Detailed explanation of Golang's reflection (example)

#golang tutorial

Reflection (example), I hope it will be helpful to friends in need!

Preface

Reflection has its wonderful uses in many languages. In computer science, reflection refers to a class of applications that enable self-describing attacks and self-controlling attacks.

This article will record the author’s notes on Golang’s reflection.

After 10 seconds, the following knowledge points will be close to you: 1. Introduction to reflection2. Why use reflection?

3. What exactly reflection can do

Text1. Introduction to reflection

Golang provides a mechanism that

does not know the type at compile time In the case of

, you can

update variables

,

view values

at runtime,

call methods

, and directly operate on their layout The mechanism is called reflection.

// 伪代码
switch value := value.(type) {
case string:
	// ...一些操作
case int:	
	// ...一些操作	
case cbsStruct: // 自定义的结构体	
	// ...一些操作

// ...
}

Have you found any problems? There is a problem here: there are many types, this function will be very long to write, and there may also be custom types, which means that this judgment may have to be changed in the future, because there is no way to know It is unknown what type the value belongs to. When you cannot see through an unknown type, the above code is actually not very reasonable. At this time, you need . 3. What exactly can reflection do? reflect provides two types to access the contents of interface variables:

2. Why use reflection?	For example, sometimes we need a function that can handle various types of values. Without knowing the type, you might write:
	Reflection to help you deal with it. Reflection uses the TypeOf and ValueOf functions from the interface Obtain the information of the target object and easily complete the purpose
	1. Obtain internal information of variables

Type

Function

reflect.ValueOf()

Get the value of the data in the input parameter interface, If it is empty, return

0

<- Note that it is 0

reflect.TypeOf()

dynamically obtains the value in the input parameter interface Type, if it is empty, return

nil

<- Note that it is nil

##The above code

package main

import (
	"fmt"
	"reflect"
)

func main() {
	var name string = "咖啡色的羊驼"

	// TypeOf会返回目标数据的类型，比如int/float/struct/指针等
	reflectType := reflect.TypeOf(name)

	// valueOf返回目标数据的的值，比如上文的"咖啡色的羊驼"
	reflectValue := reflect.ValueOf(name)

	fmt.Println("type: ", reflectType)
	fmt.Println("value: ", reflectValue)
}

Output:

type:  string
value:  咖啡色的羊驼

Deeper: When the above operation occurs, reflection converts "interface type variables" into "reflected interface type variables". For example, the above actually returns the interfaces of reflect.Value and reflect.Type. object. (You can know by tracking the return type of the relevant function according to the ide)

2.Reflection of struct

package main

import (
	"fmt"
	"reflect"
)

type Student struct {
	Id   int
	Name string
}

func (s Student) Hello(){
	fmt.Println("我是一个学生")
}

func main() {
	s := Student{Id: 1, Name: "咖啡色的羊驼"}

	// 获取目标对象
	t := reflect.TypeOf(s)
	// .Name()可以获取去这个类型的名称
	fmt.Println("这个类型的名称是:", t.Name())

	// 获取目标对象的值类型
	v := reflect.ValueOf(s)
	// .NumField()来获取其包含的字段的总数
	for i := 0; i < t.NumField(); i++ {
		// 从0开始获取Student所包含的key
		key := t.Field(i)

		// 通过interface方法来获取key所对应的值
		value := v.Field(i).Interface()

		fmt.Printf("第%d个字段是：%s:%v = %v \n", i+1, key.Name, key.Type, value)
	}

	// 通过.NumMethod()来获取Student里头的方法
	for i:=0;i<t.NumMethod(); i++ {
		m := t.Method(i)
		fmt.Printf("第%d个方法是：%s:%v\n", i+1, m.Name, m.Type)
	}
}

Output:

这个类型的名称是: Student
第1个字段是：Id:int = 1 
第2个字段是：Name:string = 咖啡色的羊驼 
第1个方法是：Hello:func(main.Student)

