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Structs with Tags

Structs with Tags

In this lesson, we'll cover how to attach a tag with the field of a struct and how to read it.

We'll cover the following...

A field in a struct can, apart from a name and a type, also optionally have a tag. It is a string (or a raw string) attached to the field, which could be documentation or some other important label. The tag-content cannot be used in normal programming; only the package reflect can access it. This package which we’ll explore deeper in the next chapter (Chapter 9), can investigate types, their properties, and methods in runtime. For example: reflect.TypeOf() on a variable gives its type. If this is a struct type, it can be indexed by Field, and then, the Tag property can be used.

The following program shows how this works:

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package main
import (
"fmt"
"reflect"
)
type TagType struct { // tags
  field1 bool "An important answer"
  field2 string `The name of the thing` 
  field3 int "How much there are"
}
func main() {
  tt := TagType{true, "Barack Obama", 1}
  for i:= 0; i < 3; i++ {
    refTag(tt, i)
  }
}
func refTag(tt TagType, ix int) {
  ttType := reflect.TypeOf(tt)
  ixField := ttType.Field(ix)       // getting field at a position ix
  fmt.Printf("%v\n", ixField.Tag)   // printing tags
}

In the above code, at line 4, we import a package reflect. At line 7, we are making a struct TagType type variable with three ...