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< div class = "book" data-level = "13.1" data-chapter-title = "unsafe.Sizeof, Alignof 和 Offsetof" data-filepath = "ch13/ch13-01.md" data-basepath = ".." data-revision = "Mon Dec 28 2015 16:03:52 GMT+0800 (中国标准时间)" >
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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 >
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致謝
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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 >
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査找重複的行
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< b > 1.4.< / b >
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GIF動畵
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< b > 1.5.< / b >
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獲取URL
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< b > 1.6.< / b >
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併發獲取多個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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< 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 >
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複數
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< b > 3.4.< / b >
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布爾型
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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 >
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複合數據類型
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< ul class = "articles" >
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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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< a href = "../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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< li class = "chapter " data-level = "5.8" data-path = "ch5/ch5-08.html" >
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< b > 5.8.< / b >
Deferred函數
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< li class = "chapter " data-level = "5.9" data-path = "ch5/ch5-09.html" >
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< b > 5.9.< / b >
Panic異常
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< li class = "chapter " data-level = "5.10" data-path = "ch5/ch5-10.html" >
< a href = "../ch5/ch5-10.html" >
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< b > 5.10.< / b >
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Recover捕獲異常
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< b > 6.< / b >
方法
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< li class = "chapter " data-level = "6.1" data-path = "ch6/ch6-01.html" >
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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" >
< a href = "../ch6/ch6-02.html" >
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< b > 6.2.< / b >
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基於指針對象的方法
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< li class = "chapter " 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 >
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方法值和方法表達式
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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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< b > 6.6.< / b >
封裝
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< b > 7.< / b >
接口
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< b > 7.1.< / b >
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接口是合約
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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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< 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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< 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 >
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示例: 表達式求值
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< b > 7.10.< / b >
類型斷言
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< b > 7.11.< / b >
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基於類型斷言識别錯誤類型
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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
< / a >
< / li >
< li class = "chapter " data-level = "7.14" data-path = "ch7/ch7-14.html" >
< a href = "../ch7/ch7-14.html" >
< i class = "fa fa-check" > < / i >
< b > 7.14.< / b >
示例: 基於標記的XML解碼
< / a >
< / li >
< li class = "chapter " data-level = "7.15" data-path = "ch7/ch7-15.html" >
< a href = "../ch7/ch7-15.html" >
< i class = "fa fa-check" > < / i >
< b > 7.15.< / b >
補充幾點
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "8" data-path = "ch8/ch8.html" >
< a href = "../ch8/ch8.html" >
< i class = "fa fa-check" > < / i >
< b > 8.< / b >
Goroutines和Channels
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "8.1" data-path = "ch8/ch8-01.html" >
< a href = "../ch8/ch8-01.html" >
< i class = "fa fa-check" > < / i >
< b > 8.1.< / b >
Goroutines
< / a >
< / li >
< li class = "chapter " data-level = "8.2" data-path = "ch8/ch8-02.html" >
< a href = "../ch8/ch8-02.html" >
< i class = "fa fa-check" > < / i >
< b > 8.2.< / b >
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示例: 併發的Clock服務
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< / 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 >
