2.4. Pointers

Basics

In addition to the basic data types Go also have pointers. A pointer contains a memory address of an actual value or nil if they do not point to anything. The zero value of a pointer is nil.

Pointer types are indicated with a star. For example:

• The type *int is a pointer to an int
• The type *bool is a pointer to a bool

Concerning pointers you mainly have to remember two operations. How to create a pointer to a value and how go obtain a value from a pointer:

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package main

import "fmt"

func main() {
	// declare myPointer as *int. Initially it is nil (zero value)
	var myPointer *int

	// myInt is of type int
	myInt := 42

	// Value To Pointer
	// myPointer becomes pointer to myInt
	myPointer = &myInt

	// Value From Pointer
	// Change myInt through myPointer. We can not assign an int to a *int hence we
	// have to dereference myPointer with * to assign a new number
	*myPointer = 12

	fmt.Println(myInt)
	fmt.Println(*myPointer)
}

Output:
12
12

If you dereference a pointer you should always be sure that it is not nil. Otherwise your program crashes with an error invalid memory address or nil pointer dereference (same as NullPointerException in Java).

Why Use Pointers

If we work with basic data types like int, bool, float64, etc. we rarely use pointers. We introduce the topic at this time because in the next chapter we will get to know about structs. Pointers are often used together with structs.

In Go function arguments are passed by value. This means they are copied in memory and a new variable is created. For large variables like structs this can be a performance problem. To overcome this we can use pointers. With pointers only the memory address to the variable value is passed and the variable itself is not copied. This is called passing values by reference.