The Proper Use of Pointers in Go (golang)

#go #programming #tutorial

Go has become increasingly popular in recent years, especially in my local area. Go has been consistently displacing other backend languages like Ruby, Python, C# and Java. Go is wanted for its simplicity, explicitness, speed, and low memory consumption.

Many developers that are new to the language, or new to a language that can handle memory directly using pointers end up misusing those pointers.

What Is a Pointer?

A pointer is a variable that stores the address of a value, rather than the value itself. If you think of a computer’s memory (RAM) as a JSON object, a pointer would be like the key, and a normal variable would be the value.

{
  "pointer": "variableValue"
}

Lets see one in action:

package main

import "fmt"

func main() {
        // create a normal string variable
    name := "original"
        // pass in a pointer to the string variable using '&'
    setName(&name, "qvault")
    fmt.Println(name)
}

func setName(ptr *string, newName string) {
        // dereference the pointer so we can modify the value
        // and set the value to "qvault"
    *ptr = newName
}

This prints:

qvault

As you can see, because we have a pointer to the address of the variable_,_ we can modify its value, even within the scope of another function. If the value were not a pointer, this would not work:

package main

import "fmt"

func main() {
    name := "original"
    setNameBroken(name, "qvault")
    fmt.Println(name)
}

func setNameBroken(ptr string, newName string) {
    ptr = newName
}

prints:

original

Pointers can be useful, but in the same way that they are useful, they can be dangerous. For example, if we dereference a pointer that has no value, the program will panic. For this reason we always check if an error value is nil before trying to print it.

Syntax

1. Creating a pointer: &

newString := ""
newStringPointer := &newString

If you print that pointer you will see a memory address.

package main

import "fmt"

func main() {
    newString := ""
    newStringPointer := &newString
    fmt.Println(newStringPointer)
}

prints: 0xc00000e1e0

Which is the memory address of that variable in your machine.

2. Describing a pointer: *

In a function signature or type definition, the * is used to designate that a value is a pointer.

func passPointer(pointer *string) {
}

3. Dereferencing a pointer: *

It can be slightly confusing, but the * is used to describe a pointer and it is also used as an operator to dereference a pointer.

func derefPointer(pointer *string) {
    newStringVariable := *pointer
        // newStringVariable is just a normal string
}

When Should I Use a Pointer?

There are probably many nuanced cases for when a pointer is a good idea, but I would guess that 95% of the time when you use a pointer, it should be for one the following reasons:

1. A function that mutates one of its parameters

When I call a function that takes a pointer as an argument, I expect that my variable will be mutated. If you aren’t mutating the variable in your function, then you probably shouldn’t be using a pointer.

2. Better Performance

If you have a string that contains an entire novel in memory it gets really expensive to copy that variable each time it is passed to a new function. It may be worthwhile to pass a pointer instead, which will save CPU and memory. This comes at the cost of readability however, so only make this optimization if you must.

3. Need a Nil Value Option

Sometimes a function needs to know what something’s value is, as well as if it exists or not. I usually use this when reading JSON to know if a field exists or not. For example, if a JSON object is:

{ "name": "qvault" } ----> *name: "qvault"

{ "name": "" } ----------> *name: ""

{} ----------------------> *name: nil

Thanks

These are some rules of thumb for when to use pointers in your code. If you are unsure, and a normal value will work just fine, I would advise to avoid the pointer. Pointers are useful tools but can lead to nasty bugs or unreadable code quite easily.

By Lane Wagner @wagslane

Download Qvault: https://qvault.io

Star our Github: https://github.com/q-vault/qvault

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