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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, 接口值, 由两个部分组成, 一个具体的类型和那个类型的值。它们被称为接口的动态类型和动态值。对于像Go语言这种静态类型的语言, 类型是编译期的概念; 因此一个类型不是一个值。在我们的概念模型中, 一些提供每个类型信息的值被称为类型描述符, 比如类型的名称和方法。在一个接口值中, 类型部分代表与之相关类型的描述符。< / p >
< p > 下面4个语句中, 变量w得到了3个不同的值。( 开始和最后的值是相同的) < / 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, 变量总是被一个定义明确的值初始化, 即使接口类型也不例外。对于一个接口的零值就是它的类型和值的部分都是nil( 图7.1)。< / 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, 所以这是一个空的接口值。你可以通过使用w==nil或者w!=nil来判断接口值是否为空。调用一个空接口值上的任意方法都会产生panic:< / 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, 这和显式的使用io.Writer(os.Stdout)是等价的。这类转换不管是显式的还是隐式的,都会刻画出操作到的类型和值。这个接口值的动态类型被设为< code > *os.File< / code > 指针的类型描述符, 它的动态值持有os.Stdout的拷贝; 这是一个代表处理标准输出的os.File类型变量的指针( 图7.2)。< / 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, 使得< 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, 我们不知道接口值的动态类型是什么, 所以一个接口上的调用必须使用动态分配。因为不是直接进行调用, 所以编译器必须把代码生成在类型描述符的方法Write上, 然后间接调用那个地址。这个调用的接收者是一个接口动态值的拷贝, os.Stdout。效果和下面这个直接调用一样: < / 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( 图7.3)。< / 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, 所以调用了(*bytes.Buffer).Write方法, 并且接收者是该缓冲区的地址。这个调用把字符串“hello”添加到缓冲区中。< / p >
< p > 最后, 第四个语句将nil赋给了接口值: < / 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, 把变量w恢复到和它之前定义时相同的状态, 在图7.1中可以看到。< / p >
< p > 一个接口值可以持有任意大的动态值。例如, 表示时间实例的time.Time类型, 这个类型有几个对外不公开的字段。我们从它上面创建一个接口值: < / 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!= 来进行比较。两个接口值相等仅当它们都是nil值, 或者它们的动态类型相同并且动态值也根据这个动态类型的==操作相等。因为接口值是可比较的, 所以它们可以用在map的键或者作为switch语句的操作数。< / p >
< p > 然而, 如果两个接口值的动态类型相同, 但是这个动态类型是不可比较的( 比如切片) , 将它们进行比较就会失败并且panic:< / 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, 接口类型是非常与众不同的。其它类型要么是安全的可比较类型( 如基本类型和指针) 要么是完全不可比较的类型( 如切片, 映射类型, 和函数) , 但是在比较接口值或者包含了接口值的聚合类型时, 我们必须要意识到潜在的panic。同样的风险也存在于使用接口作为map的键或者switch的操作数。只能比较你非常确定它们的动态值是可比较类型的接口值。< / p >
< p > 当我们处理错误或者调试的过程中, 得知接口值的动态类型是非常有帮助的。所以我们使用fmt包的%T动作:< / 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, 使用反射来获取接口动态类型的名称。我们会在第12章中学到反射相关的知识。< / 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: 一个包含nil指针的接口不是nil接口< / 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, main函数会将f函数的输出收集到一个bytes.Buffer类型中。< / 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, 但是实际上在out.Write方法调用时程序发生了panic: < / 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, 它给f函数的out参数赋了一个*bytes.Buffer的空指针, 所以out的动态值是nil。然而, 它的动态类型是*bytes.Buffer, 意思就是out变量是一个包含空指针值的非空接口( 如图7.5) , 所以防御性检查out!=nil的结果依然是true。< / 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, 但是这次的接收者的值是nil。对于一些如*os.File的类型, nil是一个有效的接收者( §6.2.1),但是*bytes.Buffer类型不在这些种类中。这个方法会被调用, 但是当它尝试去获取缓冲区时会发生panic。< / 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, 它也不满足这个接口具体的行为上的要求。特别是这个调用违反了(*bytes.Buffer).Write方法的接收者非空的隐含先觉条件, 所以将nil指针赋给这个接口是错误的。解决方案就是将main函数中的变量buf的类型改为io.Writer, 因此可以避免一开始就将一个不完整的值赋值给这个接口: < / 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, 让我们来看更多的一些在Go标准库中的重要接口类型。在下面的三章中, 我们会看到接口类型是怎样用在排序, web服务, 错误处理中的。< / p >
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