golang: Unmarshalling dynamic YAML comments

php editor Baicao brings you an article about Golang, titled "Unmarshalling Dynamic YAML Comments". This article will introduce in detail how to parse YAML files containing comments in Golang and associate the comment information with the corresponding data. Through this article, you will learn how to use the yaml.v3 package of the Go language to implement this function, and can flexibly apply it in your own projects. Whether you are a beginner or an experienced developer, this article will provide you with valuable knowledge and tips. Let’s get started!

Question content

I want to dynamically change the comments of struct and use yaml.unmarshal as follows:

package main

import (
    "fmt"
    "reflect"

    "gopkg.in/yaml.v3"
)

type User struct {
    Name string `yaml:"dummy"`
}

func (u *User) UnmarshalYAML(node *yaml.Node) error {
    value := reflect.ValueOf(*u)
    t := value.Type()
    fields := make([]reflect.StructField, 0)

    for i := 0; i < t.NumField(); i++ {
        fields = append(fields, t.Field(i))

        if t.Field(i).Name == "Name" {
            fields[i].Tag = `yaml:"name"` // Dynamic annotation
        }
    }

    newType := reflect.StructOf(fields)
    newValue := value.Convert(newType)

    err := node.Decode(newValue.Interface()) // Cause error because it's not pointer
    return err
}

var dat string = `name: abc`

func main() {
    out := User{}
    yaml.Unmarshal([]byte(dat), &out)
    fmt.Printf("%+v\n", out)
}

It causes errors like panic:reflect:reflect.value.set using unaddressable value [recovered] I think this is because node.decode does not go with the pointer use. So how do you create a new type of pointer?

Workaround

Here is a working update demo:

package main

import (
    "fmt"
    "reflect"
    "unsafe"

    "gopkg.in/yaml.v3"
)

type User struct {
    Name string `yaml:"dummy"`
}

func (u *User) UnmarshalYAML(node *yaml.Node) error {
    t := reflect.TypeOf(*u)
    fields := make([]reflect.StructField, 0)

    for i := 0; i < t.NumField(); i++ {
        fields = append(fields, t.Field(i))

        if t.Field(i).Name == "Name" {
            fields[i].Tag = `yaml:"name"` // Dynamic annotation
        }
    }

    newType := reflect.StructOf(fields)
    newValue := reflect.NewAt(newType, unsafe.Pointer(u)).Elem()

    err := node.Decode(newValue.Addr().Interface())
    return err
}

var dat string = `name: abc`

func main() {
    out := User{}
    yaml.Unmarshal([]byte(dat), &out)
    fmt.Printf("%+v\n", out)
}

Two key changes:

1. Replace newvalue.interface() with newvalue.addr().interface(). (See this example: https://www.php.cn/link/e96c7de8f6390b1e6c71556e4e0a4959 a>)

2. Replace newvalue := value.convert(newtype) with newvalue := reflect.newat(newtype, unsafe.pointer(u)).elem() .

   We do this because the value in value :=reflect.valueof(*u) is not addressable (you can use fmt.printf(" %v", value.addr()) to validate. It throws error with message panic: reflect.value.addr() for unaddressable value.

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