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The Way to Go

Structs with Tags

Explore how to define and access tags within Go structs using the reflect package. Learn to retrieve tag values dynamically and understand the key-value tag convention. This lesson helps you implement functions that analyze and manipulate struct tags for flexible Go programming.

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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:

Go (1.6.2)
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 ...