Nil pointer
- Go course
- Beginner
- Lesson 67/90
- 4 min
Learn the nil pointer concept and safe usage.
Nil pointers
A pointer that points to nothing is nil. Dereferencing a nil pointer crashes the program:
var p *int
fmt.Println(p == nil) // true
fmt.Println(*p) // panic: invalid memory address or nil pointer dereferenceThat's why pointers coming from outside or that are optional are checked before use.
nil as an optional value
A pointer's ability to be nil is used to express "no value". For example, returning nil for a product that wasn't found:
func bul(urunler []Urun, ad string) *Urun {
for i := range urunler {
if urunler[i].Ad == ad {
return &urunler[i]
}
}
return nil
}
if u := bul(liste, "silgi"); u != nil {
fmt.Println(u.Fiyat)
}If error details are needed, returning (Urun, error) instead of nil is clearer.
Practice Safe reading
Write degerVeya(p *int, varsayilan int) int: return varsayilan if p is nil, otherwise *p. For x := 7, print the results of degerVeya(&x, 0) and degerVeya(nil, -1) on one line.
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample solution
package main
import "fmt"
func degerVeya(p *int, varsayilan int) int {
if p == nil {
return varsayilan
}
return *p
}
func main() {
x := 7
fmt.Println(degerVeya(&x, 0), degerVeya(nil, -1))
}Expected output
7 -1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does accessing a struct field through a nil pointer (<code>var u *Urun; u.Ad</code>) do?
- It doesn't compile
- It panics Correct answer
- It returns an empty string
- It returns nil
Explanation: u.Ad really means (*u).Ad; since u is nil, a nil dereference panic happens at run time.
2 nil check
if p ____ nil {
fmt.Println(*p)
}if p != nil {
fmt.Println(*p)
}Explanation: Dereferencing is safe while p != nil.
3 Case and result
| Term | Meaning |
|---|---|
var p *int |
nil |
*p (p nil) |
panic |
p == nil |
true |
bul(...) bulamazsa |
returns nil |