3 .Reflection of anonymous or embedded fields

package main

import (
	"reflect"
	"fmt"
)

type Student struct {
	Id   int
	Name string
}

type People struct {
	Student // 匿名字段
}

func main() {
	p := People{Student{Id: 1, Name: "咖啡色的羊驼"}}

	t := reflect.TypeOf(p)
	// 这里需要加一个#号，可以把struct的详情都给打印出来
	// 会发现有Anonymous:true，说明是匿名字段
	fmt.Printf("%#v\n", t.Field(0))

	// 取出这个学生的名字的详情打印出来
	fmt.Printf("%#v\n", t.FieldByIndex([]int{0, 1}))

	// 获取匿名字段的值的详情
	v := reflect.ValueOf(p)
	fmt.Printf("%#v\n", v.Field(0))
}

Output:

reflect.StructField{Name:"Student", PkgPath:"", Type:(*reflect.rtype)(0x10aade0), Tag:"", Offset:0x0, Index:[]int{0}, Anonymous:true}

reflect.StructField{Name:"Name", PkgPath:"", Type:(*reflect.rtype)(0x109f4e0), Tag:"", Offset:0x8, Index:[]int{1}, Anonymous:false}

main.Student{Id:1, Name:"咖啡色的羊驼"}

4. Determine whether the incoming type is the type we want######

package main

import (
	"reflect"
	"fmt"
)

type Student struct {
	Id   int
	Name string
}

func main() {
	s := Student{Id: 1, Name: "咖啡色的羊驼"}
	t := reflect.TypeOf(s)

	// 通过.Kind()来判断对比的值是否是struct类型
	if k := t.Kind(); k == reflect.Struct {
		fmt.Println("bingo")
	}

	num := 1;
	numType := reflect.TypeOf(num)
	if k := numType.Kind(); k == reflect.Int {
		fmt.Println("bingo")
	}
}

###Output: ###

bingo
bingo

######5. Modify content through reflection######

package main

import (
	"reflect"
	"fmt"
)

type Student struct {
	Id   int
	Name string
}

func main() {
	s := &Student{Id: 1, Name: "咖啡色的羊驼"}

	v := reflect.ValueOf(s)

	// 修改值必须是指针类型否则不可行
	if v.Kind() != reflect.Ptr {
		fmt.Println("不是指针类型，没法进行修改操作")
		return
	}

	// 获取指针所指向的元素
	v = v.Elem()

	// 获取目标key的Value的封装
	name := v.FieldByName("Name")

	if name.Kind() == reflect.String {
		name.SetString("小学生")
	}

	fmt.Printf("%#v \n", *s)


	// 如果是整型的话
	test := 888
	testV := reflect.ValueOf(&test)
	testV.Elem().SetInt(666)
	fmt.Println(test)
}

###Output: ###

main.Student{Id:1, Name:"小学生"} 
666

######6. Call method through reflection# #####

package main

import (
	"fmt"
	"reflect"
)

type Student struct {
	Id   int
	Name string
}

func (s Student) EchoName(name string){
	fmt.Println("我的名字是：", name)
}

func main() {
	s := Student{Id: 1, Name: "咖啡色的羊驼"}

	v := reflect.ValueOf(s)

	// 获取方法控制权
	// 官方解释：返回v的名为name的方法的已绑定（到v的持有值的）状态的函数形式的Value封装
	mv := v.MethodByName("EchoName")
	// 拼凑参数
	args := []reflect.Value{reflect.ValueOf("咖啡色的羊驼")}

	// 调用函数
	mv.Call(args)
}

###Output: ###

我的名字是： 咖啡色的羊驼

#####4. Some small points of reflection######1. When using reflection, you need to first determine whether the value to be operated on is the expected type. , whether the "assignment" operation can be performed, otherwise the reflect package will generate a panic mercilessly. ######2. Reflection is mainly related to Golang’s interface type. Only interface types have reflection. If you are interested, you can take a look at TypeOf and ValueOf, and you will find that when the parameters are passed in, they have been converted to interface types. ###

// 以下为截取的源代码
func TypeOf(i interface{}) Type {
	eface := *(*emptyInterface)(unsafe.Pointer(&i))
	return toType(eface.typ)
}

func ValueOf(i interface{}) Value {
	if i == nil {
		return Value{}
	}
	escapes(i)

	return unpackEface(i)
}

#########

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