Pointers and functions

Learn to pass pointers to functions and modify values.

Pointers and functions

Go copies every argument. For a function to change the caller's variable, its address is passed:

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

x := 5
ikiKat(&x)
fmt.Println(x)  // 10

Seeing *int in a function signature means "this function may change the value"; it is an important hint for the reader.

Return a value or take a pointer?

In most cases returning the new value is more readable: x = ikiKat(x). A pointer parameter makes sense when updating many fields of a struct in place or filling several outputs. fmt.Sscan(&a, &b) and json.Unmarshal(data, &v) are examples of standard functions that take pointers.

Exercise Swap

Write the degistir(a, b *int) function. For x, y := 1, 2, call degistir(&x, &y) and print x and y.

Show example solution

package main

import "fmt"

func degistir(a, b *int) {
    *a, *b = *b, *a
}

func main() {
    x, y := 1, 2
    degistir(&x, &y)
    fmt.Println(x, y)
}

Test yourself

What should the signature of a function that swaps two variables be?
- func degistir(a int, b *int)
- func degistir() (int, int)
- func degistir(a, b *int)
- func degistir(a, b int)
Show answer

func degistir(a, b *int) — To change both variables, both addresses are needed; inside, write *a, *b = *b, *a.

Pass the address

x := 5
ikiKat(____x)
fmt.Println(x)  // 10

Show answer

x := 5
ikiKat(&x)
fmt.Println(x)  // 10

Signature and meaning

func f(n int)

Takes a copy
func f(n *int)

Can change it
f(&x)

Address is passed
json.Unmarshal(b, &v)

Fills the target

Next lesson: Nil pointer

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