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copied!<p>Here is a really simple example of tags being used with the <a href="http://golang.org/pkg/encoding/json/" rel="noreferrer"><code>encoding/json</code></a> package to control how fields are interpreted during encoding and decoding:</p> <p>Try live: <a href="http://play.golang.org/p/BMeR8p1cKf" rel="noreferrer">http://play.golang.org/p/BMeR8p1cKf</a></p> <pre class="lang-golang prettyprint-override"><code>package main import ( "fmt" "encoding/json" ) type Person struct { FirstName string `json:"first_name"` LastName string `json:"last_name"` MiddleName string `json:"middle_name,omitempty"` } func main() { json_string := ` { "first_name": "John", "last_name": "Smith" }` person := new(Person) json.Unmarshal([]byte(json_string), person) fmt.Println(person) new_json, _ := json.Marshal(person) fmt.Printf("%s\n", new_json) } // *Output* // &{John Smith } // {"first_name":"John","last_name":"Smith"} </code></pre> <p>The json package can look at the tags for the field and be told how to map json <=> struct field, and also extra options like whether it should ignore empty fields when serializing back to json. </p> <p>Basically, any package can use reflection on the fields to look at tag values and act on those values. There is a little more info about them in the reflect package<br> <a href="http://golang.org/pkg/reflect/#StructTag" rel="noreferrer">http://golang.org/pkg/reflect/#StructTag</a> :</p> <blockquote> <p>By convention, tag strings are a concatenation of optionally space-separated key:"value" pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ' '), quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using U+0022 '"' characters and Go string literal syntax.</p> </blockquote>

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