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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 = "75--接口值" > < a class = "header" href = "#75--接口值" > 7.5. 接口值< / a > < / h2 >
< p > 概念上讲一个接口的值， ， ， ， ； ， ， ， < / p >
< p > 下面4个语句中， （ ） < / p >
< pre > < code class = "language-go" > var w io.Writer
w = os.Stdout
w = new(bytes.Buffer)
w = nil
< / code > < / pre >
< p > 让我们进一步观察在每一个语句后的w变量的值和动态行为。第一个语句定义了变量w:< / p >
< pre > < code class = "language-go" > var w io.Writer
< / code > < / pre >
< p > 在Go语言中， ， （ < / p >
< p > < img src = "../images/ch7-01.png" alt = "" / > < / p >
< p > 一个接口值基于它的动态类型被描述为空或非空， < / p >
< pre > < code class = "language-go" > w.Write([]byte(" hello" )) // panic: nil pointer dereference
< / code > < / pre >
< p > 第二个语句将一个< code > *os.File< / code > 类型的值赋给变量w:< / p >
< pre > < code class = "language-go" > w = os.Stdout
< / code > < / pre >
< p > 这个赋值过程调用了一个具体类型到接口类型的隐式转换， < code > *os.File< / code > 指针的类型描述符， ； （ < / p >
< p > < img src = "../images/ch7-02.png" alt = "" / > < / p >
< p > 调用一个包含< code > *os.File< / code > 类型指针的接口值的Write方法， < code > (*os.File).Write< / code > 方法被调用。这个调用输出“hello”。< / p >
< pre > < code class = "language-go" > w.Write([]byte(" hello" )) // " hello"
< / code > < / pre >
< p > 通常在编译期， ， ， ， ， ： < / p >
< pre > < code class = "language-go" > os.Stdout.Write([]byte(" hello" )) // " hello"
< / code > < / pre >
< p > 第三个语句给接口值赋了一个*bytes.Buffer类型的值< / p >
< pre > < code class = "language-go" > w = new(bytes.Buffer)
< / code > < / pre >
< p > 现在动态类型是*bytes.Buffer并且动态值是一个指向新分配的缓冲区的指针（ < / p >
< p > < img src = "../images/ch7-03.png" alt = "" / > < / p >
< p > Write方法的调用也使用了和之前一样的机制： < / p >
< pre > < code class = "language-go" > w.Write([]byte(" hello" )) // writes " hello" to the bytes.Buffers
< / code > < / pre >
< p > 这次类型描述符是*bytes.Buffer， ， < / p >
< p > 最后， ： < / p >
< pre > < code class = "language-go" > w = nil
< / code > < / pre >
< p > 这个重置将它所有的部分都设为nil值， ， < / p >
< p > 一个接口值可以持有任意大的动态值。例如， ， ： < / p >
< pre > < code class = "language-go" > var x interface{} = time.Now()
< / code > < / pre >
< p > 结果可能和图7.4相似。从概念上讲，不论接口值多大，动态值总是可以容下它。（这只是一个概念上的模型；具体的实现可能会非常不同）< / p >
< p > < img src = "../images/ch7-04.png" alt = "" / > < / p >
< p > 接口值可以使用==和!＝ ， ， < / p >
< p > 然而， ， （ ） ， < / p >
< pre > < code class = "language-go" > var x interface{} = []int{1, 2, 3}
fmt.Println(x == x) // panic: comparing uncomparable type []int
< / code > < / pre >
< p > 考虑到这点， （ ） （ ， ， ） ， ， < / p >
< p > 当我们处理错误或者调试的过程中， < / p >
< pre > < code class = "language-go" > var w io.Writer
fmt.Printf(" %T\n" , w) // " < nil> "
w = os.Stdout
fmt.Printf(" %T\n" , w) // " *os.File"
w = new(bytes.Buffer)
fmt.Printf(" %T\n" , w) // " *bytes.Buffer"
< / code > < / pre >
< p > 在fmt包内部， < / p >
< h3 id = "751--警告一个包含nil指针的接口不是nil接口" > < a class = "header" href = "#751--警告一个包含nil指针的接口不是nil接口" > 7.5.1. 警告： < / a > < / h3 >
< p > 一个不包含任何值的nil接口值和一个刚好包含nil指针的接口值是不同的。这个细微区别产生了一个容易绊倒每个Go程序员的陷阱。< / p >
< p > 思考下面的程序。当debug变量设置为true时， < / p >
< pre > < code class = "language-go" > const debug = true
func main() {
var buf *bytes.Buffer
if debug {
buf = new(bytes.Buffer) // enable collection of output
}
f(buf) // NOTE: subtly incorrect!
if debug {
// ...use buf...
}
}
// If out is non-nil, output will be written to it.
func f(out io.Writer) {
// ...do something...
if out != nil {
out.Write([]byte(" done!\n" ))
}
}
< / code > < / pre >
< p > 我们可能会预计当把变量debug设置为false时可以禁止对输出的收集， ： < / p >
< pre > < code class = "language-go" > if out != nil {
out.Write([]byte(" done!\n" )) // panic: nil pointer dereference
}
< / code > < / pre >
< p > 当main函数调用函数f时， ， ， ， （ ） ， < / p >
< p > < img src = "../images/ch7-05.png" alt = "" / > < / p >
< p > 动态分配机制依然决定(*bytes.Buffer).Write的方法会被调用， ， （ ， < / p >
< p > 问题在于尽管一个nil的*bytes.Buffer指针有实现这个接口的方法， ， ， ： < / p >
< pre > < code class = "language-go" > var buf io.Writer
if debug {
buf = new(bytes.Buffer) // enable collection of output
}
f(buf) // OK
< / code > < / pre >
< p > 现在我们已经把接口值的技巧都讲完了， ， ， ， < / p >
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