Go Reflection Basics
Reflection is a way for a program to read facts about its own values while it runs. The reflect package reports the type and the kind of any value.
Type and kind
TypeOf gives the exact type of a value. Kind gives the family that the type belongs to.
package main
import (
"fmt"
"reflect"
)
type Book struct {
Title string
Pages int
}
func main() {
fmt.Println("int:", reflect.TypeOf(42), reflect.TypeOf(42).Kind())
fmt.Println("string:", reflect.TypeOf("hi"), reflect.TypeOf("hi").Kind())
fmt.Println("struct:", reflect.TypeOf(Book{}), reflect.TypeOf(Book{}).Kind())
}Reading values back
ValueOf wraps a value in a handle. Int and String pull the stored value back out.
package main
import (
"fmt"
"reflect"
)
func main() {
fmt.Println(reflect.ValueOf(42).Int())
fmt.Println(reflect.ValueOf("hello").String())
}Nil and pointers
A nil interface holds no value and no type. A pointer type has kind ptr and its Elem is the pointed type.
package main
import (
"fmt"
"reflect"
)
type Book struct {
Title string
Pages int
}
func main() {
var v any
fmt.Println(reflect.TypeOf(v) == nil)
b := Book{Title: "Go", Pages: 120}
t := reflect.TypeOf(&b)
fmt.Println(t.Kind())
fmt.Println(t.Elem())
}Example
package main
import (
"fmt"
"reflect"
)
type Book struct {
Title string
Pages int
}
func describe(v any) {
t := reflect.TypeOf(v)
fmt.Println(t, t.Kind(), reflect.ValueOf(v))
}
func main() {
describe(7)
describe("hi")
describe(Book{Title: "Go", Pages: 120})
}Key points
- reflect.TypeOf returns the type of a value.
- Kind reports the broad family of a type.
- reflect.ValueOf returns a handle to the value itself.
- Int and String read the stored value back from a handle.
- A nil interface value has no type at all.
- Elem on a pointer type reaches the type it points to.
In this example, describe takes one value of any type. It prints the type, the kind, and the value for an int. It then does the same for a string and for a struct value.
Note
Reflection reads values at run time, after the program starts.
Try it Yourself
Edit the code, then press Run.
Running is simulated · Go Playground
Exercise
What does the Kind call report for the value 42?