Is metadata or attributes allowed in Golang?

Question

How are these various validation libraries adding this meta data to structs like:

type Post struct {
    Title    string `valid:"alphanum,required"`
    Message  string `valid:"duck,ascii"`
    AuthorIP string `valid:"ipv4"`
    Date     string `valid:"-"`
}

I'm confused, the property is Title, the type is string. Beyond that how can you just add valid:"alphanum,required" Is this using reflection?

Is this like attributes in other languages?

[Required]
public int Title { get;set; }

Answer 1

Go doesn't have attributes in general sense. The strings in the struct are struct tags:

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

// 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"
}

You can't add or change them, but you can access them using the reflect package.

Another thing that kinda looks like attributes are "magic comments" like

// +build amd64

or

//go:noinline

These are compiler-specific and IIRC are not a part of the language specification.