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Go Pointers

A pointer is a value that holds the address of another value. Reading through a pointer reaches the value stored at that address.

Read Through a Pointer

The & sign takes the address of x. The pointer p holds that address, and *p reads the value there.

package main

import "fmt"

func main() {
	x := 10
	p := &x
	fmt.Println(*p)
}

Change Through a Pointer

Writing through *p changes the value at the address. The variable x then holds the new value.

package main

import "fmt"

func main() {
	x := 10
	p := &x
	*p = 20
	fmt.Println(x)
}

A Pointer Parameter

The function double takes a *int parameter. It doubles the value at that address.

package main

import "fmt"

func double(n *int) {
	*n = *n * 2
}

func main() {
	x := 21
	double(&x)
	fmt.Println(x)
}

Check a Nil Pointer

A pointer with no value behind it is nil. The check runs before any read through p.

package main

import "fmt"

func main() {
	var p *int
	if p == nil {
		fmt.Println("no value")
	} else {
		fmt.Println(*p)
	}
}

Example

package main

import "fmt"

func swap(a *int, b *int) {
	temp := *a
	*a = *b
	*b = temp
}

func main() {
	x := 3
	y := 7
	swap(&x, &y)
	fmt.Println(x, y)
}

Key points

  • Pointer: a value that holds the address of another value.
  • Use & to take the address of a value.
  • Use * before a pointer to read or change the value it points to.
  • A pointer type is written with a star, like *int for a pointer to an int.
  • The zero value of a pointer is nil, which points to nothing.
  • Check a pointer against nil before reading through it.

In this example, swap receives the addresses of x and y. The value at a is saved in temp first. The value at b is then written into a. The saved value is written into b last. Main prints x and y in their swapped order.

Note

Addresses can be stored and followed, but they cannot be moved with maths.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Go Playground

Exercise

x holds 10 and p := &x runs. What does *p = 30 do?