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< div class = "book" data-level = "3.6" data-chapter-title = "常量" data-filepath = "ch3/ch3-06.md" data-basepath = ".." data-revision = "Fri Dec 25 2015 12:32:44 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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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 >
穫取URL
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< b > 1.6.< / b >
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併發穫取多個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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< 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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< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "4.1" data-path = "ch4/ch4-01.html" >
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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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< li class = "chapter " data-level = "5" data-path = "ch5/ch5.html" >
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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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< li class = "chapter " data-level = "5.3" data-path = "ch5/ch5-03.html" >
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< b > 5.3.< / b >
多返迴值
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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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< b > 5.6.< / b >
匿名函數
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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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< li class = "chapter " data-level = "5.10" data-path = "ch5/ch5-10.html" >
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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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< 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" >
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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" >
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< b > 6.3.< / b >
通過嵌入結構體來擴展類型
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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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< 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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< 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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< 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
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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" >
< 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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併發的退齣
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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 >
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 >
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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 = "36-常量" > 3.6. 常 量 < / h2 >
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< p > 常 量 表 達 式 的 值 在 編 譯 期 計 算 , 而 不 是 在 運 行 期 . 每 種 常 量 的 潛 在 類 型 都 是 基 礎 類 型 : boolean, string, 或 數 字 .< / p >
