Pointers

A pointer is a variable that stores the memory address of a value. It allows direct access to the original value without copying it.

var p *int

Zero value: nil.

&p: Address of p.
*p: Value pointed to by p (dereferencing).

func main() {
	a := 10

	fmt.Println(a)        // 10
	fmt.Printf("%T\n", a) // int

	// Declare a pointer and assign it the address of `a`:
	ptr := &a

	fmt.Println(ptr)        // 0xed92788e008
	fmt.Printf("%T\n", ptr) // *int
	fmt.Println(*ptr)       // 10

	// Change the value through the pointer:
	*ptr = 20

	fmt.Println(a) // 20
}

Using pointers as function parameters passes a memory address instead of a value copy. This overrides Go's default pass-by-value behavior and allows modification of the original data.
func main() {
	num := 10
	fmt.Println(num) // 10

	resetVal(num)
	fmt.Println(num) // 10

	resetPtr(&num)
	fmt.Println(num) // 0
}

func resetVal(val int) {
	val = 0
}

func resetPtr(val *int) {
	*val = 0
}

Pointers as function parameters also improve efficiency for large data structures.

No unnecessary copying = Better performance.

Without pointer:

func main() {
	arr := [5]int{1, 2, 3, 4, 5}

	fmt.Printf("mem addr (arr): %p, arr: %v\n", &arr, arr) // mem addr (arr): 0xc00011e000, arr: [1 2 3 4 5]
	myFunc(arr) // mem addr (items): 0xc00011e060, items: [1 2 3 4 5]
}

func myFunc(items [5]int) [5]int {
	fmt.Printf("mem addr (items): %p, items: %v\n", &items, items)
	return items
}

With pointer:

func main() {
	arr := [5]int{1, 2, 3, 4, 5}

	fmt.Printf("mem addr (arr): %p, arr: %v\n", &arr, arr) // mem addr (arr): 0xc00011e000, arr: [1 2 3 4 5]
	myFunc(&arr) // mem addr (items): 0xc00011e000, items: [1 2 3 4 5]
}

func myFunc(items *[5]int) [5]int {
	fmt.Printf("mem addr (items): %p, items: %v\n", items, *items)
	return *items
}

Arrays are pass-by-value types in Go (like booleans, numerics, and structs). Passing them creates a full copy. Using pointers avoids that copying.