How Do Tagged Struct Fields in Go Facilitate Metadata Handling?

Tagged Struct Fields in Go

In Go, struct fields can be accompanied by string literals known as "tags." This syntax is commonly encountered in conjunction with libraries that require metadata for serialization or other purposes.

For instance, in the mgo MongoDB driver, struct fields can be tagged to indicate their corresponding object ID in MongoDB:

type Something struct {
    Id bson.ObjectId "_id,omitempty"
    Name string
}

Here, the first field Id is declared as a bson.ObjectId type with the string literal "_id,omitempty". This string represents the object ID field in MongoDB.

The Go specification explains that such tags can be accessed through the reflection interface and are otherwise ignored. This allows libraries to extract information from the tags without affecting the actual field behavior.

Here's an example from the language specification:

struct {
    microsec  uint64 "field 1"
    serverIP6 uint64 "field 2"
    process   string "field 3"
}

In this example, the fields are tagged with protocol buffer field numbers. This information can be used when interacting with protocol buffer services.

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