< p > 一 個 常 量 的 聲 明 語 句 定 義 了 常 量 的 名 字 , 和 變 量 的 聲 明 語 法 類 似 , 常 量 的 值 不 可 脩 改 , 這 樣 可 以 防 止 在 運 行 期 被 意 外 或 惡 意 的 脩 改 . 例 如 , 常 量 比 變 量 更 適 合 用 於 表 達 像 π 之 類 的 數 學 常 數 , 因 爲 它 們 的 值 不 會 變 化 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > pi = < span class = "hljs-number" > 3.14159< / span > < span class = "hljs-comment" > // approximately; math.Pi is a better approximation< / span >
< / code > < / pre >
< p > 和 變 量 聲 明 一 樣 , 可 以 批 量 聲 明 多 個 常 量 ; 這 比 較 適 合 聲 明 一 組 相 關 的 常 量 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > (
e = < span class = "hljs-number" > 2.71828182845904523536028747135266249775724709369995957496696763< / span >
pi = < span class = "hljs-number" > 3.14159265358979323846264338327950288419716939937510582097494459< / span >
)
< / code > < / pre >
< p > 許 多 常 量 的 運 算 可 以 在 編 譯 期 完 成 , 這 樣 可 以 減 少 運 行 時 的 工 作 , 也 方 便 其 他 編 譯 優 化 . 當 操 作 數 是 常 量 時 , 一 些 運 行 時 的 錯 誤 可 以 在 編 譯 時 發 現 , 例 如 整 數 除 零 , 字 符 串 索 引 越 界 , 任 何 導 致 無 效 浮 點 數 的 操 作 等 .< / p >
< p > 常 量 間 的 所 有 算 術 運 算 , 邏 輯 運 算 和 比 較 運 算 的 結 果 也 是 常 量 , 對 常 量 的 類 型 轉 換 操 作 或 以 下 函 數 調 用 都 是 返 迴 常 量 結 果 : len, cap, real, imag, complex, 和 unsafe.Sizeof(§ 13.1).< / p >
< p > 因 爲 它 們 的 值 是 在 編 譯 期 就 確 定 的 , 因 此 常 量 可 以 是 構 成 類 型 的 一 部 分 , 例 如 用 於 指 定 數 組 類 型 的 長 度 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > IPv4Len = < span class = "hljs-number" > 4< / span >
< span class = "hljs-comment" > // parseIPv4 parses an IPv4 address (d.d.d.d).< / span >
< span class = "hljs-keyword" > func< / span > parseIPv4(s < span class = "hljs-typename" > string< / span > ) IP {
< span class = "hljs-keyword" > var< / span > p [IPv4Len]< span class = "hljs-typename" > byte< / span >
< span class = "hljs-comment" > // ...< / span >
}
< / code > < / pre >
< p > 一 個 常 量 的 聲 明 也 可 以 包 含 一 個 類 型 和 一 個 值 , 但 是 如 果 沒 有 顯 式 指 明 類 型 , 那 麽 將 從 右 邊 的 表 達 式 推 斷 類 型 . 在 下 面 的 代 碼 中 , time.Duration 是 一 個 命 名 類 型 , 底 層 類 型 是 int64, time.Minute 是 對 應 類 型 的 常 量 . 下 面 聲 明 的 兩 個 常 量 都 是 time.Duration 類 型 , 可 以 通 過 %T 參 數 打 印 類 型 信 息 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > noDelay time.Duration = < span class = "hljs-number" > 0< / span >
< span class = "hljs-keyword" > const< / span > timeout = < span class = "hljs-number" > 5< / span > * time.Minute
fmt.Printf(< span class = "hljs-string" > " %T %[1]v\n" < / span > , noDelay) < span class = "hljs-comment" > // " time.Duration 0" < / span >
fmt.Printf(< span class = "hljs-string" > " %T %[1]v\n" < / span > , timeout) < span class = "hljs-comment" > // " time.Duration 5m0s< / span >
fmt.Printf(< span class = "hljs-string" > " %T %[1]v\n" < / span > , time.Minute) < span class = "hljs-comment" > // " time.Duration 1m0s" < / span >
< / code > < / pre >
< p > 如 果 是 批 量 聲 明 的 常 量 , 除 了 第 一 個 外 其 他 常 量 的 右 邊 的 表 發 生 可 以 省 略 , 如 果 省 略 則 表 示 使 用 前 面 的 表 達 式 , 對 應 的 常 量 類 型 也 一 樣 . 例 如 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > (
a = < span class = "hljs-number" > 1< / span >
b
c = < span class = "hljs-number" > 2< / span >
d
)
fmt.Println(a, b, c, d) < span class = "hljs-comment" > // " 1 1 2 2" < / span >
< / code > < / pre >
< p > 如 果 隻 是 簡 單 地 複 製 右 邊 的 常 量 表 達 式 , 併 沒 有 太 實 用 的 價 值 . 但 是 它 可 以 帶 來 其 他 的 特 性 , 那 就 是 iota 常 量 生 成 器 .< / p >
< h3 id = "361-iota-常量生成器" > 3.6.1. iota 常 量 生 成 器 < / h3 >
