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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< h2 id = "105-包的匿名导入" > < a class = "header" href = "#105-包的匿名导入" > 10.5. 包的匿名导入< / a > < / h2 >
< p > 如果只是导入一个包而并不使用导入的包将会导致一个编译错误。但是有时候我们只是想利用导入包而产生的副作用： （ ） ， < code > _< / code > 来重命名导入的包。像往常一样，下划线< code > _< / code > 为空白标识符，并不能被访问。< / p >
< pre > < code class = "language-Go" > import _ " image/png" // register PNG decoder
< / code > < / pre >
< p > 这个被称为包的匿名导入。它通常是用来实现一个编译时机制， ， ， < / p >
< p > 标准库的image图像包包含了一个< code > Decode< / code > 函数，用于从< code > io.Reader< / code > 接口读取数据并解码图像， ， < code > image.Decode< / code > 很容易编写一个图像格式的转换工具，读取一种格式的图像，然后编码为另一种图像格式：< / p >
< p > < u > < i > gopl.io/ch10/jpeg< / i > < / u > < / p >
< pre > < code class = "language-Go" > // The jpeg command reads a PNG image from the standard input
// and writes it as a JPEG image to the standard output.
package main
import (
" fmt"
" image"
" image/jpeg"
_ " image/png" // register PNG decoder
" io"
" os"
)
func main() {
if err := toJPEG(os.Stdin, os.Stdout); err != nil {
fmt.Fprintf(os.Stderr, " jpeg: %v\n" , err)
os.Exit(1)
}
}
func toJPEG(in io.Reader, out io.Writer) error {
img, kind, err := image.Decode(in)
if err != nil {
return err
}
fmt.Fprintln(os.Stderr, " Input format =" , kind)
return jpeg.Encode(out, img, & jpeg.Options{Quality: 95})
}
< / code > < / pre >
< p > 如果我们将< code > gopl.io/ch3/mandelbrot< / code > （ ） ， ， （ < / p >
< pre > < code > $ go build gopl.io/ch3/mandelbrot
$ go build gopl.io/ch10/jpeg
$ ./mandelbrot | ./jpeg > mandelbrot.jpg
Input format = png
< / code > < / pre >
< p > 要注意image/png包的匿名导入语句。如果没有这一行语句， ， ： < / p >
< pre > < code > $ go build gopl.io/ch10/jpeg
$ ./mandelbrot | ./jpeg > mandelbrot.jpg
jpeg: image: unknown format
< / code > < / pre >
< p > 下面的代码演示了它的工作机制。标准库还提供了GIF、PNG和JPEG等格式图像的解码器， ， ， ， ： ； ， ； ； ， ， ： < / p >
< pre > < code class = "language-Go" > package png // image/png
func Decode(r io.Reader) (image.Image, error)
func DecodeConfig(r io.Reader) (image.Config, error)
func init() {
const pngHeader = " \x89PNG\r\n\x1a\n"
image.RegisterFormat(" png" , pngHeader, Decode, DecodeConfig)
}
< / code > < / pre >
< p > 最终的效果是， < / p >
< p > 数据库包database/sql也是采用了类似的技术， ： < / p >
< pre > < code class = "language-Go" > import (
" database/sql"
_ " github.com/lib/pq" // enable support for Postgres
_ " github.com/go-sql-driver/mysql" // enable support for MySQL
)
db, err = sql.Open(" postgres" , dbname) // OK
db, err = sql.Open(" mysql" , dbname) // OK
db, err = sql.Open(" sqlite3" , dbname) // returns error: unknown driver " sqlite3"
< / code > < / pre >
< p > < strong > 练习 10.1： < / strong > 扩展jpeg程序， ， ， < / p >
< p > < strong > 练习 10.2： < / strong > 设计一个通用的压缩文件读取框架， （ ） （ ） ， < / p >
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