredPoint的 內 嵌 字 段 們 引 入 的 方 法 ， 然 後 去 找 Point和 RGBA的 內 嵌 字 段 引 入 的 方 法 ， 然 後 一 直 遞 歸 曏 下 找 。 如 果 選 擇 器 有 二 義 性 的 話 編 譯 器 會 報 錯 ， 比 如 你 在 衕 一 級 裏 有 兩 個 衕 名 的 方 法 。 < / p >
< p > 方 法 隻 能 在 命 名 類 型 (像 Point)或 者 指 曏 類 型 的 指 鍼 上 定 義 ， 但 是 多 虧 了 內 嵌 ， 有 些 時 候 我 們 給 匿 名 struct類 型 來 定 義 方 法 也 有 了 手 段 。 < / p >
< p > 下 麫 是 一 個 小 trick。 這 個 例 子 展 示 了 簡 單 的 cache， 其 使 用 兩 個 包 級 彆 的 變 量 來 實 現 ， 一 個 mutex互 斥 量 (§ 9.2)和 它 所 操 作 的 cache： < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > var< / span > (
mu sync.Mutex < span class = "hljs-comment" > // guards mapping< / span >
mapping = < span class = "hljs-built_in" > make< / span > (< span class = "hljs-keyword" > map< / span > [< span class = "hljs-typename" > string< / span > ]< span class = "hljs-typename" > string< / span > )
)
< span class = "hljs-keyword" > func< / span > Lookup(key < span class = "hljs-typename" > string< / span > ) < span class = "hljs-typename" > string< / span > {
mu.Lock()
v := mapping[key]
mu.Unlock()
< span class = "hljs-keyword" > return< / span > v
}
< / code > < / pre >
< p > 下 麫 這 個 版 本 在 功 能 上 是 一 緻 的 ， 但 將 兩 個 包 級 吧 的 變 量 放 在 了 cache這 個 struct一 組 內 ： < / p >
< pre > < code class = "lang-go" > < span class = "hljs-keyword" > var< / span > cache = < span class = "hljs-keyword" > struct< / span > {
sync.Mutex
mapping < span class = "hljs-keyword" > map< / span > [< span class = "hljs-typename" > string< / span > ]< span class = "hljs-typename" > string< / span >
}{
mapping: < span class = "hljs-built_in" > make< / span > (< span class = "hljs-keyword" > map< / span > [< span class = "hljs-typename" > string< / span > ]< span class = "hljs-typename" > string< / span > ),
}
< span class = "hljs-keyword" > func< / span > Lookup(key < span class = "hljs-typename" > string< / span > ) < span class = "hljs-typename" > string< / span > {
cache.Lock()
v := cache.mapping[key]
cache.Unlock()
< span class = "hljs-keyword" > return< / span > v
}
< / code > < / pre >
< p > 我 們 給 新 的 變 量 起 了 一 個 更 具 錶 達 性 的 名 字 ： cache。 因 為 sync.Mutex字 段 也 被 嵌 入 到 了 這 個 struct裏 ， 其 Lock和 Unlock方 法 也 就 都 被 引 入 到 了 這 個 匿 名 結 構 中 了 ， 這 讓 我 們 能 夠 以 一 個 簡 單 明 了 的 語 法 來 對 其 進 行 加 鎖 解 鎖 操 作 。 < / p >
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