< p > 常 量 聲 明 可 以 使 用 iota 常 量 生 成 器 , 用 於 生 成 一 組 相 似 的 常 量 值 , 但 是 不 用 每 行 都 寫 一 遍 . 在 一 個 const 聲 明 語 句 中 , 在 開 始 一 行 iota 將 會 被 置 爲 0, 然 後 在 每 一 個 有 常 量 聲 明 的 行 加 一 .< / p >
< p > 下 面 是 來 自 time 包 的 例 子 , 它 首 先 定 義 了 Weekday命 名 類 型 , 然 後 爲 一 週 的 每 天 定 義 一 個 常 量 , 從 週 日 0開 始 . 這 種 類 型 一 般 被 稱 爲 枚 舉 類 型 .< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > type< / span > Weekday < span class = "hljs-typename" > int< / span >
< span class = "hljs-keyword" > const< / span > (
Sunday Weekday = < span class = "hljs-constant" > iota< / span >
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
)
< / code > < / pre >
< p > 週 一 將 對 應 0, 週 一 爲 1, 如 此 等 等 .< / p >
< p > 我 們 也 可 以 在 複 雜 的 常 量 表 達 式 中 使 用 iota, 下 面 是 來 自 net 包 的 例 子 , 用 於 給 一 個 無 符 號 整 數 的 最 低 5bit的 每 個 bit給 定 一 個 名 字 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > type< / span > Flags < span class = "hljs-typename" > uint< / span >
< span class = "hljs-keyword" > const< / span > (
FlagUp Flags = < span class = "hljs-number" > 1< / span > < < < span class = "hljs-constant" > iota< / span > < span class = "hljs-comment" > // is up< / span >
FlagBroadcast < span class = "hljs-comment" > // supports broadcast access capability< / span >
FlagLoopback < span class = "hljs-comment" > // is a loopback interface< / span >
FlagPointToPoint < span class = "hljs-comment" > // belongs to a point-to-point link< / span >
FlagMulticast < span class = "hljs-comment" > // supports multicast access capability< / span >
)
< / code > < / pre >
< p > 隨 着 iota 的 遞 增 , 每 個 常 量 對 應 表 達 式 1 < < iota, 是 連 續 的 2的 冪 , 分 别 對 應 一 個 bit位 置 . 使 用 這 些 常 量 可 以 測 試 , 設 置 , 或 清 除 對 應 的 bit位 的 值 :< / p >
< pre > < code class = "lang-Go" > gopl.io/ch3/netflag
< span class = "hljs-keyword" > func< / span > IsUp(v Flags) < span class = "hljs-typename" > bool< / span > { < span class = "hljs-keyword" > return< / span > v& FlagUp == FlagUp }
< span class = "hljs-keyword" > func< / span > TurnDown(v *Flags) { *v & ^= FlagUp }
< span class = "hljs-keyword" > func< / span > SetBroadcast(v *Flags) { *v |= FlagBroadcast }
< span class = "hljs-keyword" > func< / span > IsCast(v Flags) < span class = "hljs-typename" > bool< / span > { < span class = "hljs-keyword" > return< / span > v& (FlagBroadcast|FlagMulticast) != < span class = "hljs-number" > 0< / span > }
unc main() {
< span class = "hljs-keyword" > var< / span > v Flags = FlagMulticast | FlagUp
fmt.Printf(< span class = "hljs-string" > " %b %t\n" < / span > , v, IsUp(v)) < span class = "hljs-comment" > // " 10001 true" < / span >
TurnDown(& v)
fmt.Printf(< span class = "hljs-string" > " %b %t\n" < / span > , v, IsUp(v)) < span class = "hljs-comment" > // " 10000 false" < / span >
SetBroadcast(& v)
fmt.Printf(< span class = "hljs-string" > " %b %t\n" < / span > , v, IsUp(v)) < span class = "hljs-comment" > // " 10010 false" < / span >
fmt.Printf(< span class = "hljs-string" > " %b %t\n" < / span > , v, IsCast(v)) < span class = "hljs-comment" > // " 10010 true" < / span >
}
< / code > < / pre >
< p > 下 面 是 一 個 更 複 雜 的 例 子 , 每 個 常 量 都 是 1024的 冪 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > (
_ = < span class = "hljs-number" > 1< / span > < < (< span class = "hljs-number" > 10< / span > * < span class = "hljs-constant" > iota< / span > )