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示例: 併發的Echo服務
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< / 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 >
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示例: 併發的Web爬蟲
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< / 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 >
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基於select的多路複用
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< / 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 >
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示例: 併發的字典遍歷
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< / 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 >
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併發的退出
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< / 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 >
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基於共享變量的併發
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< / 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 >
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內存同步
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< / 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 >
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示例: 併發的非阻塞緩存
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< / 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 >
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Goroutines和線程
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< / 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 >
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爲何需要反射?
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< / 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 >
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示例: 編碼S表達式
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< / 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 >
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示例: 解碼S表達式
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< / 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 >
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獲取結構體字段標識
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< / 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 active" 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 >
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< li class = "chapter " data-level = "14" data-path = "CONTRIBUTORS.html" >
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2015-12-24 06:47:06 +00:00
< a href = "../CONTRIBUTORS.html" >
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< i class = "fa fa-check" > < / i >
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< b > 14.< / b >
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附録
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< / 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 = "131-unsafesizeof-alignof-和-offsetof" > 13.1. unsafe.Sizeof, Alignof 和 Offsetof< / h2 >
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< p > unsafe.Sizeof函 數 返 迴 操 作 數 在 內 存 中 的 字 節 大 小 ， 參 數 可 以 是 任 意 類 型 的 表 達 式 ， 但 是 它 併 不 會 對 表 達 式 進 行 求 值 。 一 個 Sizeof函 數 調 用 是 一 個 對 應 uintptr類 型 的 常 量 表 達 式 ， 因 此 返 迴 的 結 果 可 以 用 作 數 組 類 型 的 長 度 大 小 ， 或 者 用 作 計 算 其 他 的 常 量 。 < / p >
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< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > import< / span > < span class = "hljs-string" > " unsafe" < / span >
fmt.Println(unsafe.Sizeof(< span class = "hljs-typename" > float64< / span > (< span class = "hljs-number" > 0< / span > ))) < span class = "hljs-comment" > // " 8" < / span >
< / code > < / pre >
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< p > Sizeof函 數 返 迴 的 大 小 隻 包 括 數 據 結 構 中 固 定 的 部 分 ， 例 如 字 符 串 對 應 結 構 體 中 的 指 針 和 字 符 串 長 度 部 分 ， 但 是 併 不 包 含 指 針 指 向 的 字 符 串 的 內 容 。 Go語 言 中 非 聚 合 類 型 通 常 有 一 個 固 定 的 大 小 ， 盡 管 在 不 同 工 具 鏈 下 生 成 的 實 際 大 小 可 能 會 有 所 不 同 。 考 慮 到 可 移 植 性 ， 引 用 類 型 或 包 含 引 用 類 型 的 大 小 在 32位 平 台 上 是 4個 字 節 ， 在 64位 平 台 上 是 8個 字 節 。 < / p >
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< p > 計 算 機 在 加 載 和 保 存 數 據 時 ， 如 果 內 存 地 址 合 理 地 對 齊 的 將 會 更 有 效 率 。 例 如 2字 節 大 小 的 int16類 型 的 變 量 地 址 應 該 是 偶 數 ， 一 個 4字 節 大 小 的 rune類 型 變 量 的 地 址 應 該 是 4的 倍 數 ， 一 個 8字 節 大 小 的 float64、 uint64或 64-bit指 針 類 型 變 量 的 地 址 應 該 是 8字 節 對 齊 的 。 但 是 對 於 再 大 的 地 址 對 齊 倍 數 則 是 不 需 要 的 ， 卽 使 是 complex128等 較 大 的 數 據 類 型 最 多 也 隻 是 8字 節 對 齊 。 < / p >
< p > 由 於 地 址 對 齊 這 個 因 素 ， 一 個 聚 合 類 型 （ 結 構 體 或 數 組 ） 的 大 小 至 少 是 所 有 字 段 或 元 素 大 小 的 總 和 ， 或 者 更 大 因 爲 可 能 存 在 內 存 空 洞 。 內 存 空 洞 是 編 譯 器 自 動 添 加 的 沒 有 被 使 用 的 內 存 空 間 ， 用 於 保 證 後 面 每 個 字 段 或 元 素 的 地 址 相 對 於 結 構 或 數 組 的 開 始 地 址 能 夠 合 理 地 對 齊 （ 譯 註 ： 內 存 空 洞 可 能 會 存 在 一 些 隨 機 數 據 ， 可 能 會 對 用 unsafe包 直 接 操 作 內 存 的 處 理 産 生 影 響 ） 。 < / p >
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< table >
< thead >
< tr >
< th > 類 型 < / th >
< th > 大 小 < / th >
< / tr >
< / thead >
< tbody >
< tr >
< td > bool< / td >
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< td > 1個 字 節 < / td >
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< / tr >
< tr >
< td > intN, uintN, floatN, complexN< / td >
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< td > N/8個 字 節 (例 如 float64是 8個 字 節 )< / td >
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< / tr >
< tr >
< td > int, uint, uintptr< / td >
< td > 1個 機 器 字 < / td >
< / tr >
< tr >
< td > *T< / td >
< td > 1個 機 器 字 < / td >
