What are Field Tags in Go Struct Declarations and What is Their Purpose?

Unusual Syntax in Go Struct Declarations

In the context of using the mgo library for MongoDB in Go, a particular syntax has caused confusion:

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

This raised the question: What is the purpose of the string literal after the type (e.g., "_id,omitempty" for the Id field)?

Explanation

According to the Go language specification for Struct types, this syntax is used for field tags. A field tag is an optional string literal that becomes an attribute for all the fields in the corresponding field declaration. While these tags are visible through a reflection interface, they are otherwise ignored by the Go compiler.

Example

In the protocol buffer example provided in the Go spec:

// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
    microsec  uint64 "field 1"
    serverIP6 uint64 "field 2"
    process   string "field 3"
}

The string literals serve as field tags to specify the protocol buffer field numbers.

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