KiB < span class = "hljs-comment" > // 1024< / span >
MiB < span class = "hljs-comment" > // 1048576< / span >
GiB < span class = "hljs-comment" > // 1073741824< / span >
TiB < span class = "hljs-comment" > // 1099511627776 (exceeds 1 < < 32)< / span >
PiB < span class = "hljs-comment" > // 1125899906842624< / span >
EiB < span class = "hljs-comment" > // 1152921504606846976< / span >
ZiB < span class = "hljs-comment" > // 1180591620717411303424 (exceeds 1 < < 64)< / span >
YiB < span class = "hljs-comment" > // 1208925819614629174706176< / span >
)
< / code > < / pre >
< p > iota 機 製 也 有 其 局 限 性 . 例 如 , 它 併 不 能 用 於 産 生 1000的 冪 (KB,MB,等 等 ), 因 爲 併 沒 有 計 算 冪 的 運 算 符 .< / p >
< p > < strong > 練 習 3.13:< / strong > 編 寫 KB,MB的 常 量 聲 明 , 然 後 擴 展 到 YB.< / p >
< h3 id = "362-無類型常量" > 3.6.2. 無 類 型 常 量 < / h3 >
< p > Go語 言 的 常 量 有 點 不 尋 常 . 雖 然 一 個 常 量 可 以 有 任 意 有 一 個 確 定 的 基 礎 類 型 , 例 如 int 或 float64, 或 者 是 類 似 time.Duration 這 樣 命 名 的 基 礎 類 型 , 但 是 許 多 常 量 併 沒 有 一 個 明 確 的 基 礎 類 型 . 編 譯 期 爲 這 些 沒 有 明 確 的 基 礎 類 型 的 數 字 常 量 提 供 比 基 礎 類 型 或 機 器 更 高 精 度 的 算 術 運 算 ; 你 可 以 認 爲 至 少 有 256bit的 運 算 精 度 . 這 里 有 六 種 未 明 確 類 型 的 常 量 類 型 , 分 别 是 無 類 型 的 布 爾 型 , 無 類 型 的 整 數 , 無 類 型 的 字 符 , 無 類 型 的 浮 點 數 , 無 類 型 的 複 數 , 無 類 型 的 字 符 串 .< / p >
< p > 通 過 延 遲 明 確 具 體 類 型 , 無 類 型 的 常 量 不 僅 可 以 提 供 更 高 的 精 度 , 而 且 可 以 直 接 用 於 更 多 的 表 達 式 而 不 需 要 類 型 轉 換 . 例 如 例 子 中 的 ZiB 和 YiB 的 值 已 經 超 齣 任 何 Go中 整 數 類 型 能 表 達 的 范 圍 , 但 是 它 們 依 然 是 合 法 的 常 量 , 而 且 可 以 像 下 面 表 達 式 這 樣 使 用 :< / p >
< pre > < code class = "lang-Go" > fmt.Println(YiB/ZiB) < span class = "hljs-comment" > // " 1024" < / span >
< / code > < / pre >
< p > 另 一 個 例 子 , math.Pi 無 類 型 的 浮 點 數 常 量 , 可 以 直 接 用 於 任 意 需 要 浮 點 數 或 複 數 的 地 方 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > x < span class = "hljs-typename" > float32< / span > = math.Pi
< span class = "hljs-keyword" > var< / span > y < span class = "hljs-typename" > float64< / span > = math.Pi
< span class = "hljs-keyword" > var< / span > z < span class = "hljs-typename" > complex128< / span > = math.Pi
< / code > < / pre >
< p > 如 果 math.Pi 被 確 定 爲 特 定 類 型 , 比 如 float64, 那 麽 結 果 精 度 可 能 會 不 一 樣 , 同 時 對 於 需 要 float32或 complex128類 型 值 的 地 方 會 需 要 一 個 明 確 的 類 型 轉 換 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > Pi64 < span class = "hljs-typename" > float64< / span > = math.Pi
< span class = "hljs-keyword" > var< / span > x < span class = "hljs-typename" > float32< / span > = < span class = "hljs-typename" > float32< / span > (Pi64)
< span class = "hljs-keyword" > var< / span > y < span class = "hljs-typename" > float64< / span > = Pi64
< span class = "hljs-keyword" > var< / span > z < span class = "hljs-typename" > complex128< / span > = < span class = "hljs-typename" > complex128< / span > (Pi64)
< / code > < / pre >
< p > 對 於 常 量 面 值 , 不 同 的 寫 法 對 應 不 同 的 類 型 . 例 如 0, 0.0, 0i, 和 ' \u0000' 雖 然 有 着 相 同 的 常 量 值 , 但 是 它 們 分 别 對 應 無 類 型 的 整 數 ,無 類 型 的 浮 點 數 ,無 類 型 的 複 數 ,和 無 類 型 的 字 符 等 不 同 的 常 量 類 型 . 同 樣 , true 和 false 也 是 無 類 型 的 布 爾 類 型 , 字 符 串 面 值 常 量 是 無 類 型 的 字 符 串 .< / p >
< p > 前 面 説 過 除 法 運 算 符 / 根 據 操 作 數 的 類 型 生 成 對 應 類 型 的 結 果 . 因 此 , 不 同 寫 法 的 常 量 除 法 表 達 式 可 能 對 應 不 同 的 結 果 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > f < span class = "hljs-typename" > float64< / span > = < span class = "hljs-number" > 212< / span >
