The reflect package
- Go course
- Advanced
- Lesson 1/68
- 4 min
Learn the basics of the reflect package.
The reflect package
The reflect package lets you inspect the type and contents of a value at run time. You can ask what an any value really is when its type is unknown at compile time.
var x any = 3.14
fmt.Println(reflect.TypeOf(x)) // float64Packages such as encoding/json and fmt read struct fields this way.
The cost: speed and type safety
Reflection is powerful but has two costs: it is slower than normal code, and mistakes show up as a panic at run time instead of a compile error. That is why interfaces and generics come first; reflect is used when writing truly generic tools (serializers, ORMs, validators).
Practice Print the type
Declare var x any = 42 and print the result of reflect.TypeOf(x) with fmt.Println.
Starter code
package main
import (
"fmt"
"reflect"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"reflect"
)
func main() {
var x any = 42
fmt.Println(reflect.TypeOf(x))
}Expected output
int
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When is the reflect package needed most?
- To read files
- When inspecting values whose type is unknown at compile time Correct answer
- To make loops faster
- To start goroutines
Explanation: reflect reads the real type and fields of a value passed as any at run time; JSON encoders rely on it.
2 Query the type
var x any = "Go"
fmt.Println(reflect.____(x))var x any = "Go"
fmt.Println(reflect.TypeOf(x))Explanation: reflect.TypeOf returns the type of a value (string).
3 reflect concepts
| Term | Meaning |
|---|---|
reflect.TypeOf |
Returns the type |
reflect.ValueOf |
Wraps the value |
any |
Can hold any type |
panic |
Happens on misuse of reflect |