reflect.Value

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)  // 14

Calling 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)  // 10

Elem() moves to the value the pointer points to; CanSet() tells whether it is settable.

Exercise Change it with reflect

Declare puan := 3. Reach the value with reflect.ValueOf(&puan).Elem(), call SetInt(30) and print puan.

Show example solution

package main

import (
    "fmt"
    "reflect"
)

func main() {
    puan := 3
    v := reflect.ValueOf(&puan).Elem()
    v.SetInt(30)
    fmt.Println(puan)
}

Test yourself

What happens when <code>reflect.ValueOf("Go").Int()</code> runs?
- It returns 0
- It returns "Go"
- It fails to compile
- It panics
Show answer

It panics — Int() only works on integer kinds; calling it on a string panics at run time. The compiler can't catch it.

Move from pointer to value

n := 5
v := reflect.ValueOf(&n).____()
v.SetInt(8)

Show answer

n := 5
v := reflect.ValueOf(&n).Elem()
v.SetInt(8)

Value methods

Int()

Reads an integer
SetInt()

Writes an integer
Elem()

Follows the pointer
CanSet()

Is it settable?

Next lesson: Reflecting struct fields

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