Summary - Reflection

Combine reflect.Type and Value to write a struct printer.

Reflection summary

In this group you read the type and kind with TypeOf and the contents with ValueOf; changed values through a pointer and Elem; walked struct fields with NumField; and called methods with MethodByName.

t := reflect.TypeOf(v)   // what is it?
val := reflect.ValueOf(v) // what does it hold?

Rule: consider interfaces and generics first; keep reflect for generic tools.

Reading fields by tag

A real example: a tiny encoder that prints struct fields by their tags.

for i := 0; i < t.NumField(); i++ {
    f := t.Field(i)
    fmt.Printf("%s=%v\n", f.Tag.Get("ad"), v.Field(i))
}

JSON, YAML and database libraries essentially use this loop.

Exercise Print with tags

Define type Ayar struct { Port int `ad:"port"`; Mod string `ad:"mod"` }. For Ayar{8080, "test"}, print each field as tag=value on its own line (use Tag.Get("ad")).

Show example solution

package main

import (
    "fmt"
    "reflect"
)

type Ayar struct {
    Port int    `ad:"port"`
    Mod  string `ad:"mod"`
}

func main() {
    a := Ayar{8080, "test"}
    t, v := reflect.TypeOf(a), reflect.ValueOf(a)
    for i := 0; i < t.NumField(); i++ {
        fmt.Printf("%s=%v\n", t.Field(i).Tag.Get("ad"), v.Field(i))
    }
}

Test yourself

What is needed to change a variable with reflect?
- Passing the variable's pointer and using Elem()
- Passing the variable directly to ValueOf
- Getting its type with TypeOf
- Calling NumField
Show answer

Passing the variable's pointer and using Elem() — ValueOf(x) takes a copy and is not settable; ValueOf(&x).Elem() reaches the real variable.

Read the tag

f := t.Field(0)
fmt.Println(f.Tag.____("json"))

Show answer

f := t.Field(0)
fmt.Println(f.Tag.Get("json"))

Group summary

TypeOf

Type and kind
ValueOf

Contents
Elem + SetInt

Changing the value
MethodByName + Call

Dynamic call

Next lesson: The unsafe package

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies