Welcome To Golang By Example

Pointer in Go (Golang)

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This is the  chapter 15 of the golang comprehensive tutorial series. Refer to this link for other chapters of the series – Golang Comprehensive Tutorial Series

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Previous Tutorial – Defer keyword

Now let’s check out the current tutorial. Below is the table of contents for current tutorial.

Overview

Pointer is a variable that holds a memory address of another variable.

Declaration of a Pointer

In the below example, ex is the pointer of T type.

var ex *T

In declaration the pointer value is set to its default zero value which is nil.

Initialization of a Pointer

There are two ways to initialize a pointer

Using the new operator

Pointers can  be initialized using new operator

a := new(int)
*a = 10
fmt.Println(*a) //Output will be 10

* operator can be used to dereference a pointer which means getting the value at address stored in the pointer

fmt.Println(*a) //Print the value stored at address a

Using the ambersand ‘&’ operator

&  used to get the address of a variable

a := 2
b := &a
fmt.Println(*b) //Output will be 2

Let’s see a working code covering all above points

package main

import "fmt"

func main() {
    //Declare
    var b *int
    a := 2
    b = &a
    
    //Will print a address. Output will be different everytime.
    fmt.Println(b)
    fmt.Println(*b)
    b = new(int)
    *b = 10
    fmt.Println(*b) 
}

Output:

2
10
0xc0000b0018

About * or dereferencing pointer

* operator can be used to:

package main

import "fmt"

func main() {
	a := 2
	b := &a
	fmt.Println(a)
	fmt.Println(*b)

	*b = 3
	fmt.Println(a)
	fmt.Println(*b)

	a = 4
	fmt.Println(a)
	fmt.Println(*b)
}

Output

2
2
3
3
4
4

Both a and *b refer to same variable internally.  Hence changing value of one reflects in another . Also * and & can be used together as well. But they will  cancel out each other.

Hence below two are equivalent and will print 2

Also below three are equivalent and will print the address of variable a which is stored in b variable

Note:  *a is not a valid operation as a is not a pointer

Pointer to a Pointer

It is also possible to create a pointer to a pointer in go

a := 2
b := &a
c := &b

c is a pointer to a pointer here. It stores the address of b which in turn stores the address of a. Double dereferencing using the * operator the can be used to print the value at pointer to pointer. Hence**c will print the value at which 2

Below diagram depicits pointer to pointer.

Let’s see. a program depicting pointer to pointer

package main

import "fmt"

func main() {
	a := 2
	b := &a
	c := &b

	fmt.Printf("a: %d\n", a)
	fmt.Printf("b: %x\n", b)
	fmt.Printf("c: %x\n", c)

	fmt.Println()
	fmt.Printf("a: %d\n", a)
	fmt.Printf("*&a: %d\n", *&a)
	fmt.Printf("*b: %d\n", *b)
	fmt.Printf("**c: %d\n", **c)

	fmt.Println()
	fmt.Printf("&a: %d\n", &a)
	fmt.Printf("b: %d\n", b)
	fmt.Printf("&*b: %d\n", &*b)
	fmt.Printf("*&b: %d\n", *&b)
	fmt.Printf("*c: %d\n", *c)

	fmt.Println()
	fmt.Printf("b: %d\n", &b)
	fmt.Printf("*c: %d\n", c)
}

Output

a: 2
b: c000018078
c: c00000e028

a: 2
*&a: 2
*b: 2
**c: 2

&a: 824633819256
b: 824633819256
&*b: 824633819256
*&b: 824633819256
*c: 824633819256

b: 824633778216
*c: 824633778216

From output is clear that

Below are equivalent and equal to value of variable a which is 2

Below are equivalent and equal to value of variable b which is address of a

Below are equivalent and equal to value of variable c which is address of b

Pointer Arithmetic

Pointer arithmetic is not possible in golang unlike C language. It raises compilation error.

package main
func main() {
    a := 1
    b := &a
    b = b + 1
}

Output

Above program raises compilation error

invalid operation: b + 1 (mismatched types *int and int)

Default Zero Value of Pointer

Default zero value of a pointer is nil.. Let’s see a program for it

package main

import "fmt"

func main() {
    var a *int
    fmt.Print("Default Zero Value of a pointer: ")
    fmt.Println(a)
}

Output:

Default value of pointer:

Conclusion

This is all about pointers in golang. Hope you have liked this article. Please share feedback/improvements/mistakes in comments

Next Tutorial – Struct
Previous Tutorial – Defer keyword