08 - Get to the point

Covered in this module:

• pointers
• dereferencing

pointers

Go defaults to passing parameters by value:

func main() {
    star := "Sol"
    fmt.Println(star)

    toUpper(star)
    fmt.Println(star)
}

func toUpper(input string) {
    input = strings.ToUpper(input)
}

prints

Sol
Sol

You may have noticed that toUpper didn't have an effect on the output, because it modified its input parameter, which was a copy of the string passed in by main. toUpper would only be able to copy the string initialized in main if we pass by reference, via a pointer.

referencing

You can get the address of a value with the reference syntax:

func main() {
    star := "Sol"
    fmt.Println(star)

    toUpper(&star) // pass reference to star
    fmt.Println(star)
}

func toUpper(input *string) {
    *input = strings.ToUpper(*input) // modify pointed-to value
}

prints

Sol
SOL

The example above passed a reference to star, which toUpper then dereferences so it can modify the underlying string.

operator	term	behavior
&	reference	get the address of the given value
*	dereference	get the value from the given address

Note

* is used to indicate a type as a pointer (e.g. *string) and is also used to dereference pointers.

You can get the address of a value with the reference syntax:

func main() {
    moon := "Luna"
    fmt.Println(moon)
    fmt.Println(&moon)
}

prints

Luna
0xc00000e1e0

Note

Pointer types are referred to verbally by prepending pointer to the type. For example, *string is a pointer string and *int is a pointer int.

nil

Unlike primitives, a pointer can be nil.

func main() {
    toUpper(nil)
}

func toUpper(input *string) {
    *input = strings.ToUpper(*input)
}

This example panics because strings.ToUpper is attempting to deference a nil pointer.

It's a good practice to always check nil-able values before trying to use them (unless you know for sure that the pointer can never be nil):

func main() {
    toUpper(nil)
}

func toUpper(input *string) {
    if input != nil {
        *input = strings.ToUpper(*input)
    }
}

Hands on!

1. In the repo, open the file ./intermediate/08pointers.go
2. Complete the TODOs
3. Run make 08 from project root (alternatively, type go run ./08pointers.go)
4. Example implementation available on solutions branch