Summary - Reflection
- Go course
- Advanced
- Lesson 6/68
- 4 min
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.
Practice 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")).
Starter code
package main
import (
"fmt"
"reflect"
)
func main() {
// buraya yaz
}Sample 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))
}
}Expected output
port=8080 mod=test
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is needed to change a variable with reflect?
- Passing the variable's pointer and using Elem() Correct answer
- Passing the variable directly to ValueOf
- Getting its type with TypeOf
- Calling NumField
Explanation: ValueOf(x) takes a copy and is not settable; ValueOf(&x).Elem() reaches the real variable.
2 Read the tag
f := t.Field(0)
fmt.Println(f.Tag.____("json"))f := t.Field(0)
fmt.Println(f.Tag.Get("json"))Explanation: Tag.Get returns the tag value for the given key.
3 Group summary
| Term | Meaning |
|---|---|
TypeOf |
Type and kind |
ValueOf |
Contents |
Elem + SetInt |
Changing the value |
MethodByName + Call |
Dynamic call |