What use case does pointers to pointer (eg **int) have?

Question

This is pointers to pointers

package main

import "fmt"

func main() {
    var num int

    fmt.Println(&num) //  0x...0
    makePointer(&num)
}

func makePointer(firstPointer *int) {
    fmt.Println(firstPointer)  //  0x...0
    fmt.Println(&firstPointer) //  0x...1

    makePointerToAPointer(&firstPointer)
}

func makePointerToAPointer(secondPointer **int) {
    fmt.Println(secondPointer)  //  0x...1
    fmt.Println(&secondPointer) //  0x...2

}

When would you actually use this? You can properly come up with something where it would be easier to do something else, but that is not what I asking about. I really want to know where in production you would use this?

Answer 1

Pointers to pointers make sense in function parameters sometimes; not **int probably, but a pointer to a pointer to some struct, where you want the function to be able to change what object a variable points to, not just to change the contents of the struct. For example, there are a few functions in the internals of the Go compiler that take a **Node (see cmd/compile/internal/gc/racewalk.go).

I've also written a couple of functions myself that take a **html.Node; they operate on an HTML page that may or may not have already been parsed into a tree of *html.Nodes, and they may or may not need to parse the page—but if they do, I want to keep the parsed tree around so that I don't have to parse it again. These are in github.com/andybalholm/redwood/prune.go.

They are much more common in languages that do not have multiple return values, since they can be used as a way to return an additional value that is a pointer. Many Objective-C methods take an NSError** as their last parameter so that they can optionally return an NSError*.

Answer 2

The goal to pass a pointer to something is if there is need to modify the pointed value. (We also use pointers to avoid copying large data structures when passing, but that is just for optimization.)

Like in this example:

func main() {
    var i int
    fmt.Println(i)
    inc(&i)
    fmt.Println(i)
}

func inc(i *int) {
    *i++
}

Output is the expected (try it on the Go Playground):

0
1

If parameter of inc() would receive an int only, it could only modify the copy and not the original value, and so the caller would not observe the changed value.

Same goes with pointer to pointer to something. We use pointer to pointer to something, if we need to modify the pointed value, that is the pointed pointer. Like in this example:

func main() {
    var i *int
    fmt.Println(i)
    alloc(&i, 1)
    fmt.Println(i, *i)

    setToNil(&i)
    fmt.Println(i)
}

func alloc(i **int, initial int) {
    *i = new(int)
    **i = initial
}

func setToNil(i **int) {
    *i = nil
}

Output (try it on the Go Playground):

<nil>
0x1040a130 1
<nil>

The reason why pointer to pointer is not really used is because modifying a pointed value can be substituted by returning the value, and assigning it at the caller:

func main() {
    var i *int
    fmt.Println(i)
    i = alloc(1)
    fmt.Println(i, *i)

    i = setToNil()
    fmt.Println(i)
}

func alloc(initial int) *int {
    i := new(int)
    *i = initial
    return i
}

func setToNil() *int {
    return nil // Nothing to do here, assignment happens at the caller!
}

Output is the same (address might be different) (try it on the Go Playground):

<nil>
0x1040a130 1
<nil>

This variant is easier to read and maintain, so this is clearly the favored and wide-spread alternative to functions having to modify a pointer value.

In languages where functions and methods can only have 1 return value, it usually requires additional "work" if the function also wants to return other values besides the pointer, e.g. a wrapper is to be created to accommodate the multiple return values. But since Go supports multiple return values, need for pointer to pointer basically drops to zero as it can be substituted with returning the pointer that would be set to the pointed pointer; and it does not require additional work and does not make code less readable.

This is a very similar case to the builtin append() function: it appends values to a slice. And since the slice value changes (its length increases, also the pointer in it may also change if a new backing array needs to be allocated), append() returns the new slice value which you need to assign (if you want to keep the new slice).

See this related question where a pointer to pointer is proposed (but also returning a pointer is also viable / preferred): Golang: Can the pointer in a struct pointer method be reassigned to another instance?

Answer 3

In the same way a pointer to a value lets you have many references to the same value for a consistent view of the value when it changes, a pointer to a pointer lets you have many references to the same reference for a consistent view of the pointer when it changes to point to a different location in memory.

I can't say I've ever seen it used in practice in Go that I can think of.