reflect.Value
- Go course
- Advanced
- Lesson 3/68
- 4 min
Learn the runtime value with Value.
reflect.Value
reflect.ValueOf(v) wraps the value itself. You read its contents with the method that fits the kind: Int(), String(), Float().
v := reflect.ValueOf(7)
fmt.Println(v.Int() * 2) // 14Calling the wrong method (e.g. Int() on a string) panics; checking v.Kind() first is safe.
Changing a value: Elem and Set
To change a value through reflect you must pass a pointer; otherwise you work on a copy and Set panics.
x := 1
v := reflect.ValueOf(&x).Elem()
v.SetInt(10)
fmt.Println(x) // 10Elem() moves to the value the pointer points to; CanSet() tells whether it is settable.
Practice Change it with reflect
Declare puan := 3. Reach the value with reflect.ValueOf(&puan).Elem(), call SetInt(30) and print puan.
Starter code
package main
import (
"fmt"
"reflect"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"reflect"
)
func main() {
puan := 3
v := reflect.ValueOf(&puan).Elem()
v.SetInt(30)
fmt.Println(puan)
}Expected output
30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens when <code>reflect.ValueOf("Go").Int()</code> runs?
- It returns 0
- It returns "Go"
- It fails to compile
- It panics Correct answer
Explanation: Int() only works on integer kinds; calling it on a string panics at run time. The compiler can't catch it.
2 Move from pointer to value
n := 5
v := reflect.ValueOf(&n).____()
v.SetInt(8)n := 5
v := reflect.ValueOf(&n).Elem()
v.SetInt(8)Explanation: Elem() gives the value the pointer points to; only then can SetInt be called.
3 Value methods
| Term | Meaning |
|---|---|
Int() |
Reads an integer |
SetInt() |
Writes an integer |
Elem() |
Follows the pointer |
CanSet() |
Is it settable? |