< / tr >
< tr >
< td > string< / td >
< td > 2個 機 器 字 (data,len)< / td >
< / tr >
< tr >
< td > []T< / td >
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< td > 3個 機 器 字 (data,len,cap)< / td >
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< / tr >
< tr >
< td > map< / td >
< td > 1個 機 器 字 < / td >
< / tr >
< tr >
< td > func< / td >
< td > 1個 機 器 字 < / td >
< / tr >
< tr >
< td > chan< / td >
< td > 1個 機 器 字 < / td >
< / tr >
< tr >
< td > interface< / td >
< td > 2個 機 器 字 (type,value)< / td >
< / tr >
< / tbody >
< / table >
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< p > Go語 言 的 規 范 併 沒 有 要 求 一 個 字 段 的 聲 明 順 序 和 內 存 中 的 順 序 是 一 致 的 ， 所 以 理 論 上 一 個 編 譯 器 可 以 隨 意 地 重 新 排 列 每 個 字 段 的 內 存 位 置 ， 隨 然 在 寫 作 本 書 的 時 候 編 譯 器 還 沒 有 這 麽 做 。 下 面 的 三 個 結 構 體 雖 然 有 着 相 同 的 字 段 ， 但 是 第 一 種 寫 法 比 另 外 的 兩 個 需 要 多 50%的 內 存 。 < / p >
< pre > < code class = "lang-Go" > < span class = "hljs-comment" > // 64-bit 32-bit< / span >
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< span class = "hljs-keyword" > struct< / span > { < span class = "hljs-typename" > bool< / span > ; < span class = "hljs-typename" > float64< / span > ; < span class = "hljs-typename" > int16< / span > } < span class = "hljs-comment" > // 3 words 4words< / span >
< span class = "hljs-keyword" > struct< / span > { < span class = "hljs-typename" > float64< / span > ; < span class = "hljs-typename" > int16< / span > ; < span class = "hljs-typename" > bool< / span > } < span class = "hljs-comment" > // 2 words 3words< / span >
< span class = "hljs-keyword" > struct< / span > { < span class = "hljs-typename" > bool< / span > ; < span class = "hljs-typename" > int16< / span > ; < span class = "hljs-typename" > float64< / span > } < span class = "hljs-comment" > // 2 words 3words< / span >
< / code > < / pre >
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< p > 關 於 內 存 地 址 對 齊 算 法 的 細 節 超 出 了 本 書 的 范 圍 ， 也 不 是 每 一 個 結 構 體 都 需 要 擔 心 這 個 問 題 ， 不 過 有 效 的 包 裝 可 以 使 數 據 結 構 更 加 緊 湊 （ 譯 註 ： 未 來 的 Go語 言 編 譯 器 應 該 會 默 認 優 化 結 構 體 的 順 序 ， 當 然 用 於 應 該 也 能 夠 指 定 具 體 的 內 存 布 局 ， 相 同 討 論 請 參 考 < a href = "https://github.com/golang/go/issues/10014" target = "_blank" > Issue10014< / a > ） ， 內 存 使 用 率 和 性 能 都 可 能 會 受 益 。 < / p >
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< p > < code > unsafe.Alignof< / code > 函 數 返 迴 對 應 參 數 的 類 型 需 要 對 齊 的 倍 數 . 和 Sizeof 類 似 , Alignof 也 是 返 迴 一 個 常 量 表 達 式 , 對 應 一 個 常 量 . 通 常 情 況 下 布 爾 和 數 字 類 型 需 要 對 齊 到 它 們 本 身 的 大 小 (最 多 8個 字 節 ), 其 它 的 類 型 對 齊 到 機 器 字 大 小 .< / p >
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< p > < code > unsafe.Offsetof< / code > 函 數 的 參 數 必 鬚 是 一 個 字 段 < code > x.f< / code > , 然 後 返 迴 < code > f< / code > 字 段 相 對 於 < code > x< / code > 起 始 地 址 的 偏 移 量 , 包 括 可 能 的 空 洞 .< / p >
< p > 圖 13.1 顯 示 了 一 個 結 構 體 變 量 x 以 及 其 在 32位 和 64位 機 器 上 的 典 型 的 內 存 . 灰 色 區 域 是 空 洞 .< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > x < span class = "hljs-keyword" > struct< / span > {
a < span class = "hljs-typename" > bool< / span >
b < span class = "hljs-typename" > int16< / span >
c []< span class = "hljs-typename" > int< / span >
}
< / code > < / pre >
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< p > 下 面 顯 示 了 對 x和 它 的 三 個 字 段 調 用 unsafe包 相 關 函 數 的 計 算 結 果 ： < / p >
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< p > < img src = "../images/ch13-01.png" alt = "" > < / p >
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< p > 32位 繫 統 ： < / p >
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< pre > < code > Sizeof(x) = 16 Alignof(x) = 4
Sizeof(x.a) = 1 Alignof(x.a) = 1 Offsetof(x.a) = 0
Sizeof(x.b) = 2 Alignof(x.b) = 2 Offsetof(x.b) = 2
Sizeof(x.c) = 12 Alignof(x.c) = 4 Offsetof(x.c) = 4
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< / code > < / pre > < p > 64位 繫 統 ： < / p >
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< pre > < code > Sizeof(x) = 32 Alignof(x) = 8
Sizeof(x.a) = 1 Alignof(x.a) = 1 Offsetof(x.a) = 0
Sizeof(x.b) = 2 Alignof(x.b) = 2 Offsetof(x.b) = 2
Sizeof(x.c) = 24 Alignof(x.c) = 8 Offsetof(x.c) = 8
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< / code > < / pre > < p > 雖 然 這 幾 個 函 數 在 不 安 全 的 unsafe包 ， 但 是 這 幾 個 函 數 調 用 併 不 是 眞 的 不 安 全 ， 特 别 在 需 要 優 化 內 存 空 間 時 它 們 返 迴 的 結 果 對 於 理 解 原 生 的 內 存 布 局 很 有 幫 助 。 < / p >
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< / section >
< / div >
< / div >
< / div >
< a href = "../ch13/ch13.html" class = "navigation navigation-prev " aria-label = "Previous page: 底層編程" > < i class = "fa fa-angle-left" > < / i > < / a >
< a href = "../ch13/ch13-02.html" class = "navigation navigation-next " aria-label = "Next page: unsafe.Pointer" > < i class = "fa fa-angle-right" > < / i > < / a >
< / div >
< / div >
< script src = "../gitbook/app.js" > < / script >
< script src = "../gitbook/plugins/gitbook-plugin-sharing/buttons.js" > < / script >
< script src = "../gitbook/plugins/gitbook-plugin-fontsettings/buttons.js" > < / script >
< script >
require(["gitbook"], function(gitbook) {
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var config = {"katex":{},"highlight":{},"sharing":{"facebook":true,"twitter":true,"google":false,"weibo":false,"instapaper":false,"vk":false,"all":["facebook","google","twitter","weibo","instapaper"]},"fontsettings":{"theme":"white","family":"sans","size":2}};
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gitbook.start(config);
});
< / script >
< / body >
< / html >