fmt.Println((f - < span class = "hljs-number" > 32< / span > ) * < span class = "hljs-number" > 5< / span > / < span class = "hljs-number" > 9< / span > ) < span class = "hljs-comment" > // " 100" ; (f - 32) * 5 is a float64< / span >
fmt.Println(< span class = "hljs-number" > 5< / span > / < span class = "hljs-number" > 9< / span > * (f - < span class = "hljs-number" > 32< / span > )) < span class = "hljs-comment" > // " 0" ; 5/9 is an untyped integer, 0< / span >
fmt.Println(< span class = "hljs-number" > 5.0< / span > / < span class = "hljs-number" > 9.0< / span > * (f - < span class = "hljs-number" > 32< / span > )) < span class = "hljs-comment" > // " 100" ; 5.0/9.0 is an untyped float< / span >
< / code > < / pre >
< p > 隻 有 常 量 可 以 是 無 類 型 的 . 當 一 個 無 類 型 的 常 量 被 賦 值 給 一 個 變 量 , 就 像 上 面 的 第 一 行 語 句 , 或 者 是 像 其 餘 三 個 語 句 中 右 邊 表 達 式 中 含 有 明 確 類 型 的 值 , 無 類 型 的 常 量 將 會 被 隱 式 轉 換 爲 對 應 的 類 型 , 如 果 可 能 的 話 .< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > f < span class = "hljs-typename" > float64< / span > = < span class = "hljs-number" > 3< / span > + < span class = "hljs-number" > 0i< / span > < span class = "hljs-comment" > // untyped complex -> float64< / span >
f = < span class = "hljs-number" > 2< / span > < span class = "hljs-comment" > // untyped integer -> float64< / span >
f = < span class = "hljs-number" > 1e123< / span > < span class = "hljs-comment" > // untyped floating-point -> float64< / span >
f = < span class = "hljs-string" > ' a' < / span > < span class = "hljs-comment" > // untyped rune -> float64< / span >
< / code > < / pre >
< p > 上 面 的 語 句 相 當 於 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > f < span class = "hljs-typename" > float64< / span > = < span class = "hljs-typename" > float64< / span > (< span class = "hljs-number" > 3< / span > + < span class = "hljs-number" > 0i< / span > )
f = < span class = "hljs-typename" > float64< / span > (< span class = "hljs-number" > 2< / span > )
f = < span class = "hljs-typename" > float64< / span > (< span class = "hljs-number" > 1e123< / span > )
f = < span class = "hljs-typename" > float64< / span > (< span class = "hljs-string" > ' a' < / span > )
< / code > < / pre >
< p > 無 論 是 隱 式 或 顯 式 , 將 一 種 類 型 轉 換 爲 另 一 種 類 型 要 求 目 標 可 以 表 示 原 始 值 . 對 於 浮 點 數 和 複 數 , 可 能 會 有 舍 入 處 理 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > const< / span > (
deadbeef = < span class = "hljs-number" > 0xdeadbeef< / span > < span class = "hljs-comment" > // untyped int with value 3735928559< / span >
a = < span class = "hljs-typename" > uint32< / span > (deadbeef) < span class = "hljs-comment" > // uint32 with value 3735928559< / span >
b = < span class = "hljs-typename" > float32< / span > (deadbeef) < span class = "hljs-comment" > // float32 with value 3735928576 (rounded up)< / span >
c = < span class = "hljs-typename" > float64< / span > (deadbeef) < span class = "hljs-comment" > // float64 with value 3735928559 (exact)< / span >
d = < span class = "hljs-typename" > int32< / span > (deadbeef) < span class = "hljs-comment" > // compile error: constant overflows int32< / span >
e = < span class = "hljs-typename" > float64< / span > (< span class = "hljs-number" > 1e309< / span > ) < span class = "hljs-comment" > // compile error: constant overflows float64< / span >
