Learn about golang type assertions

The following is an introduction to golang type assertions from the golang tutorial column. I hope it will be helpful to friends in need!

There is a serialize() function in php that can serialize the array into a string for storage and transmission.

If you want to deserialize this kind of string, you only need a simple function in php The unserialize() function can be completed. But it is not so easy in golang. It requires a lot of effort and writing a lot of code.

At this time I just want to sigh, php is really the best language in the world!

I encountered such a problem during today's development. I needed to use golang to parse PHP serialized strings. I found a parsing package on github, but found that the result after parsing was an interface{ }type.

I suddenly had no idea where to start. I always thought that the data was actually a map, but after parsing I got an interface{}. How should I use it?

I feel like I need to use type assertions. But I don’t know how to use it, so I went to the community and asked, hoping that the big guys can give me an answer.

It actually works really well.

Paste the code below:

package main

import (
	"github.com/yvasiyarov/php_session_decoder/php_serialize"
	"fmt"
	"log"
)


func main() {
        // 序列化后的字符串
	str := `a:3:{s:4:"name";s:3:"tom";s:3:"age";s:2:"23";s:7:"friends";a:2:{i:0;a:1:{s:4:"name";s:5:"jerry";}i:1;a:1:{s:4:"name";s:4:"jack";}}}`
	
        // 反序列化
        decoder := php_serialize.NewUnSerializer(str)
	if result, err := decoder.Decode(); err != nil {
		panic(err)
	} else {
                // 此处进行类型断言
		decodeData, ok := result.(php_serialize.PhpArray)
		if !ok {
			log.Println(err)
		}

                // 断言后，即可获取内容
		name := decodeData["name"]
		age := decodeData["age"]
		fmt.Println(name, age)

                // 内层数据仍需再次断言
		friends, ok := decodeData["friends"].(php_serialize.PhpArray)
		if !ok {
			log.Println(err)
		}

                // 断言成功后即可获取内部数据
		for _,v := range friends {
			fmt.Printf("type:%T, value:%+v\n", v,v )
			friend, ok := v.(php_serialize.PhpArray)
			if !ok {
				log.Println(err)
			}
			friendName := friend["name"]
			fmt.Println(friendName)
		}
	}
}

It can be roughly understood like this: whatever type a variable is, what type of assertion is made. After the assertion, the result can be obtained

How to determine the type of a variable?

Print it out: fmt.Printf("%T", verb)

%T placeholder can display the type of the variable

There is another example below:

package main

import (
	"github.com/yvasiyarov/php_session_decoder/php_serialize"
	"fmt"
	"log"
)

func main() {
	// 序列化后的字符串
	str := `a:3:{s:4:"name";s:3:"tom";s:3:"age";s:2:"23";s:7:"friends";a:2:{i:0;a:1:{s:4:"name";s:5:"jerry";}i:1;a:1:{s:4:"name";s:4:"jack";}}}`
	
	// 反序列化
	decoder := php_serialize.NewUnSerializer(str)
	result, err := decoder.Decode()
	if err != nil {
		panic(err)
	}
	// 类型断言
	t := result.(php_serialize.PhpArray)
	strVal := php_serialize.PhpValueString(t["name"])
	fmt.Println(strVal)
	
	switch t := result.(type) {
	default:
		fmt.Printf("unexpected type %T\n", t)
	case php_serialize.PhpArray:
		fmt.Println(t)
		fmt.Println(t["name"])
		fmt.Println(t["age"])

		switch f := t["friends"].(type) {
		default:
			fmt.Printf("unexpected type %T\n", t)
		case php_serialize.PhpArray:
			fmt.Println(f)
			fmt.Println(f[0])
			fmt.Println(f[1])
		}
	}
}

Both the above two demos can achieve the effect, but they are written in different ways.

Later, I was introduced to another package, which also achieved the effect:

package main

import (
	"fmt"
	"log"
	"github.com/wulijun/go-php-serialize/phpserialize"
)

func main() {
	str := `a:3:{s:4:"name";s:3:"tom";s:3:"age";s:2:"23";s:7:"friends";a:2:{i:0;a:1:{s:4:"name";s:5:"jerry";}i:1;a:1:{s:4:"name";s:4:"jack";}}}`
	decodedRes, err := phpserialize.Decode(str)
	if err != nil {
		panic(err)
	}
	//fmt.Printf("%T\n", decodedRes) //type is map[interface{}]interface{}

	//type assert
	decodedData, ok := decodedRes.(map[interface{}]interface{})
	if !ok {
		fmt.Printf("unexpected type %T\n", decodedRes)
	}
	fmt.Println(decodedData["name"])
	fmt.Println(decodedData["age"])

	//fmt.Printf("%T\n", decodedData["friends"]) // type is map[interface{}]interface{}
	// type assert
	friendsRes, ok := decodedData["friends"].(map[interface{}]interface{})
	if !ok {
		fmt.Printf("unexpected type %T\n", decodedData["friends"])
	}
	for _,v := range friendsRes {
		//fmt.Printf("type: %T, val: %#v\n", v,v) // type is map[interface{}]interface{}
		friend, ok := v.(map[interface{}]interface{})
		if !ok {
			fmt.Printf("unexpected type %T\n", decodedData["friends"])
		}
		//fmt.Printf("type: %T, val: %#v\n", friend,friend) // type is map[interface{}]interface{}
		fmt.Println(friend["name"])
	}
}

The types of all the results parsed by this package are map[interface{}]interface{ }, so you can be simpler and more crude when making type assertions.

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