How to add tags in golang

Golang, as a strongly typed language, is widely used in network programming and big data fields. In actual development, it is often necessary to add tags to structures and variables to facilitate data serialization and deserialization. This article will introduce how to add tags in Golang.

1. What is a tag

A tag is a built-in type in Golang that is used to identify specific properties of a structure or variable. By using tags, you can dynamically add metadata information to objects at runtime. Tags are typically used to parse data in JSON or XML format, or to store data into a database.

The format of tag definition is as follows:

`key1:"value1" key2:"value2"`

Among them, key represents the name of the label, and value represents the value corresponding to the key. Use spaces or tabs to separate multiple key-value pairs.

2. Use tags to identify structure fields

In Golang, you can add tags to the fields of a structure for use during serialization and deserialization. For example, consider the following structure definition:

type Person struct {
    Name string `json:"name" xml:"name"`
    Age int `json:"age" xml:"age"`
}

In this example, both the Name and Age fields have a json and xml tag. These tags indicate that the corresponding names need to be used when serializing and deserializing. If we encode an example Person object into JSON format, the following will be generated:

{
    "name": "John",
    "age": 30
}

These tags can also be used when storing structured data in the database. For example, in the Gorm ORM library, you can use tags to identify information such as table names, primary keys, and field types.

type User struct {
    ID        int    `gorm:"primary_key"`
    UserName  string `gorm:"type:varchar(100);unique_index"`
    Password  string `gorm:"type:varchar(100)"`
    CreatedAt time.Time
}

In this example, the ID field uses the gorm:"primary_key" tag to indicate that it is the primary key of the table. The UserName field uses the gorm:"type:varchar(100);unique_index" tag to indicate that it is a unique index type with a length of 100. The CreatedAt field indicates the record creation time.

3. Use labels to identify variables

In Golang, you can also add labels to variables. These tags are often called "static metadata" because the information they specify does not change at runtime. Labels are typically used to specify the type or value of a variable, or to specify other directives during code generation at compile time.

For example, consider the following variable definition:

var Pi float64 `math:"pi"`

In this example, the Pi variable uses the math tag to specify its value as pi. This label does not change when the program is run, but a specific value can be assigned via code generation before running.

4. How to read tags

The reflect package in the Golang standard library provides a method for reading tags. Obtain the type and field information of the structure or variable by calling the Field() or Type() method, and then use the Tag attribute to read the tag.

For example, consider the following function:

func getTag(obj interface{}, fieldName string, tagName string) string {
    t := reflect.TypeOf(obj)
    field, _ := t.FieldByName(fieldName)
    tag := field.Tag.Get(tagName)
    return tag
}

In this example, the getTag() function accepts an obj object, a fieldName field name, and a tagName tag name. It uses the reflect package to get obj's type information and field names, and then returns the requested tag value. This function can be called using:

p := Person{Name: "John", Age: 30}
fmt.Println(getTag(p, "Name", "json")) // 输出：name

In this example, the getTag() function returns the json tag value of the Name field.

5. Summary

By using tags, metadata information can be associated with structures and variables in Golang. They are typically used to serialize and deserialize JSON and XML data, store it in a database, or generate specific code at compile time. These tags can be easily read using the reflect package so that they can be used while the program is running.

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