f = < span class = "hljs-typename" > uint< / span > (-< span class = "hljs-number" > 1< / span > ) < span class = "hljs-comment" > // compile error: constant underflows uint< / span >
)
< / code > < / pre >
< p > 對 於 一 個 沒 有 顯 式 類 型 的 變 量 聲 明 (包 括 短 變 量 聲 明 語 法 ), 無 類 型 的 常 量 會 被 隱 式 轉 爲 默 認 的 變 量 類 型 , 就 像 下 面 的 例 子 :< / p >
< pre > < code class = "lang-Go" > i := < span class = "hljs-number" > 0< / span > < span class = "hljs-comment" > // untyped integer; implicit int(0)< / span >
r := < span class = "hljs-string" > ' \000' < / span > < span class = "hljs-comment" > // untyped rune; implicit rune(' \000' )< / span >
f := < span class = "hljs-number" > 0.0< / span > < span class = "hljs-comment" > // untyped floating-point; implicit float64(0.0)< / span >
c := < span class = "hljs-number" > 0i< / span > < span class = "hljs-comment" > // untyped complex; implicit complex128(0i)< / span >
< / code > < / pre >
< p > 註 意 默 認 類 型 是 規 則 的 : 無 類 型 的 整 數 常 量 默 認 轉 換 爲 int, 對 應 不 確 定 的 尺 寸 , 但 是 浮 點 數 好 複 數 常 量 則 默 認 轉 換 爲 float64和 complex128. Go語 言 本 身 併 沒 有 不 確 定 的 尺 寸 的 浮 點 數 和 複 數 類 型 , 因 爲 如 何 不 知 道 浮 點 數 類 型 的 話 很 難 寫 齣 正 確 的 數 值 算 法 .< / p >
< p > 如 果 要 給 變 量 一 個 不 同 的 類 型 , 我 們 必 鬚 顯 式 地 將 無 類 型 的 常 量 轉 化 爲 所 需 的 類 型 , 或 給 聲 明 的 變 量 指 定 類 型 , 像 下 面 例 子 這 樣 :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > var< / span > i = < span class = "hljs-typename" > int8< / span > (< span class = "hljs-number" > 0< / span > )
< span class = "hljs-keyword" > var< / span > i < span class = "hljs-typename" > int8< / span > = < span class = "hljs-number" > 0< / span >
< / code > < / pre >
< p > 當 嚐 試 將 這 些 無 類 型 的 常 量 轉 爲 一 個 接 口 值 時 (見 第 7章 ), 這 些 默 認 類 型 將 顯 得 尤 爲 重 要 , 因 爲 要 靠 它 們 明 確 接 口 對 應 的 動 態 類 型 .< / p >
< pre > < code class = "lang-Go" > fmt.Printf(< span class = "hljs-string" > " %T\n" < / span > , < span class = "hljs-number" > 0< / span > ) < span class = "hljs-comment" > // " int" < / span >
fmt.Printf(< span class = "hljs-string" > " %T\n" < / span > , < span class = "hljs-number" > 0.0< / span > ) < span class = "hljs-comment" > // " float64" < / span >
fmt.Printf(< span class = "hljs-string" > " %T\n" < / span > , < span class = "hljs-number" > 0i< / span > ) < span class = "hljs-comment" > // " complex128" < / span >
fmt.Printf(< span class = "hljs-string" > " %T\n" < / span > , < span class = "hljs-string" > ' \000' < / span > ) < span class = "hljs-comment" > // " int32" (rune)< / span >
< / code > < / pre >
< p > 現 在 我 們 已 經 講 述 了 Go語 言 中 全 部 的 基 礎 數 據 類 型 . 下 一 步 將 演 示 如 何 用 基 礎 數 據 類 型 組 合 成 數 組 或 結 構 體 等 複 雜 數 據 類 型 , 然 後 構 建 用 於 解 決 實 際 編 程 問 題 的 數 據 結 構 , 這 將 是 第 四 章 的 討 論 主 題 .< / p >
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< / section >
< / div >
< / div >
< / div >
< a href = "../ch3/ch3-05.html" class = "navigation navigation-prev " aria-label = "Previous page: 字符串" > < i class = "fa fa-angle-left" > < / i > < / a >
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< a href = "../ch4/ch4.html" class = "navigation navigation-next " aria-label = "Next page: 複合數據類型" > < i class = "fa fa-angle-right" > < / i > < / a >
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< / 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 = {"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 >
