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2023-06-26 17:30:52 +00:00
< 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-zh.html" > 译者序< / 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< /
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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 = "125-通过reflectvalue修改值" > < a class = "header" href = "#125-通过reflectvalue修改值" > 12.5. 通过reflect.Value修改值< / a > < / h2 >
< p > 到目前为止,反射还只是程序中变量的另一种读取方式。然而,在本节中我们将重点讨论如何通过反射机制来修改变量。< / p >
< p > 回想一下, Go语言中类似x、x.f[1]和*p形式的表达式都可以表示变量, 但是其它如x + 1和f(2)则不是变量。一个变量就是一个可寻址的内存空间,里面存储了一个值,并且存储的值可以通过内存地址来更新。< / p >
< p > 对于reflect.Values也有类似的区别。有一些reflect.Values是可取地址的; 其它一些则不可以。考虑以下的声明语句: < / p >
< pre > < code class = "language-Go" > x := 2 // value type variable?
a := reflect.ValueOf(2) // 2 int no
b := reflect.ValueOf(x) // 2 int no
c := reflect.ValueOf(& x) // & x *int no
d := c.Elem() // 2 int yes (x)
< / code > < / pre >
< p > 其中a对应的变量不可取地址。因为a中的值仅仅是整数2的拷贝副本。b中的值也同样不可取地址。c中的值还是不可取地址, 它只是一个指针< code > & x< / code > 的拷贝。实际上, 所有通过reflect.ValueOf(x)返回的reflect.Value都是不可取地址的。但是对于d, 它是c的解引用方式生成的, 指向另一个变量, 因此是可取地址的。我们可以通过调用reflect.ValueOf(& x).Elem(), 来获取任意变量x对应的可取地址的Value。< / p >
< p > 我们可以通过调用reflect.Value的CanAddr方法来判断其是否可以被取地址: < / p >
< pre > < code class = "language-Go" > fmt.Println(a.CanAddr()) // " false"
fmt.Println(b.CanAddr()) // " false"
fmt.Println(c.CanAddr()) // " false"
fmt.Println(d.CanAddr()) // " true"
< / code > < / pre >
< p > 每当我们通过指针间接地获取的reflect.Value都是可取地址的, 即使开始的是一个不可取地址的Value。在反射机制中, 所有关于是否支持取地址的规则都是类似的。例如, slice的索引表达式e[i]将隐式地包含一个指针, 它就是可取地址的, 即使开始的e表达式不支持也没有关系。以此类推, reflect.ValueOf(e).Index(i)对应的值也是可取地址的, 即使原始的reflect.ValueOf(e)不支持也没有关系。< / p >
< p > 要从变量对应的可取地址的reflect.Value来访问变量需要三个步骤。第一步是调用Addr()方法, 它返回一个Value, 里面保存了指向变量的指针。然后是在Value上调用Interface()方法, 也就是返回一个interface{}, 里面包含指向变量的指针。最后, 如果我们知道变量的类型, 我们可以使用类型的断言机制将得到的interface{}类型的接口强制转为普通的类型指针。这样我们就可以通过这个普通指针来更新变量了:< / p >
< pre > < code class = "language-Go" > x := 2
d := reflect.ValueOf(& x).Elem() // d refers to the variable x
px := d.Addr().Interface().(*int) // px := & x
*px = 3 // x = 3
fmt.Println(x) // " 3"
< / code > < / pre >
< p > 或者, 不使用指针, 而是通过调用可取地址的reflect.Value的reflect.Value.Set方法来更新对应的值: < / p >
< pre > < code class = "language-Go" > d.Set(reflect.ValueOf(4))
fmt.Println(x) // " 4"
< / code > < / pre >
< p > Set方法将在运行时执行和编译时进行类似的可赋值性约束的检查。以上代码, 变量和值都是int类型, 但是如果变量是int64类型, 那么程序将抛出一个panic异常, 所以关键问题是要确保改类型的变量可以接受对应的值: < / p >
< pre > < code class = "language-Go" > d.Set(reflect.ValueOf(int64(5))) // panic: int64 is not assignable to int
< / code > < / pre >
< p > 同样, 对一个不可取地址的reflect.Value调用Set方法也会导致panic异常: < / p >
< pre > < code class = "language-Go" > x := 2
b := reflect.ValueOf(x)
b.Set(reflect.ValueOf(3)) // panic: Set using unaddressable value
< / code > < / pre >
< p > 这里有很多用于基本数据类型的Set方法: SetInt、SetUint、SetString和SetFloat等。< / p >
< pre > < code class = "language-Go" > d := reflect.ValueOf(& x).Elem()
d.SetInt(3)
fmt.Println(x) // " 3"
< / code > < / pre >
< p > 从某种程度上说, 这些Set方法总是尽可能地完成任务。以SetInt为例, 只要变量是某种类型的有符号整数就可以工作, 即使是一些命名的类型、甚至只要底层数据类型是有符号整数就可以, 而且如果对于变量类型值太大的话会被自动截断。但需要谨慎的是: 对于一个引用interface{}类型的reflect.Value调用SetInt会导致panic异常, 即使那个interface{}变量对于整数类型也不行。< / p >
< pre > < code class = "language-Go" > x := 1
rx := reflect.ValueOf(& x).Elem()
rx.SetInt(2) // OK, x = 2
rx.Set(reflect.ValueOf(3)) // OK, x = 3
rx.SetString(" hello" ) // panic: string is not assignable to int
rx.Set(reflect.ValueOf(" hello" )) // panic: string is not assignable to int
var y interface{}
ry := reflect.ValueOf(& y).Elem()
ry.SetInt(2) // panic: SetInt called on interface Value
ry.Set(reflect.ValueOf(3)) // OK, y = int(3)
ry.SetString(" hello" ) // panic: SetString called on interface Value
ry.Set(reflect.ValueOf(" hello" )) // OK, y = " hello"
< / code > < / pre >
< p > 当我们用Display显示os.Stdout结构时, 我们发现反射可以越过Go语言的导出规则的限制读取结构体中未导出的成员, 比如在类Unix系统上os.File结构体中的fd int成员。然而, 利用反射机制并不能修改这些未导出的成员: < / p >
< pre > < code class = "language-Go" > stdout := reflect.ValueOf(os.Stdout).Elem() // *os.Stdout, an os.File var
fmt.Println(stdout.Type()) // " os.File"
fd := stdout.FieldByName(" fd" )
fmt.Println(fd.Int()) // " 1"
fd.SetInt(2) // panic: unexported field
< / code > < / pre >
< p > 一个可取地址的reflect.Value会记录一个结构体成员是否是未导出成员, 如果是的话则拒绝修改操作。因此, CanAddr方法并不能正确反映一个变量是否是可以被修改的。另一个相关的方法CanSet是用于检查对应的reflect.Value是否是可取地址并可被修改的: < / p >
< pre > < code class = "language-Go" > fmt.Println(fd.CanAddr(), fd.CanSet()) // " true false"
< / code > < / pre >
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